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CVE-2025-61920 — Authlib is vulnerable to Denial of Service via Oversized JOSE Segments

2026-09-30T21:09:08.123359+00:00

Affected: authlib

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.

GHSA-pq5p-34cr-23v9 — Authlib is vulnerable to Denial of Service via Oversized JOSE Segments

2026-09-30T21:09:08.123467+00:00

Affected: PyPI: authlib

**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 [jws_segment_dos_demo.py](https://github.com/user-attachments/files/22450820/jws_segment_dos_demo.py) the PoC in direcotry authlib/ Run following Command ``` python3 jws_segment_dos_demo.py --variant both --sizes "500MB" --fork-per-case

``` Environment: Python 3.13.6, Authlib 1.6.4, Linux x86_64, CPUs=8 Sample output: Refined

PYSEC-2026-1203 — Authlib is vulnerable to Denial of Service via Oversized JOSE Segments

2026-09-30T21:09:08.123574+00:00