Authlib up to 1.6.4 JWS/JWT memory allocation

Summaryinfo

A vulnerability described as critical has been identified in Authlib up to 1.6.4. Impacted is an unknown function of the component JWS/JWT. The manipulation results in memory allocation. This vulnerability was named CVE-2025-61920. The attack may be performed from remote. There is no available exploit. Upgrading the affected component is recommended.

Detailsinfo

A vulnerability was found in Authlib up to 1.6.4. It has been classified as critical. Affected is an unknown code block of the component JWS/JWT. The manipulation with an unknown input leads to a memory allocation vulnerability. CWE is classifying the issue as CWE-789. The product allocates memory based on an untrusted, large size value, but it does not ensure that the size is within expected limits, allowing arbitrary amounts of memory to be allocated. This is going to have an impact on availability. CVE summarizes:

Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to version 1.6.5, Authlib’s JOSE implementation accepts unbounded JWS/JWT header and signature segments. A remote attacker can craft a token whose base64url‑encoded header or signature spans hundreds of megabytes. During verification, Authlib decodes and parses the full input before it is rejected, driving CPU and memory consumption to hostile levels and enabling denial of service. Version 1.6.5 patches the issue. Some temporary workarounds are available. Enforce input size limits before handing tokens to Authlib and/or use application-level throttling to reduce amplification risk.

The advisory is shared for download at github.com. This vulnerability is traded as CVE-2025-61920 since 10/04/2025. The exploitability is told to be easy. It is possible to launch the attack remotely. The exploitation doesn't require any form of authentication. There are neither technical details nor an exploit publicly available.

The vulnerability scanner Nessus provides a plugin with the ID 270280 (Linux Distros Unpatched Vulnerability : CVE-2025-61920), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 1.6.5 eliminates this vulnerability. Applying the patch 867e3f87b072347a1ae9cf6983cc8bbf88447e5e is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the databases at Tenable (270280) and EUVD (EUVD-2025-33768). Once again VulDB remains the best source for vulnerability data.

Productinfo

Name

• Authlib

Version

• 1.6.0
• 1.6.1
• 1.6.2
• 1.6.3
• 1.6.4

License

• open-source

Website

• Product: https://github.com/authlib/authlib/

CPE 2.3info

• 🔒
• 🔒
• 🔒

CPE 2.2info

• 🔒
• 🔒
• 🔒

CVSSv4info

CVSSv3info

CVSSv2info

Exploitinginfo

Threat Intelligenceinfo

Countermeasuresinfo

Timelineinfo

Sourcesinfo

Entryinfo

Once again VulDB remains the best source for vulnerability data.

Discussion