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CVE Record

CVE-2026-27962: Authlib JWS JWK Header Injection: Signature Verification Bypass

Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to version 1.6.9, a JWK Header Injection vulnerability in authlib's JWS implementation allows an unauthenticated attacker to forge arbitrary JWT tokens that pass signature verification. When key=None is passed to any JWS deserialization function, the library extracts and uses the cryptographic key embedded in the attacker-controlled JWT jwk header field. An attacker can sign a token with their own private key, embed the matching public key in the header, and have the server accept the forged token as cryptographically valid — bypassing authentication and authorization entirely. 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

CVE-2026-27962 lets an unauthenticated attacker bypass JWT signature verification in vulnerable Authlib versions. If an application relies on this path for authentication or authorization, forged tokens could be accepted as valid. The issue is critical because it can directly undermine identity controls, but the supplied sources do not show confirmed active exploitation.

Executive priority

Treat this as urgent for any internet-facing or identity-critical service using Authlib. A successful attack could bypass login and authorization checks without credentials. Patch validation should be prioritized before routine maintenance, with business focus on systems protecting customer, employee, or administrative access.

Technical view

Authlib before 1.6.9 has a JWS JWK header injection flaw. When JWS deserialization is called with key=None, Authlib may use a key supplied in the attacker-controlled JWT jwk header, allowing arbitrary tokens to verify successfully. This maps to CWE-347 and has CVSS 9.1.

Likely exposure

Exposure is most likely in Python applications using Authlib versions earlier than 1.6.9 for JWT/JWS verification, especially OAuth or OpenID Connect services, and specifically where JWS deserialization functions are called with key=None.

Exploitation context

The source bundle states the flaw is remotely reachable, unauthenticated, low complexity, and requires no user interaction. KEV is false, and the provided sources do not establish active exploitation in the wild.

Researcher notes

The key condition is not only package version, but whether vulnerable JWS deserialization paths are reachable with key=None. The supplied evidence names the patch release and commit, but does not provide confirmed exploit telemetry. Red Hat references indicate downstream tracking and errata should be checked separately.

Mitigation direction

  • Upgrade Authlib to version 1.6.9 or later.
  • Identify downstream Red Hat packages and apply relevant RHSA updates.
  • Avoid verification flows that allow attacker-controlled keys to be trusted implicitly.
  • Review vendor advisories for environment-specific remediation guidance.
  • Prioritize services where JWTs protect administrative or sensitive data access.

Validation and detection

  • Inventory deployed Authlib versions across applications and containers.
  • Search code for JWS deserialization calls using key=None.
  • Confirm production builds resolve Authlib to 1.6.9 or later.
  • Add regression tests rejecting JWTs with untrusted jwk headers.
  • Review authentication logs for anomalous token acceptance patterns.
Prepared
Confidence
high
Sources
8

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

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cwe · low confidence lookup

CWE-347: Exact CWE lookup

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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.

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cve · low confidence lookup

CVE-2026-27962 mapping review

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

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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
11Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: pocAutomatable: yesTechnical Impact: total

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:N3.95.2GitHub_M
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

Vulnerability scoring details

Base CVSS 3.1 score

9.1Critical
CVSS 3.1 vector shape for CVE-2026-27962Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

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

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
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 due to JWK Header Injection vulnerability
other:Red Hat severity ratingcvssV3_1
  • 2026-03-16T18:02:07.041Z: Reported to Red Hat.
  • 2026-03-16T17:34:38.946Z: Made public.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
authlibauthlib< 1.6.9Listed
Weakness

CWE details

CWE-347

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

CWE-347 · source CWE mapping

Improper Verification of Cryptographic Signature

Improper Verification of Cryptographic Signature represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs