https://www.ietf.org/rfc/rfc4251.txt
Network Working Group T. Ylonen
Request for Comments: 4251 SSH Communications Security Corp
Category: Standards Track C. Lonvick, Ed.
Cisco Systems, Inc.
January 2006
The Secure Shell (SSH) Protocol Architecture
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the “Internet
Official Protocol Standards” (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright © The Internet Society (2006).
Abstract
The Secure Shell (SSH) Protocol is a protocol for secure remote login
and other secure network services over an insecure network. This
document describes the architecture of the SSH protocol, as well as
the notation and terminology used in SSH protocol documents. It also
discusses the SSH algorithm naming system that allows local
extensions. The SSH protocol consists of three major components: The
Transport Layer Protocol provides server authentication,
confidentiality, and integrity with perfect forward secrecy. The
User Authentication Protocol authenticates the client to the server.
The Connection Protocol multiplexes the encrypted tunnel into several
logical channels. Details of these protocols are described in
separate documents.
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RFC 4251 SSH Protocol Architecture January 2006
Table of Contents
- Introduction …3
- Contributors …3
- Conventions Used in This Document …4
- Architecture …4
4.1. Host Keys …4
4.2. Extensibility …6
4.3. Policy Issues …6
4.4. Security Properties …7
4.5. Localization and Character Set Support …7 - Data Type Representations Used in the SSH Protocols …8
- Algorithm and Method Naming …10
- Message Numbers …11
- IANA Considerations …12
- Security Considerations …13
9.1. Pseudo-Random Number Generation …13
9.2. Control Character Filtering …14
9.3. Transport …14
9.3.1. Confidentiality …14
9.3.2. Data Integrity …16
9.3.3. Replay …16
9.3.4. Man-in-the-middle …17
9.3.5. Denial of Service …19
9.3.6. Covert Channels …20
9.3.7. Forward Secrecy …20
9.3.8. Ordering of Key Exchange Methods …20
9.3.9. Traffic Analysis …21
9.4. Authentication Protocol …21
9.4.1. Weak Transport …21
9.4.2. Debug Messages …22
9.4.3. Local Security Policy …22
9.4.4. Public Key Authentication …23
9.4.5. Password Authentication …23
9.4.6. Host-Based Authentication …23
9.5. Connection Protocol …24
9.5.1. End Point Security …24
9.5.2. Proxy Forwarding …24
9.5.3. X11 Forwarding …24 - References …26
10.1. Normative References …26
10.2. Informative References …26
Authors’ Addresses …29
Trademark Notice …29
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- Introduction
Secure Shell (SSH) is a protocol for secure remote login and other
secure network services over an insecure network. It consists of
three major components:
o The Transport Layer Protocol [SSH-TRANS] provides server
authentication, confidentiality, and integrity. It may optionally
also provide compression. The transport layer will typically be
run over a TCP/IP connection, but might also be used on top of any
other reliable data stream.
o The User Authentication Protocol [SSH-USERAUTH] authenticates the
client-side user to the server. It runs over the transport layer
protocol.
o The Connection Protocol [SSH-CONNECT] multiplexes the encrypted
tunnel into several logical channels. It runs over the user
authentication protocol.
The client sends a service request once a secure transport layer
connection has been established. A second service request is sent
after user authentication is complete. This allows new protocols to
be defined and coexist with the protocols listed above.
The connection protocol provides channels that can be used for a wide
range of purposes. Standard methods are provided for setting up
secure interactive shell sessions and for forwarding (“tunneling”)
arbitrary TCP/IP ports and X11 connections.
- Contributors
The major original contributors of this set of documents have been:
Tatu Ylonen, Tero Kivinen, Timo J. Rinne, Sami Lehtinen (all of SSH
Communications Security Corp), and Markku-Juhani O. Saarinen
(University of Jyvaskyla). Darren Moffat was the original editor of
this set of documents and also made very substantial contributions.
Many people contributed to the development of this document over the
years. People who should be acknowledged include Mats Andersson, Ben
Harris, Bill Sommerfeld, Brent McClure, Niels Moller, Damien Miller,
Derek Fawcus, Frank Cusack, Heikki Nousiainen, Jakob Schlyter, Jeff
Van Dyke, Jeffrey Altman, Jeffrey Hutzelman, Jon Bright, Joseph
Galbraith, Ken Hornstein, Markus Friedl, Martin Forssen, Nicolas
Williams, Niels Provos, Perry Metzger, Peter Gutmann, Simon
Josefsson, Simon Tatham, Wei Dai, Denis Bider, der Mouse, and
Tadayoshi Kohno. Listing their names here does not mean that they
endorse this document, but that they have contributed to it.
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- Conventions Used in This Document
All documents related to the SSH protocols shall use the keywords
“MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”,
“SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” to describe
requirements. These keywords are to be interpreted as described in
[RFC2119].
The keywords “PRIVATE USE”, “HIERARCHICAL ALLOCATION”, “FIRST COME
FIRST SERVED”, “EXPERT REVIEW”, “SPECIFICATION REQUIRED”, “IESG
APPROVAL”, “IETF CONSENSUS”, and “STANDARDS ACTION” that appear in
this document when used to describe namespace allocation are to be
interpreted as described in [RFC2434].
Protocol fields and possible values to fill them are defined in this
set of documents. Protocol fields will be defined in the message
definitions. As an example, SSH_MSG_CHANNEL_DATA is defined as
follows.
byte SSH_MSG_CHANNEL_DATA
uint32 recipient channel
string data
Throughout these documents, when the fields are referenced, they will
appear within single quotes. When values to fill those fields are
referenced, they will appear within double quotes. Using the above
example, possible values for ‘data’ are “foo” and “bar”.
- Architecture
4.1. Host Keys
Each server host SHOULD have a host key. Hosts MAY have multiple
host keys using multiple different algorithms. Multiple hosts MAY
share the same host key. If a host has keys at all, it MUST have at
least one key that uses each REQUIRED public key algorithm (DSS
[FIPS-186-2]).
The server host key is used during key exchange to verify that the
client is really talking to the correct server. For this to be
possible, the client must have a priori knowledge of the server’s
public host key.
