Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to version 1.6.9, a library-level vulnerability was identified in the Authlib Python library concerning the validation of OpenID Connect (OIDC) ID Tokens. Specifically, the internal hash verification logic (_verify_hash) responsible for validating the at_hash (Access Token Hash) and c_hash (Authorization Code Hash) claims exhibits a fail-open behavior when encountering an unsupported or unknown cryptographic algorithm. This flaw allows an attacker to bypass mandatory integrity protections by supplying a forged ID Token with a deliberately unrecognized alg header parameter. The library intercepts the unsupported state and silently returns True (validation passed), inherently violating fundamental cryptographic design principles and direct OIDC specifications. This issue has been patched in version 1.6.9.

The remote Debian 11 host has packages installed that are affected by multiple vulnerabilities as referenced in the dla-4579 advisory.

    -------------------------------------------------------------------------     Debian LTS Advisory DLA-4579-1                debian-lts@lists.debian.org     https://www.debian.org/lts/security/                       Emmanuel Arias     May 11, 2026                                  https://wiki.debian.org/LTS
    -------------------------------------------------------------------------

    Package        : python-authlib     Version        : 0.15.4-1+deb11u2     CVE ID         : CVE-2026-27962 CVE-2026-28490 CVE-2026-28498

    Three security vulnerabilities were discovered in python-authlib, a python     library which builds OAuth and OpenID Connect servers, that can cause     authentication bypass or information leaks.

    CVE-2026-27962

      Fix authentication and authorization bypass vulnerability by embedding a       crafted public key in the jwk header field when key=None is passed to JWS       deserialization functions.

    CVE-2026-28490

      Authlib exposed distinguishable error responses between invalid PKCS#1 v1.5       padding and invalid AES-GCM tag, enabling Bleichenbacher-style attacks.

    CVE-2026-28498

      Fix OIDC ID Token validation bypass in at_hash and c_hash verification.
      _verify_hash() silently returned True when create_half_hash() received an       unknown algorithm, allowing forged ID          Tokens to pass validation.

    For Debian 11 bullseye, these problems have been fixed in version     0.15.4-1+deb11u2.

    We recommend that you upgrade your python-authlib packages.

    For the detailed security status of python-authlib please refer to     its security tracker page at:
    https://security-tracker.debian.org/tracker/python-authlib

    Further information about Debian LTS security advisories, how to apply     these updates to your system and frequently asked questions can be     found at: https://wiki.debian.org/LTS     Attachment:
    signature.asc     Description: PGP signature

Tenable has extracted the preceding description block directly from the Debian security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The remote SUSE Linux SLED15 / SLED_SAP15 / SLES15 / SLES_SAP15 / openSUSE 15 host has a package installed that is affected by multiple vulnerabilities as referenced in the SUSE-SU-2026:0975-1 advisory.

    - CVE-2026-27962: JWS `deserialize_compact()` allows for signature bypass by accepting user-controlled     embedded JWK as       verification key (bsc#1259738).
    - CVE-2026-28490: cryptographic padding oracle in JWE RSA1_5 key management algorithm (bsc#1259736).
    - CVE-2026-28498: fail-open in behavior OIDC hash validation allows for bypass mandatory integrity     protections       (bsc#1259737).

Tenable has extracted the preceding description block directly from the SUSE security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

There are packages installed that are affected by a vulnerability referenced in the following CVE:

  - Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to version 1.6.9, a
    library-level vulnerability was identified in the Authlib Python library concerning the validation of
    OpenID Connect (OIDC) ID Tokens. Specifically, the internal hash verification logic (_verify_hash)
    responsible for validating the at_hash (Access Token Hash) and c_hash (Authorization Code Hash) claims
    exhibits a fail-open behavior when encountering an unsupported or unknown cryptographic algorithm. This
    flaw allows an attacker to bypass mandatory integrity protections by supplying a forged ID Token with a
    deliberately unrecognized alg header parameter. The library intercepts the unsupported state and silently
    returns True (validation passed), inherently violating fundamental cryptographic design principles and
    direct OIDC specifications. This issue has been patched in version 1.6.9. (CVE-2026-28498)

The Linux/Unix host has one or more packages installed that are impacted by a vulnerability without a vendor supplied patch available.

  - Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to version 1.6.9, a     library-level vulnerability was identified in the Authlib Python library concerning the validation of     OpenID Connect (OIDC) ID Tokens. Specifically, the internal hash verification logic (_verify_hash)     responsible for validating the at_hash (Access Token Hash) and c_hash (Authorization Code Hash) claims     exhibits a fail-open behavior when encountering an unsupported or unknown cryptographic algorithm. This     flaw allows an attacker to bypass mandatory integrity protections by supplying a forged ID Token with a     deliberately unrecognized alg header parameter. The library intercepts the unsupported state and silently     returns True (validation passed), inherently violating fundamental cryptographic design principles and     direct OIDC specifications. This issue has been patched in version 1.6.9. (CVE-2026-28498)

Note that Nessus relies on the presence of the package as reported by the vendor.

ID	Name	Product	Family	Severity
314275	Debian dla-4579 : python-authlib-doc - security update	Nessus	Debian Local Security Checks	high
303588	SUSE SLED15 / SLES15 / openSUSE 15 Security Update : python-Authlib (SUSE-SU-2026:0975-1)	Nessus	SuSE Local Security Checks	high
438849	SCA: security update for authlib (GHSA-m344-f55w-2m6j)	Tenable Cloud Security	SCA Checks	high
438849	SCA: security update for authlib (GHSA-m344-f55w-2m6j)	Tenable Self-Hosted Container Security	SCA Checks	high
302537	Linux Distros Unpatched Vulnerability : CVE-2026-28498	Nessus	Misc.	high

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