JWT authentication bypass via unverified signature | Dec 26, 2022

Introduction

Welcome to my another writeup! In this Portswigger Labs lab, you’ll learn: JWT authentication bypass via unverified signature! Without further ado, let’s dive in.

• Overall difficulty for me (From 1-10 stars): ★☆☆☆☆☆☆☆☆☆

Background

This lab uses a JWT-based mechanism for handling sessions. Due to implementation flaws, the server doesn’t verify the signature of any JWTs that it receives.

To solve the lab, modify your session token to gain access to the admin panel at /admin, then delete the user carlos.

You can log in to your own account using the following credentials: wiener:peter

Exploitation

Login as user wiener:

Session cookie:

eyJraWQiOiI0NWQyYmU4Zi03MmNjLTRiYjQtOGQyNy1jY2FmODU0MjA5Y2UiLCJhbGciOiJSUzI1NiJ9.eyJpc3MiOiJwb3J0c3dpZ2dlciIsInN1YiI6IndpZW5lciIsImV4cCI6MTY3MjA0MDgxNX0.oMDhwyF-Cd6DxHjPxlfZ1_7PJqLT-6PtDaNXtZs2bFVN3HvIS_xidTq8KX7OuQGSng9FtZhijD_DV16Xw0axAMasA4_Gqj3rBqUEE_I_KVt51qQ1VvDMxcx4mhQGi_wGXs3oyv8_Rhiz4uYHvryKDL5mS7AWzmL-ObmhWc3XOhzZ8TsksZkRMjiIJ3vKhIIhqtX8P-NLEp8k-eY0CR9OcVQAFzhirYD97Ul4b3aD05DY6qD_bKycL8hbGKP9vYKLwUuPH08MTVZGkk77ggGzvKbWcEV2hxZt6KF_XOQx3NBTotR_Ds-slBtd-XmpSgtmIeSMMiJJpA3oPaERCEK9Ew

As you can see, this session cookie is a JWT(JSON Web Token), as a JWT consists of 3 parts: a header, a payload, and a signature. They are each separated by a dot.

Let’s copy and paste that to token.dev, which is an online tool that encode or decode JWTs:

As you can see, in the header part, the signature’s algorithm is RS256(RSA + SHA-256), and it has a kid(Key ID). In the payload part, it has an issuer(portswigger), subject(wiener), and expires(1672040815).

Now, in the lab’s background, it said:

Due to implementation flaws, the server doesn’t verify the signature of any JWTs that it receives.

Armed with above information, we can just simply modify payload’s subject to administrator:

Then we copy and paste the modified JWT string to the session cookie:

Finally, refresh the page:

We’re administrator, and have access to the admin panel!

Let’s delete user carlos!

Nice!

What we’ve learned:

1. JWT authentication bypass via unverified signature