Two different trust models can be used:
o The client has a local database that associates each host name (as
typed by the user) with the corresponding public host key. This
method requires no centrally administered infrastructure, and no
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third-party coordination. The downside is that the database of
name-to-key associations may become burdensome to maintain.
o The host name-to-key association is certified by a trusted
certification authority (CA). The client only knows the CA root
key, and can verify the validity of all host keys certified by
accepted CAs.
The second alternative eases the maintenance problem, since ideally
only a single CA key needs to be securely stored on the client. On
the other hand, each host key must be appropriately certified by a
central authority before authorization is possible. Also, a lot of
trust is placed on the central infrastructure.
The protocol provides the option that the server name - host key
association is not checked when connecting to the host for the first
time. This allows communication without prior communication of host
keys or certification. The connection still provides protection
against passive listening; however, it becomes vulnerable to active
man-in-the-middle attacks. Implementations SHOULD NOT normally allow
such connections by default, as they pose a potential security
problem. However, as there is no widely deployed key infrastructure
available on the Internet at the time of this writing, this option
makes the protocol much more usable during the transition time until
such an infrastructure emerges, while still providing a much higher
level of security than that offered by older solutions (e.g., telnet
[RFC0854] and rlogin [RFC1282]).
Implementations SHOULD try to make the best effort to check host
keys. An example of a possible strategy is to only accept a host key
without checking the first time a host is connected, save the key in
a local database, and compare against that key on all future
connections to that host.
Implementations MAY provide additional methods for verifying the
correctness of host keys, e.g., a hexadecimal fingerprint derived
from the SHA-1 hash [FIPS-180-2] of the public key. Such
fingerprints can easily be verified by using telephone or other
external communication channels.
All implementations SHOULD provide an option not to accept host keys
that cannot be verified.
The members of this Working Group believe that ‘ease of use’ is
critical to end-user acceptance of security solutions, and no
improvement in security is gained if the new solutions are not used.
Thus, providing the option not to check the server host key is
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believed to improve the overall security of the Internet, even though
it reduces the security of the protocol in configurations where it is
allowed.
4.2. Extensibility
We believe that the protocol will evolve over time, and some
organizations will want to use their own encryption, authentication,
and/or key exchange methods. Central registration of all extensions
is cumbersome, especially for experimental or classified features.
On the other hand, having no central registration leads to conflicts
in method identifiers, making interoperability difficult.
We have chosen to identify algorithms, methods, formats, and
extension protocols with textual names that are of a specific format.
DNS names are used to create local namespaces where experimental or
classified extensions can be defined without fear of conflicts with
other implementations.
One design goal has been to keep the base protocol as simple as
possible, and to require as few algorithms as possible. However, all
implementations MUST support a minimal set of algorithms to ensure
interoperability (this does not imply that the local policy on all
hosts would necessarily allow these algorithms). The mandatory
algorithms are specified in the relevant protocol documents.
Additional algorithms, methods, formats, and extension protocols can
be defined in separate documents. See Section 6, Algorithm Naming,
for more information.
4.3. Policy Issues
The protocol allows full negotiation of encryption, integrity, key
exchange, compression, and public key algorithms and formats.
Encryption, integrity, public key, and compression algorithms can be
different for each direction.
The following policy issues SHOULD be addressed in the configuration
mechanisms of each implementation:
o Encryption, integrity, and compression algorithms, separately for
each direction. The policy MUST specify which is the preferred
algorithm (e.g., the first algorithm listed in each category).
o Public key algorithms and key exchange method to be used for host
authentication. The existence of trusted host keys for different
public key algorithms also affects this choice.
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o The authentication methods that are to be required by the server
for each user. The server’s policy MAY require multiple
authentication for some or all users. The required algorithms MAY
depend on the location from where the user is trying to gain
access.
o The operations that the user is allowed to perform using the
connection protocol. Some issues are related to security; for
example, the policy SHOULD NOT allow the server to start sessions
or run commands on the client machine, and MUST NOT allow
connections to the authentication agent unless forwarding such
connections has been requested. Other issues, such as which
TCP/IP ports can be forwarded and by whom, are clearly issues of
local policy. Many of these issues may involve traversing or
bypassing firewalls, and are interrelated with the local security
policy.
4.4. Security Properties
The primary goal of the SSH protocol is to improve security on the
Internet. It attempts to do this in a way that is easy to deploy,
even at the cost of absolute security.
o All encryption, integrity, and public key algorithms used are
well-known, well-established algorithms.
o All algorithms are used with cryptographically sound key sizes
that are believed to provide protection against even the strongest
cryptanalytic attacks for decades.
o All algorithms are negotiated, and in case some algorithm is
broken, it is easy to switch to some other algorithm without
modifying the base protocol.
Specific concessions were made to make widespread, fast deployment
easier. The particular case where this comes up is verifying that
the server host key really belongs to the desired host; the protocol
allows the verification to be left out, but this is NOT RECOMMENDED.
This is believed to significantly improve usability in the short
term, until widespread Internet public key infrastructures emerge.
4.5. Localization and Character Set Support
For the most part, the SSH protocols do not directly pass text that
would be displayed to the user. However, there are some places where
such data might be passed. When applicable, the character set for
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the data MUST be explicitly specified. In most places, ISO-10646
UTF-8 encoding is used [RFC3629]. When applicable, a field is also
provided for a language tag [RFC3066].
One big issue is the character set of the interactive session. There
is no clear solution, as different applications may display data in
different formats. Different types of terminal emulation may also be
employed in the client, and the character set to be used is
effectively determined by the terminal emulation. Thus, no place is
provided for directly specifying the character set or encoding for
terminal session data. However, the terminal emulation type (e.g.,
“vt100”) is transmitted to the remote site, and it implicitly
specifies the character set and encoding. Applications typically use
the terminal type to determine what character set they use, or the
character set is determined using some external means. The terminal
emulation may also allow configuring the default character set. In
any case, the character set for the terminal session is considered
primarily a client local issue.
Internal names used to identify algorithms or protocols are normally
never displayed to users, and must be in US-ASCII.
The client and server user names are inherently constrained by what
the server is prepared to accept. They might, however, occasionally
be displayed in logs, reports, etc. They MUST be encoded using ISO
10646 UTF-8, but other encodings may be required in some cases. It
is up to the server to decide how to map user names to accepted user
names. Straight bit-wise, binary comparison is RECOMMENDED.
