Authlib Vulnerable to JWE RSA1_5 Bleichenbacher Padding Oracle

Exploitation

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

CVSS 4.0

8.3 · CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:L/VA:N/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-28490: 0.14% estimated exploitation activity in the next 30 days; percentile 3.72 (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.

1. Executive Summary A cryptographic padding oracle vulnerability was identified in the Authlib Python library concerning the implementation of the JSON Web Encryption (JWE) RSA15 key management algorithm. Authlib registers RSA15 in its default algorithm registry without requiring explicit opt-in, and actively destroys the constant-time Bleichenbacher mitigation that the underlying cryptography library implements correctly. When cryptography encounters an invalid PKCS#1 v1.5 padding, it returns a randomized byte string instead of raising an exception — the correct behavior per RFC 3218 §2.3.2. Authlib ignores this contract and raises ValueError('Invalid "cek" length') immediately after decryption, before reaching AES-GCM tag validation. This creates a clean, reliable Exception Oracle: - Invalid padding → cryptography returns random bytes → Authlib length check fails → ValueError: Invalid "cek" length - Valid padding, wrong MAC → decryption succeeds → length check passes → AES-GCM…

Published by the source

16 Mar 2026, 15:17 UTC

Last modified by the source

13 Jul 2026, 07:26 UTC

Source last checked

26 Oct 2026, 13:37 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-7432-952r-cw78

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-7432-952r-cw78

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# Patch Next advisory brief: CVE-2026-28490

Advisory: Authlib Vulnerable to JWE RSA1_5 Bleichenbacher Padding Oracle
IDs: CVE-2026-28490, GHSA-7432-952r-cw78
Record: https://patchnext.com/advisories/cve-2026-28490 (revision 3, source checked 26 Oct 2026, 13:37 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: authlib 1.6.9. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI authlib: affected <= 1.6.8; fixed in 1.6.9

## 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-7432-952r-cw78 (OSV record GHSA-7432-952r-cw78 lists affected ranges: PyPI authlib >=0 <1.6.9 (fixed))
- Verified: https://github.com/advisories/GHSA-7432-952r-cw78 (GitHub-reviewed advisory GHSA-7432-952r-cw78 lists affected ranges: PyPI authlib >=0 <=1.6.8)

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