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BREW-azure-cli-CVE-2026-102266 — PyJWT: PyJWK accepts empty HMAC keys, bypassing PyJWT's empty-key validation

Affected: Homebrew: azure-cli

### Summary

A service that verifies HS256 tokens using an empty oct JWK through PyJWK, including a PyJWK obtained from PyJWKSet, can therefore accept attacker-generated tokens as authenticated.

PyJWT 2.13.0 rejects an empty HMAC key when it is supplied through the raw `str`/`bytes` key path, but accepts the same zero-length key when it is supplied as a symmetric `PyJWK`.

An `oct` JWK containing an empty Base64URL key value:

```json {"kty":"oct","k":""} ```

is decoded to `b""`. During signature verification, the `PyJWK` path uses this decoded value directly and does not invoke the empty-key validation in `HMACAlgorithm.prepare_key`. With the default `enforce_minimum_key_length=False`, PyJWT emits a warning and proceeds with HMAC verification using the zero-length key.

An attacker can independently calculate `HMAC-SHA256(b"", signing_input)` and create valid HS256 JWTs with arbitrary claims. A service that verifies HS256 tokens using an empty `oct` JWK through `PyJWK` or `PyJWKSet` can therefore accept attacker-generated tokens as authenticated.

The application must already contain an empty HMAC JWK, for example because a missing Base64URL configuration or secret-manager value was serialized as `"k": ""`. The attacker does not need control of the JWK/JWKS source, a signing oracle, a private key, or a mixed algorithm allow-list.

### Details

The vulnerability is caused by inconsistent validation of the same HMAC key material depending on whether it enters PyJWT as a raw…

fkie_cve-2026-102266

PyJWT is a Python implementation of JSON Web Token standards. From 2.13.0 until 2.14.0, HMACAlgorithm.from_jwk is affected because PyJWK verification path used the decoded key without applying prepare_key validation. This occurs when a trusted JWK Set contains an oct entry with an empty k value. As a result, an attacker signs an HMAC token with the same zero-length key accepted by PyJWT. Consequently, forged token can carry arbitrary authenticated claims. This issue is fixed in version 2.14.0.

GHSA-9j54-fg26-wv3r — PyJWT: PyJWK accepts empty HMAC keys, bypassing PyJWT's empty-key validation

Affected: PyPI: PyJWT