For localization purposes, the protocol attempts to minimize the
number of textual messages transmitted. When present, such messages
typically relate to errors, debugging information, or some externally
configured data. For data that is normally displayed, it SHOULD be
possible to fetch a localized message instead of the transmitted
message by using a numerical code. The remaining messages SHOULD be
configurable.
- Data Type Representations Used in the SSH Protocols
byte
A byte represents an arbitrary 8-bit value (octet). Fixed length
data is sometimes represented as an array of bytes, written
byte[n], where n is the number of bytes in the array.
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boolean
A boolean value is stored as a single byte. The value 0
represents FALSE, and the value 1 represents TRUE. All non-zero
values MUST be interpreted as TRUE; however, applications MUST NOT
store values other than 0 and 1.
uint32
Represents a 32-bit unsigned integer. Stored as four bytes in the
order of decreasing significance (network byte order). For
example: the value 699921578 (0x29b7f4aa) is stored as 29 b7 f4
aa.
uint64
Represents a 64-bit unsigned integer. Stored as eight bytes in
the order of decreasing significance (network byte order).
string
Arbitrary length binary string. Strings are allowed to contain
arbitrary binary data, including null characters and 8-bit
characters. They are stored as a uint32 containing its length
(number of bytes that follow) and zero (= empty string) or more
bytes that are the value of the string. Terminating null
characters are not used.
Strings are also used to store text. In that case, US-ASCII is
used for internal names, and ISO-10646 UTF-8 for text that might
be displayed to the user. The terminating null character SHOULD
NOT normally be stored in the string. For example: the US-ASCII
string "testing" is represented as 00 00 00 07 t e s t i n g. The
UTF-8 mapping does not alter the encoding of US-ASCII characters.
mpint
Represents multiple precision integers in two's complement format,
stored as a string, 8 bits per byte, MSB first. Negative numbers
have the value 1 as the most significant bit of the first byte of
the data partition. If the most significant bit would be set for
a positive number, the number MUST be preceded by a zero byte.
Unnecessary leading bytes with the value 0 or 255 MUST NOT be
included. The value zero MUST be stored as a string with zero
bytes of data.
By convention, a number that is used in modular computations in
Z_n SHOULD be represented in the range 0 <= x < n.
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Examples:
value (hex) representation (hex)
----------- --------------------
0 00 00 00 00
9a378f9b2e332a7 00 00 00 08 09 a3 78 f9 b2 e3 32 a7
80 00 00 00 02 00 80
-1234 00 00 00 02 ed cc
-deadbeef 00 00 00 05 ff 21 52 41 11
name-list
A string containing a comma-separated list of names. A name-list
is represented as a uint32 containing its length (number of bytes
that follow) followed by a comma-separated list of zero or more
names. A name MUST have a non-zero length, and it MUST NOT
contain a comma (","). As this is a list of names, all of the
elements contained are names and MUST be in US-ASCII. Context may
impose additional restrictions on the names. For example, the
names in a name-list may have to be a list of valid algorithm
identifiers (see Section 6 below), or a list of [RFC3066] language
tags. The order of the names in a name-list may or may not be
significant. Again, this depends on the context in which the list
is used. Terminating null characters MUST NOT be used, neither
for the individual names, nor for the list as a whole.
Examples:
value representation (hex)
----- --------------------
(), the empty name-list 00 00 00 00
("zlib") 00 00 00 04 7a 6c 69 62
("zlib,none") 00 00 00 09 7a 6c 69 62 2c 6e 6f 6e 65
- Algorithm and Method Naming
The SSH protocols refer to particular hash, encryption, integrity,
compression, and key exchange algorithms or methods by name. There
are some standard algorithms and methods that all implementations
MUST support. There are also algorithms and methods that are defined
in the protocol specification, but are OPTIONAL. Furthermore, it is
expected that some organizations will want to use their own
algorithms or methods.
In this protocol, all algorithm and method identifiers MUST be
printable US-ASCII, non-empty strings no longer than 64 characters.
Names MUST be case-sensitive.
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There are two formats for algorithm and method names:
o Names that do not contain an at-sign ("@") are reserved to be
assigned by IETF CONSENSUS. Examples include “3des-cbc”, “sha-1”,
“hmac-sha1”, and “zlib” (the doublequotes are not part of the
name). Names of this format are only valid if they are first
registered with the IANA. Registered names MUST NOT contain an
at-sign ("@"), comma (","), whitespace, control characters (ASCII
codes 32 or less), or the ASCII code 127 (DEL). Names are case-
sensitive, and MUST NOT be longer than 64 characters.
o Anyone can define additional algorithms or methods by using names
in the format name@domainname, e.g., “ourcipher-cbc@example.com”.
The format of the part preceding the at-sign is not specified;
however, these names MUST be printable US-ASCII strings, and MUST
NOT contain the comma character (","), whitespace, control
characters (ASCII codes 32 or less), or the ASCII code 127 (DEL).
They MUST have only a single at-sign in them. The part following
the at-sign MUST be a valid, fully qualified domain name [RFC1034]
controlled by the person or organization defining the name. Names
are case-sensitive, and MUST NOT be longer than 64 characters. It
is up to each domain how it manages its local namespace. It
should be noted that these names resemble STD 11 [RFC0822] email
addresses. This is purely coincidental and has nothing to do with
STD 11 [RFC0822].
- Message Numbers
SSH packets have message numbers in the range 1 to 255. These
numbers have been allocated as follows:
Transport layer protocol:
1 to 19 Transport layer generic (e.g., disconnect, ignore,
debug, etc.)
20 to 29 Algorithm negotiation
30 to 49 Key exchange method specific (numbers can be reused
for different authentication methods)
User authentication protocol:
50 to 59 User authentication generic
60 to 79 User authentication method specific (numbers can be
reused for different authentication methods)
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Connection protocol:
80 to 89 Connection protocol generic
90 to 127 Channel related messages
Reserved for client protocols:
128 to 191 Reserved
Local extensions:
192 to 255 Local extensions
- IANA Considerations
This document is part of a set. The instructions for the IANA for
the SSH protocol, as defined in this document, [SSH-USERAUTH],
[SSH-TRANS], and [SSH-CONNECT], are detailed in [SSH-NUMBERS]. The
following is a brief summary for convenience, but note well that
[SSH-NUMBERS] contains the actual instructions to the IANA, which may
be superseded in the future.
