pwnable.kr-cmd1-Writeup

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pwnable.kr-cmd1-Writeup

这道题挺有意思，这里详细的记录一下；

• 首先还是ssh远程登录，ls -l查看文件，然后cat cmd1.c读C代码如下：

 #include <stdio.h>
 #include <string.h>

 int filter(char* cmd){
     int r=;
     r += strstr(cmd, "flag")!=;
     r += strstr(cmd, "sh")!=;
     r += strstr(cmd, "tmp")!=;
     return r;
 }
 int main(int argc, char* argv[], char** envp){
     putenv("PATH=/fuckyouverymuch");
     if(filter(argv[])) return ;
     system( argv[] );
     return ;
 }

• 先查到主函数main的第三个参数envp[][]和环境变量有关；
• putenv()函数是改变环境变量的，因此更改了环境变量后，惯用的ls, cat等命令都不能直接使用，但我们通过完整路径来使用，如/bin/cat;
• strstr(str1，str2)函数，当str2包含在str1中，返回str2在str1中的索引；否则返回0；
• 然后分析r += strstr(cmd, "flag")!=0, 根据C语言中 =运算符 从又向左读的规则，该行代码可以分解为：
  

  int tmp = strstr(cmd,"flag")? 0 : 1;
  r += tmp;

• 则有：
  

其中，argv[1] == "/bin/cat f* "中使用了通配符来避免出现关键字符串flag；

• 传递参数如下：
  

则flag即为 mommy now I get what PATH environment is for :)

2017-2-11 14:52;38