Improper Verification of Cryptographic Signature in pyjwt - #VU150525
Published: September 17, 2026
Vulnerability identifier: #VU150525
CSH Severity: High
CVSS v4: 9.3 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N]
CVE-ID: N/A
CWE-ID: CWE-347
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows a remote attacker to forge arbitrary-claim tokens.
The vulnerability exists due to improper verification of cryptographic signatures in HMACAlgorithm.prepare_key and is_pem_format when processing a loader-accepted PEM public key that is not recognized as PEM under a mixed algorithm allow-list. A remote attacker can sign an HS256 token using the public key as an HMAC secret to forge arbitrary-claim tokens.
Exploitation requires knowledge of the public verification key, cryptography to be installed, and use of the raw PEM non-PyJWK verification path.
Affected software
pyjwt
Remediation
Install security update from vendor's website.
pyjwt - update to 2.14.0