Allocation of the following types of names in the SSH protocols is
assigned by IETF consensus:
o Service Names
* Authentication Methods
* Connection Protocol Channel Names
* Connection Protocol Global Request Names
* Connection Protocol Channel Request Names
o Key Exchange Method Names
o Assigned Algorithm Names
* Encryption Algorithm Names
* MAC Algorithm Names
* Public Key Algorithm Names
* Compression Algorithm Names
These names MUST be printable US-ASCII strings, and MUST NOT contain
the characters at-sign ("@"), comma (","), whitespace, control
characters (ASCII codes 32 or less), or the ASCII code 127 (DEL).
Names are case-sensitive, and MUST NOT be longer than 64 characters.
Names with the at-sign ("@") are locally defined extensions and are
not controlled by the IANA.
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Each category of names listed above has a separate namespace.
However, using the same name in multiple categories SHOULD be avoided
to minimize confusion.
Message numbers (see Section 7) in the range of 0 to 191 are
allocated via IETF CONSENSUS, as described in [RFC2434]. Message
numbers in the 192 to 255 range (local extensions) are reserved for
PRIVATE USE, also as described in [RFC2434].
- Security Considerations
In order to make the entire body of Security Considerations more
accessible, Security Considerations for the transport,
authentication, and connection documents have been gathered here.
The transport protocol [SSH-TRANS] provides a confidential channel
over an insecure network. It performs server host authentication,
key exchange, encryption, and integrity protection. It also derives
a unique session id that may be used by higher-level protocols.
The authentication protocol [SSH-USERAUTH] provides a suite of
mechanisms that can be used to authenticate the client user to the
server. Individual mechanisms specified in the authentication
protocol use the session id provided by the transport protocol and/or
depend on the security and integrity guarantees of the transport
protocol.
The connection protocol [SSH-CONNECT] specifies a mechanism to
multiplex multiple streams (channels) of data over the confidential
and authenticated transport. It also specifies channels for
accessing an interactive shell, for proxy-forwarding various external
protocols over the secure transport (including arbitrary TCP/IP
protocols), and for accessing secure subsystems on the server host.
9.1. Pseudo-Random Number Generation
This protocol binds each session key to the session by including
random, session specific data in the hash used to produce session
keys. Special care should be taken to ensure that all of the random
numbers are of good quality. If the random data here (e.g., Diffie-
Hellman (DH) parameters) are pseudo-random, then the pseudo-random
number generator should be cryptographically secure (i.e., its next
output not easily guessed even when knowing all previous outputs)
and, furthermore, proper entropy needs to be added to the pseudo-
random number generator. [RFC4086] offers suggestions for sources of
random numbers and entropy. Implementers should note the importance
of entropy and the well-meant, anecdotal warning about the difficulty
in properly implementing pseudo-random number generating functions.
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The amount of entropy available to a given client or server may
sometimes be less than what is required. In this case, one must
either resort to pseudo-random number generation regardless of
insufficient entropy or refuse to run the protocol. The latter is
preferable.
9.2. Control Character Filtering
When displaying text to a user, such as error or debug messages, the
client software SHOULD replace any control characters (except tab,
carriage return, and newline) with safe sequences to avoid attacks by
sending terminal control characters.
9.3. Transport
9.3.1. Confidentiality
It is beyond the scope of this document and the Secure Shell Working
Group to analyze or recommend specific ciphers other than the ones
that have been established and accepted within the industry. At the
time of this writing, commonly used ciphers include 3DES, ARCFOUR,
twofish, serpent, and blowfish. AES has been published by The US
Federal Information Processing Standards as [FIPS-197], and the
cryptographic community has accepted AES as well. As always,
implementers and users should check current literature to ensure that
no recent vulnerabilities have been found in ciphers used within
products. Implementers should also check to see which ciphers are
considered to be relatively stronger than others and should recommend
their use to users over relatively weaker ciphers. It would be
considered good form for an implementation to politely and
unobtrusively notify a user that a stronger cipher is available and
should be used when a weaker one is actively chosen.
The “none” cipher is provided for debugging and SHOULD NOT be used
except for that purpose. Its cryptographic properties are
sufficiently described in [RFC2410], which will show that its use
does not meet the intent of this protocol.
The relative merits of these and other ciphers may also be found in
current literature. Two references that may provide information on
the subject are [SCHNEIER] and [KAUFMAN]. Both of these describe the
CBC mode of operation of certain ciphers and the weakness of this
scheme. Essentially, this mode is theoretically vulnerable to chosen
cipher-text attacks because of the high predictability of the start
of packet sequence. However, this attack is deemed difficult and not
considered fully practicable, especially if relatively long block
sizes are used.
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Additionally, another CBC mode attack may be mitigated through the
insertion of packets containing SSH_MSG_IGNORE. Without this
technique, a specific attack may be successful. For this attack
(commonly known as the Rogaway attack [ROGAWAY], [DAI], [BELLARE]) to
work, the attacker would need to know the Initialization Vector (IV)
of the next block that is going to be encrypted. In CBC mode that is
the output of the encryption of the previous block. If the attacker
does not have any way to see the packet yet (i.e., it is in the
internal buffers of the SSH implementation or even in the kernel),
then this attack will not work. If the last packet has been sent out
to the network (i.e., the attacker has access to it), then he can use
the attack.
In the optimal case, an implementer would need to add an extra packet
only if the packet has been sent out onto the network and there are
no other packets waiting for transmission. Implementers may wish to
check if there are any unsent packets awaiting transmission;
unfortunately, it is not normally easy to obtain this information
from the kernel or buffers. If there are no unsent packets, then a
packet containing SSH_MSG_IGNORE SHOULD be sent. If a new packet is
added to the stream every time the attacker knows the IV that is
supposed to be used for the next packet, then the attacker will not
be able to guess the correct IV, thus the attack will never be
successful.
