CVE-2026-49852: authlib joserfc Improper Authentication

Vulnerability summary

CVE-2026-49852 is a high (CVSS 8.7) Improper Authentication vulnerability in authlib joserfc. EPSS estimates a 0.19% chance of exploitation in the next 30 days, higher than 8.5% of scored vulnerabilities.

Timeline

1. 17-07-2026
   PublishedVulnerability first disclosed.
2. 20-07-2026
   Last modifiedVulnerability information updated.

Description

joserfc is a Python library that provides an implementation of several JSON Object Signing and Encryption (JOSE) standards. Prior to 1.6.8, joserfc.jwt.decode accepts attacker-forged HMAC-signed tokens when the caller-supplied verification key is the empty string or None, because HMACAlgorithm.sign and HMACAlgorithm.verify in src/joserfc/_rfc7518/jws_algs.py pass the output of OctKey.get_op_key(...) to hmac.new(...) and OctKey.import_key in src/joserfc/_rfc7518/oct_key.py only emits a SecurityWarning for keys shorter than 14 bytes without rejecting zero-length input. This issue is fixed in version 1.6.8.

• CWE-287Improper AuthenticationWhen an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct.
• CWE-326Inadequate Encryption StrengthThe product stores or transmits sensitive data using an encryption scheme that is theoretically sound, but is not strong enough for the level of protection required.
• CWE-1391Use of Weak CredentialsThe product uses weak credentials (such as a default key or hard-coded password) that can be calculated, derived, reused, or guessed by an attacker.