CVE-2025-61920
A remote attacker can craft a token whose base64url‑encoded header or signature spans hundreds of megabytes.
Does this matter?
High impact if exploited, but EPSS currently rates exploitation as unlikely (0.64%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.
Description
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.
- CVSS 3.1
- 7.5 HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- EPSS
- 0.64% probability · 49th percentile
- CISA KEV
- Not listed
- Weakness
- CWE-20, CWE-400, CWE-770
- Affected
- authlib/authlib
- Source
- security-advisories@github.com
References
Source: NVD record, EPSS from FIRST.org, KEV from CISA. Refreshed daily.