As an example, consider the following case:
Client Server
------ ------
TCP(seq=x, len=500) ---->
contains Record 1
[500 ms passes, no ACK]
TCP(seq=x, len=1000) ---->
contains Records 1,2
ACK
-
The Nagle algorithm + TCP retransmits mean that the two records
get coalesced into a single TCP segment. -
Record 2 is not at the beginning of the TCP segment and never will
be because it gets ACKed. -
Yet, the attack is possible because Record 1 has already been
seen.
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As this example indicates, it is unsafe to use the existence of
unflushed data in the TCP buffers proper as a guide to whether an
empty packet is needed, since when the second write() is performed
the buffers will contain the un-ACKed Record 1.
On the other hand, it is perfectly safe to have the following
situation:
Client Server
------ ------
TCP(seq=x, len=500) ---->
contains SSH_MSG_IGNORE
TCP(seq=y, len=500) ---->
contains Data
Provided that the IV for the second SSH Record is fixed after the
data for the Data packet is determined, then the following should
be performed:
read from user
encrypt null packet
encrypt data packet
9.3.2. Data Integrity
This protocol does allow the Data Integrity mechanism to be disabled.
Implementers SHOULD be wary of exposing this feature for any purpose
other than debugging. Users and administrators SHOULD be explicitly
warned anytime the “none” MAC is enabled.
So long as the “none” MAC is not used, this protocol provides data
integrity.
Because MACs use a 32-bit sequence number, they might start to leak
information after 232 packets have been sent. However, following
the rekeying recommendations should prevent this attack. The
transport protocol [SSH-TRANS] recommends rekeying after one gigabyte
of data, and the smallest possible packet is 16 bytes. Therefore,
rekeying SHOULD happen after 228 packets at the very most.
9.3.3. Replay
The use of a MAC other than “none” provides integrity and
authentication. In addition, the transport protocol provides a
unique session identifier (bound in part to pseudo-random data that
is part of the algorithm and key exchange process) that can be used
by higher level protocols to bind data to a given session and prevent
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replay of data from prior sessions. For example, the authentication
protocol ([SSH-USERAUTH]) uses this to prevent replay of signatures
from previous sessions. Because public key authentication exchanges
are cryptographically bound to the session (i.e., to the initial key
exchange), they cannot be successfully replayed in other sessions.
Note that the session id can be made public without harming the
security of the protocol.
If two sessions have the same session id (hash of key exchanges),
then packets from one can be replayed against the other. It must be
stressed that the chances of such an occurrence are, needless to say,
minimal when using modern cryptographic methods. This is all the
more true when specifying larger hash function outputs and DH
parameters.
Replay detection using monotonically increasing sequence numbers as
input to the MAC, or HMAC in some cases, is described in [RFC2085],
[RFC2246], [RFC2743], [RFC1964], [RFC2025], and [RFC4120]. The
underlying construct is discussed in [RFC2104]. Essentially, a
different sequence number in each packet ensures that at least this
one input to the MAC function will be unique and will provide a
nonrecurring MAC output that is not predictable to an attacker. If
the session stays active long enough, however, this sequence number
will wrap. This event may provide an attacker an opportunity to
replay a previously recorded packet with an identical sequence number
but only if the peers have not rekeyed since the transmission of the
first packet with that sequence number. If the peers have rekeyed,
then the replay will be detected since the MAC check will fail. For
this reason, it must be emphasized that peers MUST rekey before a
wrap of the sequence numbers. Naturally, if an attacker does attempt
to replay a captured packet before the peers have rekeyed, then the
receiver of the duplicate packet will not be able to validate the MAC
and it will be discarded. The reason that the MAC will fail is
because the receiver will formulate a MAC based upon the packet
contents, the shared secret, and the expected sequence number. Since
the replayed packet will not be using that expected sequence number
(the sequence number of the replayed packet will have already been
passed by the receiver), the calculated MAC will not match the MAC
received with the packet.
9.3.4. Man-in-the-middle
This protocol makes no assumptions or provisions for an
infrastructure or means for distributing the public keys of hosts.
It is expected that this protocol will sometimes be used without
first verifying the association between the server host key and the
server host name. Such usage is vulnerable to man-in-the-middle
attacks. This section describes this and encourages administrators
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and users to understand the importance of verifying this association
before any session is initiated.
There are three cases of man-in-the-middle attacks to consider. The
first is where an attacker places a device between the client and the
server before the session is initiated. In this case, the attack
device is trying to mimic the legitimate server and will offer its
public key to the client when the client initiates a session. If it
were to offer the public key of the server, then it would not be able
to decrypt or sign the transmissions between the legitimate server
and the client unless it also had access to the private key of the
host. The attack device will also, simultaneously to this, initiate
a session to the legitimate server, masquerading itself as the
client. If the public key of the server had been securely
distributed to the client prior to that session initiation, the key
offered to the client by the attack device will not match the key
stored on the client. In that case, the user SHOULD be given a
warning that the offered host key does not match the host key cached
on the client. As described in Section 4.1, the user may be free to
accept the new key and continue the session. It is RECOMMENDED that
the warning provide sufficient information to the user of the client
device so the user may make an informed decision. If the user
chooses to continue the session with the stored public key of the
server (not the public key offered at the start of the session), then
the session-specific data between the attacker and server will be
different between the client-to-attacker session and the attacker-
to-server sessions due to the randomness discussed above. From this,
the attacker will not be able to make this attack work since the
attacker will not be able to correctly sign packets containing this
session-specific data from the server, since he does not have the
private key of that server.
The second case that should be considered is similar to the first
case in that it also happens at the time of connection, but this case
points out the need for the secure distribution of server public
keys. If the server public keys are not securely distributed, then
the client cannot know if it is talking to the intended server. An
attacker may use social engineering techniques to pass off server
keys to unsuspecting users and may then place a man-in-the-middle
attack device between the legitimate server and the clients. If this
is allowed to happen, then the clients will form client-to-attacker
sessions, and the attacker will form attacker-to-server sessions and
will be able to monitor and manipulate all of the traffic between the
clients and the legitimate servers. Server administrators are
encouraged to make host key fingerprints available for checking by
some means whose security does not rely on the integrity of the
actual host keys. Possible mechanisms are discussed in Section 4.1
and may also include secured Web pages, physical pieces of paper,
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etc. Implementers SHOULD provide recommendations on how best to do
this with their implementation. Because the protocol is extensible,
future extensions to the protocol may provide better mechanisms for
dealing with the need to know the server’s host key before
connecting. For example, making the host key fingerprint available
through a secure DNS lookup, or using Kerberos ([RFC4120]) over
GSS-API ([RFC1964]) during key exchange to authenticate the server
are possibilities.
