pwnable.kr-collision -Writeup

bof

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ul li > :first-child,
ol li > :first-child,
ul li ul:first-of-type,
ol li ol:first-of-type,
ul li ol:first-of-type,
ol li ul:first-of-type {
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img {
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-->

pwnable.kr-collision -Writeup

同第一题fd,
ssh连接,ls -l查看文件, cat col.c得到代码如下

 #include <stdio.h>
#include <string.h>
unsigned long hashcode = 0x21DD09EC;
unsigned long check_password(const char* p){
int* ip = (int*)p;
int i;
int res=;
for(i=; i<; i++){
res += ip[i];
}
return res;
} int main(int argc, char* argv[]){
if(argc<){
printf("usage : %s [passcode]\n", argv[]);
return ;
}
if(strlen(argv[]) != ){
printf("passcode length should be 20 bytes\n");
return ;
} if(hashcode == check_password( argv[] )){
system("/bin/cat flag");
return ;
}
else
printf("wrong passcode.\n");
return ;
}
  • 分析过程如下:
    pwnable.kr-collision -Writeup
  • 分析check_password()函数,当strlen(argv[1])==20时,要保证check_password()函数的返回值为0x21DD09EC;

    经分析,check_password()函数的作用为将长度为20的argv[1]分为5段,每段有4个字符,这些字符是以小端的形式存储的,以int *指针的形式每次读取4个字符,将5次读取的值求和

  • 最简单的想法是前16位均为\x00,后4位为\xEC\x09\xDD\x21,但经过测试答案错误,查表得\x09为制表符tab,会截断输入,同时\x00也会截断输入;
  • 因此构造另一种输入'\x01'*16+'\xE8\x05\xD9\x1D',分析过程如下:
  • 理解字节序
  • pwnable.kr-collision -Writeup
  • 则可构造语句

    python -c "print '\x01'*16+'\xE8\x05\xD9\x1D'"

\x和0x表示16进制的区别:pwnable.kr-collision -Writeup

  • 运行,结果如下,flag为daddy! I just managed to create a hash collision :)
    pwnable.kr-collision -Writeup
    2017-2-5 0:55;56

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