CVE-2025-61920 Authlib is vulnerable to Denial of Service via Oversized JOSE Segments
Remote attackers can cause denial of service in Authlib by sending oversized JOSE segments, exhausting CPU and memory resources.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-61920?
CVE-2025-61920 is a high-severity denial of service vulnerability in authlib, published on 10 Oct 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Which versions are affected by CVE-2025-61920?
3 open source packages ship an affected version, including authlib. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
authlib PyPI | All versions | 1.6.5 |
authlib PyPI | All versions | 1.6.5 |
authlib PyPI | All versions | 1.6.5 |
Linux distribution packages (4)
python-Authlib SUSE:Linux Enterprise Module for Python 3 15 SP6 | All versions | 1.3.1-150600.3.6.1 |
python-Authlib SUSE:Linux Enterprise Module for Python 3 15 SP7 | All versions | 1.3.1-150600.3.6.1 |
python-Authlib openSUSE:Leap 15.6 | All versions | 1.3.1-150600.3.6.1 |
python-Authlib openSUSE:Tumbleweed | All versions | 1.6.5-1.1 |
Products include Authlib Authlib.
How do I fix CVE-2025-61920?
Upgrade authlib to 1.6.5 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2025-61920 being exploited?
There is no known exploitation of CVE-2025-61920 in the wild and no public exploit code.
Other vulnerabilities in authlib
- CVE-2026-27962 Authlib JWS JWK Header Injection: Signature Verification Bypass
- CVE-2026-28490 Authlib Vulnerable to JWE RSA1_5 Bleichenbacher Padding Oracle
- CVE-2026-28498 Authlib: Fail-Open Cryptographic Verification in OIDC Hash Binding
- CVE-2026-28802 Authlib: Setting `alg: none` and a blank signature appears to bypass signature verification
- CVE-2025-59420 Authlib: JWS/JWT accepts unknown crit headers (RFC violation → possible authz bypass)
Sources
Also known as DEBIAN-CVE-2025-61920, ECHO-9171-850d-97d8, GHSA-pq5p-34cr-23v9, PYSEC-2026-1203, ROOT-APP-PYPI-CVE-2025-61920, SUSE-SU-2025:3754-1 and 2 more.