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CVE Record

CVE-2026-28498: Authlib: Fail-Open Cryptographic Verification in OIDC Hash Binding

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.

CriticalCVSS 9.1Not KEV-listedUpdated
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Glexia's TakeAutomated analysiscritical

Security readout for executives and security teams

Plain-English summary

Authlib versions before 1.6.9 can mistakenly accept forged OpenID Connect token bindings when an unknown cryptographic algorithm is supplied. For organizations using Authlib in login or SSO flows, this could undermine trust decisions around identity tokens. The source bundle names a fix in Authlib 1.6.9.

Executive priority

High priority for any business system using Authlib in authentication. Patch quickly because the issue can affect identity assurance and access control decisions. Do not assume compromise from the provided evidence; focus on rapid inventory, upgrade, and validation.

Technical view

The flaw is in Authlib's internal OIDC _verify_hash handling for at_hash and c_hash. Unsupported or unknown algorithms fail open, returning validation success instead of rejecting the token hash check. This weakens integrity binding between ID Tokens and access tokens or authorization codes. A patch is identified in version 1.6.9.

Likely exposure

Most likely exposed systems are Python applications using Authlib before 1.6.9 for OpenID Connect ID Token validation, especially flows relying on at_hash or c_hash. Exposure depends on whether Authlib handles token validation in the deployed authentication path. Red Hat-listed packages should be checked against their advisories.

Exploitation context

The source bundle describes a network-reachable, unauthenticated, low-complexity bypass with high confidentiality and integrity impact. It does not provide evidence of active exploitation, and KEV status is false. Treat as urgent because it affects authentication trust, not because exploitation is confirmed.

Researcher notes

Key evidence is the Authlib advisory, patch commit, and 1.6.9 release reference. The bundle attributes CWE-325, CWE-354, and CWE-573. Avoid broad claims beyond Authlib before 1.6.9 and vendor-packaged variants unless confirmed by the cited advisories.

Mitigation direction

  • Upgrade Authlib to version 1.6.9 or later.
  • Check Red Hat advisories for backported package fixes.
  • Prioritize internet-facing and SSO-dependent applications first.
  • Review vendor guidance before applying compensating controls.
  • Monitor Authlib and distribution advisories for updates.

Validation and detection

  • Inventory deployed Authlib versions across Python environments.
  • Identify OIDC flows that validate ID Tokens with Authlib.
  • Confirm deployed builds include Authlib 1.6.9 or a vendor backport.
  • Review dependency locks, container images, and base images.
  • Retest authentication flows after upgrading packages.
Prepared
Confidence
high
Sources
12

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · low confidence lookup

CWE-325: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
cwe · low confidence lookup

CWE-354: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
cwe · low confidence lookup

CWE-573: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
description · low confidence lookup

Credential and access behavior lookup

The CVE wording references authentication or credential exposure, so valid-account and credential-access review may help. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.

Open ATT&CK lookup
cve · low confidence lookup

CVE-2026-28498 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

Open ATT&CK lookup
Vulnerability profileCVE Program record
Severity
Critical
CVSS
9.1 (3.1)
Known Exploited
No
Published

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

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

2CVSS vectors
5Timeline events
2ADP providers
13Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: pocAutomatable: noTechnical Impact: partial

CVSS vector scores

2 official scores

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
9.1CVSS 3.1CriticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N3.95.2redhat-SADP
8.2CVSS 4.0HighCVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N——GitHub_M

Vulnerability scoring details

Base CVSS 4.0 score

8.2High
CVSS 4.0 vector shape for CVE-2026-28498Attack VectorAttack ComplexityAttack RequirementsPrivileges RequiredUser InteractionVS ConfidentialityVS IntegrityVS AvailabilitySS ConfidentialitySS IntegritySS Availability

Vector: CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Attack Requirements
NonePresent
Privileges Required
NoneLowHigh
User Interaction
NonePassiveActive
VS Confidentiality
HighLowNone
VS Integrity
HighLowNone
VS Availability
HighLowNone
SS Confidentiality
HighLowNone
SS Integrity
HighLowNone
SS Availability
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. ADP timelineredhat-SADP

    Made public.

  3. CVE publishedCVE Program

    The CVE record was published.

  4. ADP timelineredhat-SADP

    Reported to Red Hat.

  5. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
redhat-SADPauthlib: Authlib: Authentication bypass via forged OpenID Connect ID Tokens
other:Red Hat severity ratingcvssV3_1
  • 2026-03-16T19:02:00.128Z: Reported to Red Hat.
  • 2026-03-16T18:03:28.821Z: Made public.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
authlibauthlib< 1.6.9Listed
Weakness

CWE details

CWE-325CWE-354CWE-573

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-325 · source CWE mapping

Missing Cryptographic Step

Missing Cryptographic Step represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs
CWE-354 · source CWE mapping

Improper Validation of Integrity Check Value

Improper Validation of Integrity Check Value represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs
CWE-573 · source CWE mapping

Improper Following of Specification by Caller

Improper Following of Specification by Caller represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs