vulnerability / NVI-00010689 / Revision 1
PyJWT: HS256/384/512 verify accepts empty HMAC key via oct JWK (PyJWK/PyJWKClient) enabling token forgery
GitHub lists this reviewed advisory for pyjwt. Published version ranges and source prerequisites are shown below; this is not an inspection of a visitor’s application.
This report has not inspected your inventory, configuration, deployed build, reachability or evidence. A catalog listing and a severity score do not establish exposure or compromise.
01 / Meaning
Why it matters
Published affected context helps an owner decide what evidence to gather. This report has not examined your application.
7.4 high
GitHub source severity; exploitability and local consequence require separate evidence.
As of · GitHub Advisory Database
02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
pyjwt
source listed<= 2.13.0
### Summary `HMACAlgorithm.from_jwk` accepts a symmetric (`kty=oct`) JWK whose `k` value is the empty string and returns a zero-length key without raising. When a token is then verified with that key object (a `PyJWK`, or a `PyJWK` resolved by `PyJWKSet` / `PyJWKClient`), the HMAC verifier computes `HMAC(key=b"", signing_input)` and accepts any token an attacker forged with the same empty key. The attacker needs no secret knowledge: they reproduce the digest locally with an empty key and set the token's `kid` to the empty-`k` JWK. The empty-key rejection that `HMACAlgorithm.prepare_key` performs for string/bytes secrets (added in 2.13.0) was never propagated to the sibling `from_jwk` code path, so the JWK/JWKS verification route stays exploitable. Under default decode options this path only emits an `InsecureKeyLengthWarning`, not an error, so verification still succeeds. This is the same "empty/nil HMAC key is accepted by verify" defect the JWT ecosystem is currently closing library-side: fast-jwt CVE-2026-44351 (CVSS 9.1) and ruby-jwt CVE-2026-45363 both added a library-side reject-empty-HMAC-key precondition. ### Affected versions `PyJWT` (PyPI package `pyjwt`) `<= 2.13.0`,
03 / Response
What to do next
- investigate
Identify the exact component and deployed version, examine the linked publisher’s conditions, and retain evidence of any mitigation.
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04 / Limits
What this report cannot establish
- Customer inventory, deployment, reachability and exploitation have not been inspected.
05 / Provenance
Sources you can inspect
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- GHSA-pxh4-856f-4h89 ↗
GitHub Advisory Database · vendor advisory
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Public revision history
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- Revision 1 ·
First indexed by Null; source disclosure date is shown separately.