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CVE-2026-34950— fast-jwt has an incomplete fix for CVE-2023-48223: JWT Algorithm Confusion via Whitespace-Prefixed RSA Public Key

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是Nearform开源的一个 JSON Web Token 实现。 fast-jwt 6.1.0及之前版本存在加密问题漏洞，该漏洞源于publicKeyPemMatcher正则表达式的^锚定符可被密钥字符串中的前导空格绕过，可能导致JWT算法混淆攻击。

CVSS 9.1 · Critical EPSS 0.24% · P15

Possible ATT&CK Techniques 1 AI

T1556.006 · Multi-Factor Authentication
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I. Basic Information for CVE-2026-34950

Vulnerability Information
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Vulnerability Title
fast-jwt has an incomplete fix for CVE-2023-48223: JWT Algorithm Confusion via Whitespace-Prefixed RSA Public Key
Source: CVE Program / CVE List V5
Vulnerability Description
fast-jwt provides fast JSON Web Token (JWT) implementation. In 6.1.0 and earlier, the publicKeyPemMatcher regex in fast-jwt/src/crypto.js uses a ^ anchor that is defeated by any leading whitespace in the key string, re-enabling the exact same JWT algorithm confusion attack that CVE-2023-48223 patched.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
使用已被攻破或存在风险的密码学算法
Source: CVE Program / CVE List V5
Vulnerability Title
fast-jwt 加密问题漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
fast-jwt是Nearform开源的一个 JSON Web Token 实现。 fast-jwt 6.1.0及之前版本存在加密问题漏洞，该漏洞源于publicKeyPemMatcher正则表达式的^锚定符可被密钥字符串中的前导空格绕过，可能导致JWT算法混淆攻击。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

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Affected Products

Vendor Product Affected Versions CPE Subscribe
nearform fast-jwt <= 6.1.0 -

II. Public POCs for CVE-2026-34950

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III. Intelligence Information for CVE-2026-34950

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Vendor Advisories for CVE-2026-34950 (1)

Same Patch Batch · nearform · 2026-04-06 · 3 CVEs total

CVE-2026-35039 9.1 CRITICAL fast-jwt Affected by Cache Confusion via cacheKeyBuilder Collisions Can Return Claims From
CVE-2026-35042 7.5 HIGH fast-jwt accepts unknown `crit` header extensions (RFC 7515 §4.1.11 MUST violation)

IV. Related Vulnerabilities

Same product: fast-jwt
Same vendor: nearform
Same weakness: CWE-327

V. Comments for CVE-2026-34950

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