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CVE-2026-76172— fast-uri vulnerable to host confusion via percent-encoded scheme normalization

Quick assessment

Affected
fast-uri fast-uri
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 fast-uri是Fastify团队的一个URI解析工具。 Fastify fast-uri 2.3.1至2.4.5之前版本、3.0.0至3.1.6之前版本和4.0.0至4.1.3之前版本存在输入验证错误漏洞，该漏洞源于对URI的scheme组件进行解码后未重新转义，可能导致站外重定向、服务端请求伪造或地址策略绕过。

CVSS 7.5 · High EPSS 0.23% · P13

Possible ATT&CK Techniques 1 AI

T1190 · Exploit Public-Facing Application

Affected Version Matrix 6

VendorProduct Version RangeStatus
fast-uri fast-uri 2.3.1< 2.4.5 affected
2.4.5 unaffected
3.0.0< 3.1.6 affected
3.1.6 unaffected
4.0.0< 4.1.3 affected
4.1.3 unaffected
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I. Basic Information for CVE-2026-76172

Vulnerability Information
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Vulnerability Title
fast-uri vulnerable to host confusion via percent-encoded scheme normalization
Source: CVE Program / CVE List V5
Vulnerability Description
fast-uri is a URI parser for Node.js. During parsing it runs a legacy decoding pass over the scheme component and never re-escapes the result, and serialization writes the scheme back out verbatim, unlike the host component which is re-escaped. As a result an input whose scheme carries percent-encoded slashes parses as a scheme with no authority, so the parsed host and error are both undefined, yet resolving or normalizing that same input emits a network-path reference whose authority is attacker-chosen and re-parses to that host. An application that allowlists on the parsed host, or treats a reference with no authority as safe to resolve against its base, gets the opposite of what it checked, giving an off-site redirect, server-side request forgery, or address-policy bypass. The legacy decoder also expands non-standard escape forms, widening the issue past upstream filters, and control characters in the scheme can reach the output as raw carriage return and line feed. The affected versions are 2.3.1 up to but not including 2.4.5, 3.0.0 up to but not including 3.1.6, and 4.0.0 up to but not including 4.1.3. The issue is fixed in 2.4.5, 3.1.6, and 4.1.3, which reject a scheme that is not valid after decoding. Users should upgrade to a patched version.
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
URL编码处理不恰当(Hex编码)
Source: CVE Program / CVE List V5
Vulnerability Title
Fastify fast-uri 输入验证错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Fastify fast-uri是Fastify团队的一个URI解析工具。 Fastify fast-uri 2.3.1至2.4.5之前版本、3.0.0至3.1.6之前版本和4.0.0至4.1.3之前版本存在输入验证错误漏洞，该漏洞源于对URI的scheme组件进行解码后未重新转义，可能导致站外重定向、服务端请求伪造或地址策略绕过。
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
fast-uri fast-uri 2.3.1 ~ 2.4.5 -

II. Public POCs for CVE-2026-76172

# POC Description Source Link Shenlong Link
AI-Generated POC Verified env Premium
Reproduced successfully in a real sandbox · Below is the actual recording of building the environment and exploiting the vulnerability.
Success marker: VULNERABLE: proof_38e5d2e652b08d41 - attacker host exfiltrated through the resolved reference (PROOF_38e5d2e652b08d41)
Reproduction recording is a Pro+ exclusive
Watch the full sandbox build + live exploit recording for this CVE. Limited-time ¥499/mo.
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claude_code · 10407 chars
GitHub 复现环境（可 clone 直接跑）
Pro+ exclusive includes:
Vulnerability reproduction recording (real sandbox build + trigger, exclusive)
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POC zip download
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III. Intelligence Information for CVE-2026-76172

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Vendor Advisories for CVE-2026-76172 (1)

Same Patch Batch · fast-uri · 2026-08-24 · 4 CVEs total

CVE-2026-75899 7.5 HIGH fast-uri vulnerable to server-side request forgery via repeated hostname percent-decoding
CVE-2026-75931 7.5 HIGH fast-uri vulnerable to host confusion via skipped IDN canonicalization on scheme-relative
CVE-2026-75975 7.5 HIGH fast-uri vulnerable to server-side request forgery via malformed IPv6 normalization

IV. Related Vulnerabilities

Same product: fast-uri
Same vendor: fast-uri
Same weakness: CWE-177

V. Comments for CVE-2026-76172

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