# Problem DescriptionWe are presented with a SUID binary `/sbin/grhealth`. It has a retry logic where on failure it will execute `execv(argv[0], argv);` if it encounters an error. It takes two arguments, the first is what restart number we are currently on, the second is the max number of restarts. Both are parsed with strtol()
We want to read the file /flag.txt. We are given access to a read only system where we can start the deamon but not create any files.
# Attack PlanWe are going to try and mess with argv[0] in order to make the restart start a different program that will read the flag for us.  In bash, we can do this by using the `-a` option of the `exec` builtin. There are two things we need to overcome - first the second argument must parse as an integer > 1. Second we need to ensure the program encounters an error so that it restarts.
We can force it to restart by limiting the max number of file descriptors with ulimit -n. When it recieves a new incoming connection, a new file descriptor will be allocated, putting it over the limit. We can make a new connection using bash's pseudo network devices /dev/tcp/127.0.0.1/80 opens a tcp connection to local host on port 80 when written to.
strtol() will stop parsing once it hits the first non-integer character. So "2foo" is considered 2. We cannot create any files, but all we need is file path starting with a number that evaluates to flag.txt. /proc is useful for this since it has numbered directories in it.
# ExploitPutting this altogether:
```bashcd /proculimit -n 7exec -a /bin/cat 0 '1/../self/root/flag.txt' &echo foo > /dev/tcp/127.0.0.1/80
```
You may need to hit enter a few times after that to wait for the job to complete and so the jobs output will be put on screen.
The result is `gctf{31230_b4ckd00r3d_pr1v4t3_m1l1t4r1_c0mp4n1_74123}`