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CVE-2026-84428— fastify vulnerable to header validation bypass via incomplete schema case normalization

Quick assessment

Affected
fastify fastify
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.

Fastify 在 5.12.2 版本之前，通过将路由 header 模式（schema）中的名称转换为小写，来实现 HTTP 头部名称的“不区分大小写”特性。但该转换并不完整：它仅对属性键（properties keys）和根级别的 数组进行了小写化处理，却未对 JSON Schema Draft 7 中 关键字内部的 （触发条件）和 （依赖项）名称进行小写化。 由于 Node.js 以全小写形式存储请求头部名称，像“当存在特权模式头部时，必须同时存在认证头部”这类使用标准大小写写法的依赖关系将无法匹配，相应的存

CVSS 7.5 · High

Possible ATT&CK Techniques 1 AI

T1190 · Exploit Public-Facing Application

Affected Version Matrix 2

VendorProduct Version RangeStatus
fastify fastify < 5.12.2 affected
5.12.2 unaffected
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I. Basic Information for CVE-2026-84428

Vulnerability Information
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Vulnerability Title
fastify vulnerable to header validation bypass via incomplete schema case normalization
Source: CVE Program / CVE List V5
Vulnerability Description
fastify versions before 5.12.2 implement the case-insensitive nature of HTTP header names by lowercasing names in a route's header schema before compiling it, but the transformation is incomplete: it lowercases the properties keys and the root-level required array, and does not lowercase the trigger and dependent names inside the JSON Schema Draft 7 dependencies keyword. Because Node stores request header names in lowercase, a canonical-case dependency such as requiring an authentication header whenever a privileged-mode header is present never matches, and the presence assertion is silently skipped. An unauthenticated remote client can therefore send the header that activates a privileged branch while omitting the header the dependency was meant to require, bypassing the conditional check. Users should upgrade to fastify 5.12.2 or later.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
大小写敏感处理不恰当
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
fastify fastify 0 ~ 5.12.2 -

II. Public POCs for CVE-2026-84428

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III. Intelligence Information for CVE-2026-84428

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Vendor Advisories for CVE-2026-84428 (1)

Same Patch Batch · fastify · 2026-09-04 · 4 CVEs total

CVE-2026-84504 8.1 HIGH fastify vulnerable to request body replacement via an async validation result collision
CVE-2026-76169 7.5 HIGH fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-f
CVE-2026-84469 7.5 HIGH fastify vulnerable to request validation bypass via skipped boolean false schemas

IV. Related Vulnerabilities

Same product: fastify
Same vendor: fastify
Same weakness: CWE-178

V. Comments for CVE-2026-84428

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