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CVE-2026-107720— fast-jwt: createVerifier accepts unsigned JWTs when key is '' or null and algorithms is explicitly set

Quick assessment

Affected
nearform fast-jwt
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

fast-jwt 提供了一个快速实现的 JSON Web Token（JWT）库。在 6.3.1 版本之前，当密钥为空字符串或 null，且 algorithms 参数为非空白名单时，createVerifier 方法会接受未签名的 JWT。此时，虚假（falsy）同步密钥会绕过 prepareKeyOrSecret 处理流程，allowedAlgorithms 仍保持启用状态，hasKey 为 false，且空签名会绕过 verifySignature 验证网关。因此，攻击者可以提交包含任意声明（claims）的

CVSS 7.4 · High

Possible ATT&CK Techniques 1 AI

T1199 · Trusted Relationship
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I. Basic Information for CVE-2026-107720

Vulnerability Information
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Vulnerability Title
fast-jwt: createVerifier accepts unsigned JWTs when key is '' or null and algorithms is explicitly set
Source: CVE Program / CVE List V5
Vulnerability Description
fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.3.1, fast-jwt createVerifier accepts an unsigned JWT when key is an empty string or null and algorithms is a non-empty allowlist. Falsy synchronous keys bypass prepareKeyOrSecret, allowedAlgorithms remains active, hasKey is false, and the empty signature avoids the verifySignature gate. An attacker can therefore submit a token containing arbitrary claims without possessing a signing key, resulting in authentication or authorization bypass. Claim validators still run, and non-empty keys, an empty key without algorithms, and the async key resolver path do not have this behavior. This issue is fixed in version 6.3.1.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
输入验证不恰当
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
nearform fast-jwt < 6.3.1 -

II. Public POCs for CVE-2026-107720

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III. Intelligence Information for CVE-2026-107720

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Patches & Fixes for CVE-2026-107720 (2)

Vendor Advisories for CVE-2026-107720 (1)

Other References for CVE-2026-107720 (1)

Same Patch Batch · nearform · 2026-10-08 · 6 CVEs total

CVE-2026-107722 9.8 CRITICAL fast-jwt: Incomplete patch of CVE-2026-34950: Non-whitespace key-prefix re-enables RSA→HS2
CVE-2026-107723 8.1 HIGH fast-jwt : Silent claim-validator bypass when JWT payload is a JSON array
CVE-2026-107724 7.4 HIGH fast-jwt treats raw public JWK JSON as an HMAC secret, enabling HS256 token forgery
CVE-2026-107721 5.9 MEDIUM fast-jwt clockTolerance: Infinity silently bypasses both exp and nbf validation (and persi
CVE-2026-107719 4.2 MEDIUM fast-jwt: Verifier cache accepts expired JWTs without iat.

IV. Related Vulnerabilities

Same product: fast-jwt
Same vendor: nearform
Same weakness: CWE-20

V. Comments for CVE-2026-107720

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