JWT and token security testing
A JSON Web Token (JWT) carries who you are and what you can do, trusted on every request. If your app validates it weakly, an attacker can forge identity or privilege outright. Sintropyc tests signature, algorithm, claim and expiry handling — and proves each token that can be forged, replayed or tampered.
How JWT validation goes wrong
alg:none— a token with no signature the server still accepts- Algorithm confusion — an
RS256verifier tricked into treating the public key as anHS256secret - Weak HMAC secret — a guessable or default signing key, brute-forced offline
- Unverified claims — trusting
role,tenant_idorsubfrom the token without re-checking server-side - No expiry or revocation — tokens that live forever or survive logout and password change
Why tokens are a high-value target
A JWT is a bearer credential: whoever holds a valid one is that user, everywhere it's accepted. A single validation flaw isn't a small bug — it's a master key. And because the token looks opaque, these flaws are easy to ship and easy to miss.
Fast-built and template-started apps frequently ship a hard-coded or example signing secret, skip expiry, or trust claims the client can set. The token looks fine; the validation isn't.
How Sintropyc tests it
Sintropyc is an autonomous AI penetration testing agent. It signs in as real users across different roles, drives your product the way a customer would, and proves every finding with a working exploit — never a scanner guess. Each finding is checked by a human before it reaches you, comes with the exact fix, and is attacked again after you ship it to confirm it held.
For JWTs, Sintropyc inspects your tokens, attempts alg:none and algorithm-confusion forgeries, tests for weak secrets, tampers privilege and tenant claims, and replays tokens past logout and expiry. A finding is reported only when a forged or tampered token is actually accepted — proven with the token and the access it granted.
What you get
- Signature, algorithm, claim and expiry handling tested
- Proven forgeries — a tampered token the server accepted, not a theoretical risk
- The fix (strict algorithm, strong secret / keys, claim re-validation, expiry) per finding
- A retest after you ship it
Common questions
Do you brute-force our signing key?
Only offline, against a captured token, to prove a weak or guessable secret — never by hammering your server. A strong random key is safe from this.
We use a library — aren't we fine?
Libraries help, but misuse is common: accepting multiple algorithms, skipping expiry, or trusting claims without re-checking ownership. Sintropyc tests your actual validation, not the library in the abstract.
Is this part of API authorization testing?
It's complementary. JWT testing checks the token itself; API authorization testing checks whether endpoints enforce what the (valid) token is allowed to do.