The WPICTF 2018 “guess5” challenge:

The URL presented us with a guessing game, we have to pick 6 numbers. If we picked the correct numbers we’ll get a flag:

However submitting our picks never worked. Investigating this for a bit it looks like this requires to run a local Ethereum node. Before trying to set that up we’ve looked more into what the web application does. Interestingly it fetches the URL https://glgaines.github.io/guess5/Guess6.json (mirror here). In there we can find the ETH contract including in plain text for some reason. Which contains the flag:

The flag is: WPI{All_Hail_The_Mighty_Vitalik}

The WPICTF 2018 “Shell-JAIL-2” challenge:

This is almost the same challenge as Shell-JAIL-1 (see my write-up here for explanation of details) with the exception of one extra line in access.c (full mirror here):

        setenv("PATH", "", 1);

This means that before dropping the arguments to system() the $PATH environment variable is unset. Also the blacklist filter of the previous challenge remains the same. With that only built in sh commands will continue to work and since / is also blacklisted we cannot provide full paths to binaries either. For example id will now not work while pwd still executes:

But the . (or source) command still works. With that we can tell the shell to try to execute the flag.txt file and the error message will reveal its content. We still use the ? wildcard to circumvent the other blacklist by passing . "fl?g.t?t" to it:

The flag is: wpi{p0s1x_sh3Lls_ar3_w13rD}

The WPICTF 2018 “Shell-JAIL-1” challenge:

After downloading the linked private key and connecting to the remote server we are dropped into a limited user account and the directory /home/pc_owner. In that folder there are only 3 files – including flag.txt to which our user has no access:

The access file is basically a setuid executable which will run as the pc_owner user. The source of the executable is also available in access.c (mirror here). The program will take all arguments and pass it to system() unless it contains blacklisted strings, relevant parts in the source code:

int filter(const char *cmd){
	int valid = 1;
	valid &= strstr(cmd, "*") == NULL;
	valid &= strstr(cmd, "sh") == NULL;
	valid &= strstr(cmd, "/") == NULL;
	valid &= strstr(cmd, "home") == NULL;
	valid &= strstr(cmd, "pc_owner") == NULL;
	valid &= strstr(cmd, "flag") == NULL;
	valid &= strstr(cmd, "txt") == NULL;
	return valid;
}


int main(int argc, const char **argv){
	setreuid(UID, UID);
	char *cmd = gen_cmd(argc, argv);
	if (!filter(cmd)){
		exit(-1);
	}
	system(cmd);
}

This means passing id to it will work but cat flag.txt will not:

Of course circumventing that filter is rather easy, the * wildcard is forbidden, but ? is not. We can use those wildcards to read flag.txt by passing cat "fla?.tx?" to it:

The flag is: wpi{MaNY_WayS_T0_r3Ad}

The Nuit du Hack CTF 2018 CoinGame challenge:

The URL presented us basically only with a simple webform, which fetches a resource we can specify via cURL:

After a bit of trying, we figured out that file:/// URLs also work, like file:///etc/passwd:

Fetching a lot of files from the server yielded not a lot of success. After a while we noticed the text on the main site: “DESIGNED BY TOTHEYELLOWMOON”

Searching for this and CoinGame a GitHub repo was found: https://github.com/totheyellowmoon/CoinGame
The description of that repo read: “Congrats it was the first step ! Welcome on my Github, this is my new game but I haven’t pushed the modifications …”

From the description of the challenge and the GitHub repo we gather that “CoinGame” is being developed on this server and some changes aren’t pushed yet to the repo.
From /etc/passwd and /var/log/dpkg.log on the server we’ve also figured out that probably a tftp server is running on that system.

Requesting http://coingame.challs.malice.fr/curl.php?way=tftp://127.0.0.1/README.md we found the local repository:

Next we cloned the public GitHub repo, with that we had a list of all existing files in the repository. We looped over all the files and downloaded them via tftp from the system. Then simply ran a diff on the checkout and downloaded files. None of the code had any differences, but a few pictures didn’t match:

In any of the gameAnimationImages/background*.png images the flag was visible:

The flag was: flag{_Rends_L'Arg3nt_!}