Authlib is vulnerable to Denial of Service via Oversized JOSE Segments

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2025-61920: 0.64% estimated exploitation activity in the next 30 days; percentile 50.64 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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…

Published by the source

10 Oct 2025, 20:26 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:35 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-pq5p-34cr-23v9

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-pq5p-34cr-23v9

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# Patch Next advisory brief: CVE-2025-61920

Advisory: Authlib is vulnerable to Denial of Service via Oversized JOSE Segments
IDs: CVE-2025-61920, GHSA-pq5p-34cr-23v9
Record: https://patchnext.com/advisories/cve-2025-61920 (revision 2, source checked 26 Oct 2026, 13:35 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: authlib 1.6.5. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI authlib: affected < 1.6.5; fixed in 1.6.5

## Suggested inspection (read-only)
- `python -m pip show authlib`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on authlib.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-pq5p-34cr-23v9 (OSV record GHSA-pq5p-34cr-23v9 lists affected ranges: PyPI authlib >=0 <1.6.5 (fixed))
- Verified: https://github.com/advisories/GHSA-pq5p-34cr-23v9 (GitHub-reviewed advisory GHSA-pq5p-34cr-23v9 lists affected ranges: PyPI authlib >=0 <1.6.5)

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