Authlib: Default JWT bearer grant accepts assertions for the wrong audience

Exploitation

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

CVSS

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

EPSS

• CVE-2026-114618: 0.35% estimated exploitation activity in the next 30 days; percentile 28.82 (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.

Description

Summary Authlib's default RFC 7523 JWT bearer grant leaves `get_audiences()` empty, so `verify_claims()` only checks that an `aud` claim *exists* instead of requiring it to match the current authorization server's token endpoint. An attacker holding a legitimately issued JWT assertion for *any* audience can present it to a different authorization server sharing the same signing key and obtain an access token. To be completely transparent, AI tools helped surface it. However, we have independently reproduced and carefully validated the issue. We follow responsible disclosure practices and are sharing this privately.

Affected Entrypoints - `authlib.oauth2.rfc7523.JWTBearerGrant` — default `get_audiences()` returns empty list - `authlib.jose.jwt.decode()` with `claims_options` for audience validation - `authlib.integrations.flask_oauth2.ResourceProtector.validate_request()` when audience checking is enabled

Exploit Path ``` 1. Attacker authenticates to Service…

Published by the source

27 Oct 2026, 15:39 UTC

Last modified by the source

30 Oct 2026, 10:11 UTC

Source last checked

31 Oct 2026, 06:15 UTC

1. Revision 4 · amended · 31 Oct 2026

   Amended from osv record PYSEC-2026-4358

2. Revision 3 · amended · 31 Oct 2026

   Amended from osv record GHSA-fch4-3c6x-j4gc

3. Revision 2 · amended · 28 Oct 2026

   Amended from osv record GHSA-fch4-3c6x-j4gc

4. Revision 1 · created · 27 Oct 2026

   Published from ghsa record GHSA-fch4-3c6x-j4gc

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# Patch Next advisory brief: CVE-2026-114618

Advisory: Authlib: Default JWT bearer grant accepts assertions for the wrong audience
IDs: CVE-2026-114618, GHSA-fch4-3c6x-j4gc
Record: https://patchnext.com/advisories/cve-2026-114618 (revision 4, source checked 31 Oct 2026, 06:15 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: authlib 1.9.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI authlib: affected < 1.9.0; fixed in 1.9.0

## 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-fch4-3c6x-j4gc (OSV record GHSA-fch4-3c6x-j4gc lists affected ranges: PyPI authlib >=0 <1.9.0 (fixed))
- Verified: https://github.com/advisories/GHSA-fch4-3c6x-j4gc (GitHub-reviewed advisory GHSA-fch4-3c6x-j4gc lists affected ranges: PyPI authlib >=0 <1.9.0)
- Verified: https://osv.dev/vulnerability/PYSEC-2026-4358 (OSV record PYSEC-2026-4358 lists affected ranges: PyPI authlib >=0 <1.9.0 (fixed))

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