In the third man-in-the-middle case, attackers may attempt to
manipulate packets in transit between peers after the session has
been established. As described in Section 9.3.3, a successful attack
of this nature is very improbable. As in Section 9.3.3, this
reasoning does assume that the MAC is secure and that it is
infeasible to construct inputs to a MAC algorithm to give a known
output. This is discussed in much greater detail in Section 6 of
[RFC2104]. If the MAC algorithm has a vulnerability or is weak
enough, then the attacker may be able to specify certain inputs to
yield a known MAC. With that, they may be able to alter the contents
of a packet in transit. Alternatively, the attacker may be able to
exploit the algorithm vulnerability or weakness to find the shared
secret by reviewing the MACs from captured packets. In either of
those cases, an attacker could construct a packet or packets that
could be inserted into an SSH stream. To prevent this, implementers
are encouraged to utilize commonly accepted MAC algorithms, and
administrators are encouraged to watch current literature and
discussions of cryptography to ensure that they are not using a MAC
algorithm that has a recently found vulnerability or weakness.
In summary, the use of this protocol without a reliable association
of the binding between a host and its host keys is inherently
insecure and is NOT RECOMMENDED. However, it may be necessary in
non-security-critical environments, and will still provide protection
against passive attacks. Implementers of protocols and applications
running on top of this protocol should keep this possibility in mind.
9.3.5. Denial of Service
This protocol is designed to be used over a reliable transport. If
transmission errors or message manipulation occur, the connection is
closed. The connection SHOULD be re-established if this occurs.
Denial of service attacks of this type (wire cutter) are almost
impossible to avoid.
In addition, this protocol is vulnerable to denial of service attacks
because an attacker can force the server to go through the CPU and
memory intensive tasks of connection setup and key exchange without
authenticating. Implementers SHOULD provide features that make this
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more difficult, for example, only allowing connections from a subset
of clients known to have valid users.
9.3.6. Covert Channels
The protocol was not designed to eliminate covert channels. For
example, the padding, SSH_MSG_IGNORE messages, and several other
places in the protocol can be used to pass covert information, and
the recipient has no reliable way of verifying whether such
information is being sent.
9.3.7. Forward Secrecy
It should be noted that the Diffie-Hellman key exchanges may provide
perfect forward secrecy (PFS). PFS is essentially defined as the
cryptographic property of a key-establishment protocol in which the
compromise of a session key or long-term private key after a given
session does not cause the compromise of any earlier session
[ANSI-T1.523-2001]. SSH sessions resulting from a key exchange using
the diffie-hellman methods described in the section Diffie-Hellman
Key Exchange of [SSH-TRANS] (including “diffie-hellman-group1-sha1”
and “diffie-hellman-group14-sha1”) are secure even if private
keying/authentication material is later revealed, but not if the
session keys are revealed. So, given this definition of PFS, SSH
does have PFS. However, this property is not commuted to any of the
applications or protocols using SSH as a transport. The transport
layer of SSH provides confidentiality for password authentication and
other methods that rely on secret data.
Of course, if the DH private parameters for the client and server are
revealed, then the session key is revealed, but these items can be
thrown away after the key exchange completes. It’s worth pointing
out that these items should not be allowed to end up on swap space
and that they should be erased from memory as soon as the key
exchange completes.
9.3.8. Ordering of Key Exchange Methods
As stated in the section on Algorithm Negotiation of [SSH-TRANS],
each device will send a list of preferred methods for key exchange.
The most-preferred method is the first in the list. It is
RECOMMENDED that the algorithms be sorted by cryptographic strength,
strongest first. Some additional guidance for this is given in
[RFC3766].
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9.3.9. Traffic Analysis
Passive monitoring of any protocol may give an attacker some
information about the session, the user, or protocol specific
information that they would otherwise not be able to garner. For
example, it has been shown that traffic analysis of an SSH session
can yield information about the length of the password - [Openwall]
and [USENIX]. Implementers should use the SSH_MSG_IGNORE packet,
along with the inclusion of random lengths of padding, to thwart
attempts at traffic analysis. Other methods may also be found and
implemented.
9.4. Authentication Protocol
The purpose of this protocol is to perform client user
authentication. It assumes that this runs over a secure transport
layer protocol, which has already authenticated the server machine,
established an encrypted communications channel, and computed a
unique session identifier for this session.
Several authentication methods with different security
characteristics are allowed. It is up to the server’s local policy
to decide which methods (or combinations of methods) it is willing to
accept for each user. Authentication is no stronger than the weakest
combination allowed.
The server may go into a sleep period after repeated unsuccessful
authentication attempts to make key search more difficult for
attackers. Care should be taken so that this doesn’t become a self-
denial of service vector.
9.4.1. Weak Transport
If the transport layer does not provide confidentiality,
authentication methods that rely on secret data SHOULD be disabled.
If it does not provide strong integrity protection, requests to
change authentication data (e.g., a password change) SHOULD be
disabled to prevent an attacker from modifying the ciphertext without
being noticed, or rendering the new authentication data unusable
(denial of service).
The assumption stated above, that the Authentication Protocol only
runs over a secure transport that has previously authenticated the
server, is very important to note. People deploying SSH are reminded
of the consequences of man-in-the-middle attacks if the client does
not have a very strong a priori association of the server with the
host key of that server. Specifically, for the case of the
Authentication Protocol, the client may form a session to a man-in-
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the-middle attack device and divulge user credentials such as their
username and password. Even in the cases of authentication where no
user credentials are divulged, an attacker may still gain information
they shouldn’t have by capturing key-strokes in much the same way
that a honeypot works.
9.4.2. Debug Messages
Special care should be taken when designing debug messages. These
messages may reveal surprising amounts of information about the host
if not properly designed. Debug messages can be disabled (during
user authentication phase) if high security is required.
