PyJWT 2.13.0 contains an incomplete defense against algorithm confusion when an application mixes symmetric and asymmetric algorithms in one verification path. A public RSA, EC, or OKP JWK can be accepted as an HMAC secret when it is wrapped in a JWKS object, nested in an array, or represented in another container form that does not expose a top-level kty member.

An attacker who knows the public key material can forge HS256/HS384/HS512 tokens if the application simultaneously:

This can allow forged JWT claims in affected application configurations. The issue does not affect applications that keep symmetric and asymmetric verification paths separate and follow PyJWT's algorithm-selection guidance.

The fix is on master in commit 801cd12 (fix: reject public JWK container HMAC keys). HMACAlgorithm.prepare key now rejects public JWK members found in objects, arrays, nested containers, BOM/UTF variants, and recursion-limit inputs. It also recognizes escaped JSON member names without treating ordinary string values as JWKs. Ordinary JSON secrets remain accepted byte-for-byte.

The change was tested with focused regression tests and the full local tox matrix. Available Python 3.9, 3.12, and 3.13 crypto/no-crypto suites, mypy, package metadata, and coverage passed; unavailable interpreters were skipped by the project configuration. A fresh independent Astra/max security review accepted the final diff with no blocking findings.

The affected range is = 2.13.0. The fix is on the unreleased development branch; the patched version will be recorded when a released 2.x version containing the fix is available. This advisory is being moved to draft pending that release.

Credit: Charles Vosburgh / Trilobyte.

The original report and reproduction package are retained in the private advisory record.

The verified fix for this advisory is included in PyJWT 2.14.0, released on 2026-09-11 and available on PyPI. PyJWT 2.14.0 is the first release containing the fix. This advisory is now published with 2.14.0 recorded as the patched version.