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GHSA-w2cx-738m-mc7w
pyjwtSummary
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.
Impact
An attacker who knows the public key material can forge HS256/HS384/HS512 tokens if the application simultaneously:
- allows both HS* and asymmetric algorithms;
- passes raw public JWK/JWKS JSON as
key=; and - uses that same value as the HMAC secret.
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.
Fix status
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.
Reporter credit
Credit: Charles Vosburgh / Trilobyte.
Original report
The original report and reproduction package are retained in the private advisory record.
Maintainer update — 2026-09-11
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.
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The vulnerability can be exploited over the network without needing physical access. It is difficult for an attacker to exploit this vulnerability and may require special conditions. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a high impact on the confidentiality of the information. There is a high impact on the integrity of the data.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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