Administrators of host machines should make all attempts to
compartmentalize all event notification messages and protect them
from unwarranted observation. Developers should be aware of the
sensitive nature of some of the normal event and debug messages, and
may want to provide guidance to administrators on ways to keep this
information away from unauthorized people. Developers should
consider minimizing the amount of sensitive information obtainable by
users during the authentication phase, in accordance with the local
policies. For this reason, it is RECOMMENDED that debug messages be
initially disabled at the time of deployment and require an active
decision by an administrator to allow them to be enabled. It is also
RECOMMENDED that a message expressing this concern be presented to
the administrator of a system when the action is taken to enable
debugging messages.
9.4.3. Local Security Policy
The implementer MUST ensure that the credentials provided validate
the professed user and also MUST ensure that the local policy of the
server permits the user the access requested. In particular, because
of the flexible nature of the SSH connection protocol, it may not be
possible to determine the local security policy, if any, that should
apply at the time of authentication because the kind of service being
requested is not clear at that instant. For example, local policy
might allow a user to access files on the server, but not start an
interactive shell. However, during the authentication protocol, it
is not known whether the user will be accessing files, attempting to
use an interactive shell, or even both. In any event, where local
security policy for the server host exists, it MUST be applied and
enforced correctly.
Implementers are encouraged to provide a default local policy and
make its parameters known to administrators and users. At the
discretion of the implementers, this default policy may be along the
lines of anything-goes where there are no restrictions placed upon
users, or it may be along the lines of excessively-restrictive, in
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which case, the administrators will have to actively make changes to
the initial default parameters to meet their needs. Alternatively,
it may be some attempt at providing something practical and
immediately useful to the administrators of the system so they don’t
have to put in much effort to get SSH working. Whatever choice is
made must be applied and enforced as required above.
9.4.4 Public Key Authentication
The use of public key authentication assumes that the client host has
not been compromised. It also assumes that the private key of the
server host has not been compromised.
This risk can be mitigated by the use of passphrases on private keys;
however, this is not an enforceable policy. The use of smartcards,
or other technology to make passphrases an enforceable policy is
suggested.
The server could require both password and public key authentication;
however, this requires the client to expose its password to the
server (see the section on Password Authentication below.)
9.4.5. Password Authentication
The password mechanism, as specified in the authentication protocol,
assumes that the server has not been compromised. If the server has
been compromised, using password authentication will reveal a valid
username/password combination to the attacker, which may lead to
further compromises.
This vulnerability can be mitigated by using an alternative form of
authentication. For example, public key authentication makes no
assumptions about security on the server.
9.4.6. Host-Based Authentication
Host-based authentication assumes that the client has not been
compromised. There are no mitigating strategies, other than to use
host-based authentication in combination with another authentication
method.
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9.5. Connection Protocol
9.5.1. End Point Security
End point security is assumed by the connection protocol. If the
server has been compromised, any terminal sessions, port forwarding,
or systems accessed on the host are compromised. There are no
mitigating factors for this.
If the client has been compromised, and the server fails to stop the
attacker at the authentication protocol, all services exposed (either
as subsystems or through forwarding) will be vulnerable to attack.
Implementers SHOULD provide mechanisms for administrators to control
which services are exposed to limit the vulnerability of other
services. These controls might include controlling which machines
and ports can be targeted in port-forwarding operations, which users
are allowed to use interactive shell facilities, or which users are
allowed to use exposed subsystems.
9.5.2. Proxy Forwarding
The SSH connection protocol allows for proxy forwarding of other
protocols such as SMTP, POP3, and HTTP. This may be a concern for
network administrators who wish to control the access of certain
applications by users located outside of their physical location.
Essentially, the forwarding of these protocols may violate site-
specific security policies, as they may be undetectably tunneled
through a firewall. Implementers SHOULD provide an administrative
mechanism to control the proxy forwarding functionality so that
site-specific security policies may be upheld.
In addition, a reverse proxy forwarding functionality is available,
which, again, can be used to bypass firewall controls.
As indicated above, end-point security is assumed during proxy
forwarding operations. Failure of end-point security will compromise
all data passed over proxy forwarding.
9.5.3. X11 Forwarding
Another form of proxy forwarding provided by the SSH connection
protocol is the forwarding of the X11 protocol. If end-point
security has been compromised, X11 forwarding may allow attacks
against the X11 server. Users and administrators should, as a matter
of course, use appropriate X11 security mechanisms to prevent
unauthorized use of the X11 server. Implementers, administrators,
and users who wish to further explore the security mechanisms of X11
are invited to read [SCHEIFLER] and analyze previously reported
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RFC 4251 SSH Protocol Architecture January 2006
problems with the interactions between SSH forwarding and X11 in CERT
vulnerabilities VU#363181 and VU#118892 [CERT].
X11 display forwarding with SSH, by itself, is not sufficient to
correct well known problems with X11 security [VENEMA]. However, X11
display forwarding in SSH (or other secure protocols), combined with
actual and pseudo-displays that accept connections only over local
IPC mechanisms authorized by permissions or access control lists
(ACLs), does correct many X11 security problems, as long as the
“none” MAC is not used. It is RECOMMENDED that X11 display
implementations default to allow the display to open only over local
IPC. It is RECOMMENDED that SSH server implementations that support
X11 forwarding default to allow the display to open only over local
IPC. On single-user systems, it might be reasonable to default to
allow the local display to open over TCP/IP.
Implementers of the X11 forwarding protocol SHOULD implement the
magic cookie access-checking spoofing mechanism, as described in
[SSH-CONNECT], as an additional mechanism to prevent unauthorized use
of the proxy.
Ylonen & Lonvick Standards Track [Page 25]
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- References
10.1. Normative References
[SSH-TRANS] Ylonen, T. and C. Lonvick, Ed., “The Secure Shell
(SSH) Transport Layer Protocol”, RFC 4253, January
2006.
[SSH-USERAUTH] Ylonen, T. and C. Lonvick, Ed., “The Secure Shell
(SSH) Authentication Protocol”, RFC 4252, January
2006.
[SSH-CONNECT] Ylonen, T. and C. Lonvick, Ed., “The Secure Shell
(SSH) Connection Protocol”, RFC 4254, January
2006.
