CVE-2025-61920: Authlib is vulnerable to Denial of Service via Oversized JOSE Segments
Summary 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.
Impact
- Attack vector: unauthenticated network attacker submits a malicious JWS/JWT.
- Effect: base64 decode + JSON/crypto processing of huge buffers pegs CPU and allocates large amounts of RAM; a single request can exhaust service capacity.
- Observed behaviour: on a test host, the legacy code verified a 500 MB header, consuming ~4 GB RSS and ~9 s CPU before failing.
- Severity: High. CVSS v3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H (7.5).
Affected Versions Authlib ≤ 1.6.3 (and earlier) when verifying JWS/JWT tokens. Later snapshots with 256 KB header/signature limits are not affected.
Proof of concept
Local demo (do not run against third-party systems): Download jwssegmentdosdemo.py the PoC in direcotry authlib/ Run following Command python3 jwssegmentdosdemo.py --variant both --sizes "500MB" --fork-per-case
Environment: Python 3.13.6, Authlib 1.6.4, Linux x8664, CPUs=8 Sample output: Refined <img width="1295" height="306" alt="image" src="https://github.com/user-attachments/assets/6dd8410f-bc36-4717-8cee-649bac9bf291" />
The compilation script prints separate “[ATTACKER]” (token construction) and “[SERVER]” (Authlib verification) RSS deltas so defenders can distinguish client-side preparation from server-side amplification. Regression tests authlib/tests/dos/testjosedos.py further capture the issue; the saved originalutil.py/originaljws.py reproductions still accept the malicious payload.
Remediation
- Apply the upstream patch that introduces decoded size limits:
- MAXHEADERSEGMENTBYTES = 256 KB
- MAXSIGNATURESEGMENTBYTES = 256 KB
- Enforce Limits in authlib/jose/util.extractsegment and extractsignature.
- Deploy the patched release immediately.
- For additional defence in depth, reject JWS/JWT inputs above a few kilobytes at the proxy or WAF layer, and rate-limit verification endpoints.
Workarounds (temporary)
- Enforce input size limits before handing tokens to Authlib.
- Use application-level throttling to reduce amplification risk.
Resources
- Demo script: jwssegmentdosdemo.py
- Tests: authlib/tests/dos/testjosedos.py
- OWASP JWT Cheat Sheet (DoS guidance)
Other sources
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.
— MITRE
Affected Software
Remediation
Event History
Frequently Asked Questions
What is the severity of CVE-2025-61920?
CVE-2025-61920 has been classified as a moderate severity vulnerability due to potential exposure to security risks from crafted tokens.
How do I fix CVE-2025-61920?
To mitigate CVE-2025-61920, upgrade Authlib to version 1.6.5 or later, which addresses the unbounded JWS/JWT header and signature issue.
What type of attack does CVE-2025-61920 allow?
CVE-2025-61920 allows remote attackers to craft malicious tokens that exploit the unbounded header or signature segments.
Which versions of Authlib are affected by CVE-2025-61920?
Authlib versions prior to 1.6.5 are affected by CVE-2025-61920.
What is Authlib and why is CVE-2025-61920 important?
Authlib is a Python library for building OAuth and OpenID Connect servers, and CVE-2025-61920 is important because it can lead to significant security vulnerabilities if not addressed.