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CVE-2026-6556— @fastify/express vulnerable to middleware bypass via non-string mount paths in prefixed plugins

Quick assessment

Affected
@fastify/express @fastify/express
Exploitation
Public or AI PoC available; prioritize validation
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Fastify是Fastify团队开源的一个 Web 框架。 Fastify 4.0.6及之前版本存在授权问题漏洞，该漏洞源于在带前缀的插件作用域中，非字符串挂载路径（路径数组和正则表达式）无法得到前缀重写，导致路径作用域中间件被绕过，可能影响认证、授权、限流或审计功能。

CVSS 9.1 · Critical EPSS 0.52% · P44

Possible ATT&CK Techniques 1 AI

T1190 · Exploit Public-Facing Application

Affected Version Matrix 2

VendorProduct Version RangeStatus
@fastify/express @fastify/express < 4.0.7 affected
4.0.7 unaffected
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I. Basic Information for CVE-2026-6556

Vulnerability Information
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Vulnerability Title
@fastify/express vulnerable to middleware bypass via non-string mount paths in prefixed plugins
Source: CVE Program / CVE List V5
Vulnerability Description
@fastify/express versions 4.0.6 and earlier only rewrite the plugin prefix for middleware mount paths when the path argument is a string. Non-string mount paths (arrays of paths and regular expressions) are left unprefixed inside prefixed plugin scopes, so middleware registered with those forms does not match the actual prefixed request path. Applications that use path-scoped middleware for authentication, authorization, rate limiting, or auditing on routes inside a prefixed scope can be bypassed by sending a request to the prefixed route, because Fastify still matches the route but the middleware is skipped. Patches: upgrade to @fastify/express 4.0.7. Workarounds: use string mount paths instead of arrays or regular expressions in prefixed plugins, or register one use call per path.
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
授权机制不恰当
Source: CVE Program / CVE List V5
Vulnerability Title
Fastify 授权问题漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Fastify是Fastify团队开源的一个 Web 框架。 Fastify 4.0.6及之前版本存在授权问题漏洞，该漏洞源于在带前缀的插件作用域中，非字符串挂载路径（路径数组和正则表达式）无法得到前缀重写，导致路径作用域中间件被绕过，可能影响认证、授权、限流或审计功能。
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)

Affected Products

Vendor Product Affected Versions CPE Subscribe
@fastify/express @fastify/express 0 ~ 4.0.7 -

II. Public POCs for CVE-2026-6556

# POC Description Source Link Shenlong Link
AI-Generated POC Premium
Qwen3.6-35B-A3B · 7834 chars
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Vulnerability reproduction recording (real sandbox build + trigger, exclusive)
In-depth vulnerability mechanism
Trigger conditions & impact
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Exploit chain & mitigation
POC zip download
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III. Intelligence Information for CVE-2026-6556

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Vendor Advisories for CVE-2026-6556 (1)

IV. Related Vulnerabilities

Same product: @fastify/express
Same vendor: @fastify/express
Same weakness: CWE-285