[SSH-NUMBERS] Lehtinen, S. and C. Lonvick, Ed., “The Secure
Shell (SSH) Protocol Assigned Numbers”, RFC 4250,
January 2006.
[RFC2119] Bradner, S., “Key words for use in RFCs to
Indicate Requirement Levels”, BCP 14, RFC 2119,
March 1997.
[RFC2434] Narten, T. and H. Alvestrand, “Guidelines for
Writing an IANA Considerations Section in RFCs”,
BCP 26, RFC 2434, October 1998.
[RFC3066] Alvestrand, H., “Tags for the Identification of
Languages”, BCP 47, RFC 3066, January 2001.
[RFC3629] Yergeau, F., “UTF-8, a transformation format of
ISO 10646”, STD 63, RFC 3629, November 2003.
10.2. Informative References
[RFC0822] Crocker, D., “Standard for the format of ARPA
Internet text messages”, STD 11, RFC 822, August
1982.
[RFC0854] Postel, J. and J. Reynolds, “Telnet Protocol
Specification”, STD 8, RFC 854, May 1983.
[RFC1034] Mockapetris, P., “Domain names - concepts and
facilities”, STD 13, RFC 1034, November 1987.
Ylonen & Lonvick Standards Track [Page 26]
RFC 4251 SSH Protocol Architecture January 2006
[RFC1282] Kantor, B., “BSD Rlogin”, RFC 1282, December 1991.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn,
“The Kerberos Network Authentication Service
(V5)”, RFC 4120, July 2005.
[RFC1964] Linn, J., “The Kerberos Version 5 GSS-API
Mechanism”, RFC 1964, June 1996.
[RFC2025] Adams, C., “The Simple Public-Key GSS-API
Mechanism (SPKM)”, RFC 2025, October 1996.
[RFC2085] Oehler, M. and R. Glenn, “HMAC-MD5 IP
Authentication with Replay Prevention”, RFC 2085,
February 1997.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, “HMAC:
Keyed-Hashing for Message Authentication”, RFC
2104, February 1997.
[RFC2246] Dierks, T. and C. Allen, “The TLS Protocol Version
1.0”, RFC 2246, January 1999.
[RFC2410] Glenn, R. and S. Kent, “The NULL Encryption
Algorithm and Its Use With IPsec”, RFC 2410,
November 1998.
[RFC2743] Linn, J., “Generic Security Service Application
Program Interface Version 2, Update 1”, RFC 2743,
January 2000.
[RFC3766] Orman, H. and P. Hoffman, “Determining Strengths
For Public Keys Used For Exchanging Symmetric
Keys”, BCP 86, RFC 3766, April 2004.
[RFC4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
“Randomness Requirements for Security”, BCP 106,
RFC 4086, June 2005.
[FIPS-180-2] US National Institute of Standards and Technology,
“Secure Hash Standard (SHS)”, Federal Information
Processing Standards Publication 180-2, August
2002.
[FIPS-186-2] US National Institute of Standards and Technology,
“Digital Signature Standard (DSS)”, Federal
Information Processing Standards Publication 186-
2, January 2000.
Ylonen & Lonvick Standards Track [Page 27]
RFC 4251 SSH Protocol Architecture January 2006
[FIPS-197] US National Institute of Standards and Technology,
“Advanced Encryption Standard (AES)”, Federal
Information Processing Standards Publication 197,
November 2001.
[ANSI-T1.523-2001] American National Standards Institute, Inc.,
“Telecom Glossary 2000”, ANSI T1.523-2001,
February 2001.
[SCHNEIER] Schneier, B., “Applied Cryptography Second
Edition: protocols algorithms and source in code
in C”, John Wiley and Sons, New York, NY, 1996.
[SCHEIFLER] Scheifler, R., “X Window System : The Complete
Reference to Xlib, X Protocol, Icccm, Xlfd, 3rd
edition.”, Digital Press, ISBN 1555580882,
February 1992.
[KAUFMAN] Kaufman, C., Perlman, R., and M. Speciner,
“Network Security: PRIVATE Communication in a
PUBLIC World”, Prentice Hall Publisher, 1995.
[CERT] CERT Coordination Center, The.,
“http://www.cert.org/nav/index_red.html”.
[VENEMA] Venema, W., “Murphy’s Law and Computer Security”,
Proceedings of 6th USENIX Security Symposium, San
Jose CA
http://www.usenix.org/publications/library/
proceedings/sec96/venema.html, July 1996.
[ROGAWAY] Rogaway, P., “Problems with Proposed IP
Cryptography”, Unpublished paper
http://www.cs.ucdavis.edu/~rogaway/ papers/draft-
rogaway-ipsec-comments-00.txt, 1996.
[DAI] Dai, W., “An attack against SSH2 protocol”, Email
to the SECSH Working Group ietf-ssh@netbsd.org
ftp:// ftp.ietf.org/ietf-mail-archive/secsh/2002-
02.mail, Feb 2002.
[BELLARE] Bellaire, M., Kohno, T., and C. Namprempre,
“Authenticated Encryption in SSH: Fixing the SSH
Binary Packet Protocol”, Proceedings of the 9th
ACM Conference on Computer and Communications
Security, Sept 2002.
Ylonen & Lonvick Standards Track [Page 28]
RFC 4251 SSH Protocol Architecture January 2006
[Openwall] Solar Designer and D. Song, “SSH Traffic Analysis
Attacks”, Presentation given at HAL2001 and
NordU2002 Conferences, Sept 2001.
[USENIX] Song, X.D., Wagner, D., and X. Tian, “Timing
Analysis of Keystrokes and SSH Timing Attacks”,
Paper given at 10th USENIX Security Symposium,
2001.
Authors’ Addresses
Tatu Ylonen
SSH Communications Security Corp
Valimotie 17
00380 Helsinki
Finland
EMail: ylo@ssh.com
Chris Lonvick (editor)
Cisco Systems, Inc.
12515 Research Blvd.
Austin 78759
USA
EMail: clonvick@cisco.com
Trademark Notice
“ssh” is a registered trademark in the United States and/or other
countries.
Ylonen & Lonvick Standards Track [Page 29]
RFC 4251 SSH Protocol Architecture January 2006
Full Copyright Statement
Copyright © The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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Ylonen & Lonvick Standards Track [Page 30]