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TI-2026-69684 Aliases CVE-2026-107723GHSA-5hjw-83fp-phq9

Claim validator bypass in fast-jwt when JWT payload is a JSON array

Priority HIGH 70CWE-1287Published · Updated

Key facts

Attack vector
Network
Privileges required
Low
User interaction
None
Attack complexity
Low
Impact type
—
Confidentiality · integrity · availability
High · High · None
Fix status
Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA and GitHub.

Summary

fast-jwt versions before 6.3.0 accept a validly signed JWT whose payload is a JSON array, because the decoder checks for an object but does not reject arrays (CWE-1287). The claim validator then finds none of the named claims and skips its checks for exp, nbf, iss, aud, sub, jti and nonce. An attacker who can produce or influence a validly signed token can bypass expiry, issuer, audience, subject and replay checks. Signature verification itself is not bypassed.

Written with Claude Haiku 5.5 from the sources below · 2026-10-29

Description from the CNA

fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.3.0, fast-jwt createVerifier accepts a validly signed JWT whose payload is a JSON array because src/decoder.js checks that the payload is an object but does not reject arrays. The claim validator loop then finds no named exp, nbf, iss, aud, sub, jti, or nonce properties and silently skips those configured checks, returning the array as a successfully verified payload. An attacker who can produce or influence a validly signed token may bypass expiry, issuer, audience, subject, revocation, and replay protections. The opt-in requiredClaims option can block missing claims, and signature verification itself is not bypassed. This issue is fixed in version 6.3.0.

Why it matters now

Exploitation Proof of concept CISA SSVC · not in CISA KEV
EPSS 0.00455 39th percentileScored 2026-11-07
Fix Fixed fast-jwt (nearform): 6.3.0fast-jwt (npm): 6.3.0

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
Poc
Automatable
No
Technical impact
Total

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
nearformfast-jwtbefore 6.3.06.3.0Patch availableCNA
—fast-jwt (npm)before 6.3.06.3.0Patch availableGHSA-5hjw-83fp-phq9
  • fast-jwt Vendornearform Affected before 6.3.0 Fixed in 6.3.0 Patch available CNA
  • fast-jwt (npm) Vendor— Affected before 6.3.0 Fixed in 6.3.0 Patch available GHSA-5hjw-83fp-phq9

CPE names Threatist (inferred) · 1

Inferred from how NVD named this product before; not given by a source.

CPEVersionsRoleVendor
cpe:2.3:a:nearform:fast-jwt:*:*:*:*:*:*:*:*——nearform
  • cpe:2.3:a:nearform:fast-jwt:*:*:*:*:*:*:*:* Versions — — ·nearform

Remediation

Upgrade the npm package fast-jwt to version 6.3.0. The sources list no fixed version. Until upgrading, set the requiredClaims option to block tokens with missing claims.

Written with Claude Haiku 5.5 from the sources below · 2026-10-29

  • Fix · CNA fast-jwt (nearform) Fixed in 6.3.0
  • Fix · GHSA-5hjw-83fp-phq9 fast-jwt (npm) Fixed in 6.3.0 Advisory →

Timeline

  1. 2026-10-09 Threatist record created from CVE-2026-107723
  2. 2026-10-09 Alias added: GHSA-5hjw-83fp-phq9
  3. 2026-10-10 Priority changed: MEDIUM → LOW
  4. 2026-10-14 CISA SSVC decision points changed
  5. 2026-10-14 Priority changed: LOW → HIGH

References

Other 5

  • TI-2026-69702 Incomplete fix in fast-jwt enables RSA to HS256 algorithm confusion CVSS 3.1CRITICAL 9.8
  • TI-2026-27174 fast-jwt has an incomplete fix for CVE-2023-48223: JWT Algorithm Confusion via Whitespace-Prefixed RSA Public Key CVSS 3.1CRITICAL 9.1
  • TI-2025-37324 get-jwks poisoned JWKS cache allows post-fetch issuer validation bypass CVSS 3.0CRITICAL 9.4
  • TI-2026-27245 fast-jwt accepts unknown `crit` header extensions (RFC 7515 §4.1.11 MUST violation) CVSS 3.1HIGH 7.5
  • TI-2026-69695 fast-jwt accepts public JWK JSON as HMAC key, enabling token forgery CVSS 3.1HIGH 7.4
  • TI-2026-69689 fast-jwt accepts unsigned JWTs when the key is empty or null CVSS 3.1HIGH 7.4
See all 15 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences