CVE-2026-35040Public exploitNetwork attack vector

Authentication disruption via stateful claim regex reuse

Published Apr 9, 2026 · Updated Apr 9, 2026

Expected behavior violation in Nearform fast-jwt before 6.2.1 allows remote attackers to disrupt authentication via valid JWTs. In src/verifier.js, validateClaimValues repeatedly calls RegExp.test on a reused allowed-claim matcher without resetting lastIndex, so global or sticky patterns alternate between matching and failing. Only applications configuring allowedAud, allowedIss, allowedSub, allowedJti, or allowedNonce with /g or /y are affected, and valid requests are rejected on every second verification attempt.

CVSS severity5.3
Medium
EPSS probability0.38%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Expected behavior violation in Nearform fast-jwt before 6.2.1 allows remote attackers to disrupt authentication via valid JWTs. In src/verifier.js, validateClaimValues repeatedly calls RegExp.test on a reused allowed-claim matcher without resetting lastIndex, so global or sticky patterns alternate between matching and failing. Only applications configuring allowedAud, allowedIss, allowedSub, allowedJti, or allowedNonce with /g or /y are affected, and valid requests are rejected on every second verification attempt.

The record

CVE
CVE-2026-35040
Published
Apr 9, 2026
Updated
Apr 9, 2026
Vendor
Nearform
Product
fast-jwt
Classifications
CWE-440, CWE-697, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 9, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 9, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 6.2.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Nearform · fast-jwtversion=< 6.2.1

Published CVSS scores

5.3GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-440Expected Behavior Violation
CWE-697Incorrect Comparison
T1499Endpoint Denial of Service

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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