CVE-2026-48523: PyJWT: Algorithm allow-list bypass when decoding with `PyJWK` / `PyJWKClient` keys

Severity: medium (CVSS 5.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-05-28 · updated 2026-10-25
Weakness: CWE-347

Description

PyJWT is a JSON Web Token implementation in Python. From 2.9.0 to 2.12.1, there is a verifier-side algorithm allow-list bypass when jwt.decode() or jwt.decode_complete() are called with a PyJWK key. The token header alg is checked against the caller-supplied algorithms allow-list, but signature verification is performed with the algorithm bound to the PyJWK object instead of the header algorithm. An attacker who controls a registered JWK/JWKS private key can sign with a disallowed algorithm, advertise an allowed algorithm in the JWT header, and still be accepted. The issue affects the documented PyJWKClient.get_signing_key_from_jwt(...) flow. This vulnerability is fixed in 2.13.0.

Affected products

jpadilla pyjwt: from 2.9.0 before 2.13.0 — fixed in 2.13.0

How to fix

Upgrade to a fixed version: jpadilla pyjwt 2.13.0. Check the vendor advisory for mitigations.

References

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