crypto / authy

Question

Check out this new storage application that your government has started! It's supposed to be pretty secure since everything is authenticated...

curl crypto.chal.csaw.io:5003

Provided Files

handout.py

Solution

Background

The target is a plaintext note storage service. Notes are submitted to POST /new with an author value and a note value. This endpoint returns a base64 version of the note and a SHA1 hash of the metadata.

Notes can be retrieved from POST /view with the note id (the base64 string) and integrity (the SHA1 hash). If the id sets admin and access_sensitive to true and has entrynum 7, then the flag will be printed.

The id data is stored and parsed using URL parameter notation (param1=value1&param2=value&...). All parameters are concatenated together before parsing. However, the code does not sanitize for & within a note.

# handout.py L93-97
identifier = base64.b64decode(info["id"]).decode()
checksum = info["integrity"]

params = identifier.replace('&', ' ').split(" ")
note_dict = { param.split("=")[0]: param.split("=")[1]  for param in params }

This allows us to arbitrarily set values of note_dict, which is used to verify the admin, access_sensitive, and entrynum properties.

Exploit

We can now craft a note with the required fields (and entrynum=none to have something to overwrite):

$ curl -X POST \
  -F "author=1337" \
  -F "entrynum=none" \
  -F "note=AAAAAAAA&admin=True&access_sensitive=True&entrynum=7" \
  crypto.chal.csaw.io:5003/new

Successfully added YWRtaW49RmFsc2UmYWNjZXNzX3NlbnNpdGl2ZT1GYWxzZSZhdXRob3I9MTMzNyZlbnRyeW51bT03ODMmbm90ZT1BQUFBQUFBQSZhZG1pbj1UcnVlJmFjY2Vzc19zZW5zaXRpdmU9VHJ1ZSZlbnRyeW51bT03:1a51c1aa28c65fb763539c8055ae270b4c231a11

And access it, triggering the flag print statement:

$ curl -X POST \
  -F "id=YWRtaW49RmFsc2UmYWNjZXNzX3NlbnNpdGl2ZT1GYWxzZSZhdXRob3I9MTMzNyZlbnRyeW51bT03ODMmbm90ZT1BQUFBQUFBQSZhZG1pbj1UcnVlJmFjY2Vzc19zZW5zaXRpdmU9VHJ1ZSZlbnRyeW51bT03"
  -F "integrity=1a51c1aa28c65fb763539c8055ae270b4c231a11" \
  crypto.chal.csaw.io:5003/view

Author: admin
Note: You disobeyed our rules, but here's the note: flag{h4ck_th3_h4sh}

Flag

flag{h4ck_th3_h4sh}

Original writeup (https://github.com/TeamOm3ga/writeups/blob/master/2020/csaw/crypto/authy.md).

authy