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CVE-2026-75931— fast-uri vulnerable to host confusion via skipped IDN canonicalization on scheme-relative references

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 2.4.2至2.4.5之前版本、3.1.3至3.1.6之前版本和4.0.1至4.1.3之前版本存在处理逻辑错误漏洞，该漏洞源于主机规范化处理不完善，仅当输入携带显式scheme时才将主机规范化为ASCII形式，导致不同入口函数对同一输入返回不同主机，可能造成主机混淆和安全策略绕过。

CVSS 7.5 · High EPSS 0.40% · P32

Possible ATT&CK Techniques 1 AI

T1518.001 · Security Software Discovery

Affected Version Matrix 6

VendorProduct Version RangeStatus
fast-uri fast-uri 2.4.2< 2.4.5 affected
2.4.5 unaffected
3.1.3< 3.1.6 affected
3.1.6 unaffected
4.0.1< 4.1.3 affected
4.1.3 unaffected
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I. Basic Information for CVE-2026-75931

Vulnerability Information
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Vulnerability Title
fast-uri vulnerable to host confusion via skipped IDN canonicalization on scheme-relative references
Source: CVE Program / CVE List V5
Vulnerability Description
fast-uri is a URI parser for Node.js. It canonicalizes a host to its ASCII form only when the input carries an explicit scheme, so a scheme-relative reference such as a host preceded by two slashes is returned with its host verbatim and no error set. As a result fast-uri's own entry points disagree with each other: parse, resolve, normalize, and equal can yield different hosts for the same input depending only on whether a scheme is written out, and equal can return opposite verdicts for the same pair of hosts. An application that extracts a host with fast-uri to check it against a policy list and then resolves the same reference can make its decision on one host while the destination is another, enabling host confusion and policy bypass. The affected versions are 2.4.2 up to but not including 2.4.5, 3.1.3 up to but not including 3.1.6, and 4.0.1 up to but not including 4.1.3. The issue is fixed in 2.4.5, 3.1.6, and 4.1.3, which canonicalize the host consistently across the resolve path. 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
解释冲突
Source: CVE Program / CVE List V5
Vulnerability Title
Fastify fast-uri 处理逻辑错误漏洞
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 2.4.2至2.4.5之前版本、3.1.3至3.1.6之前版本和4.0.1至4.1.3之前版本存在处理逻辑错误漏洞，该漏洞源于主机规范化处理不完善，仅当输入携带显式scheme时才将主机规范化为ASCII形式，导致不同入口函数对同一输入返回不同主机，可能造成主机混淆和安全策略绕过。
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 2.4.2 ~ 2.4.5 -

II. Public POCs for CVE-2026-75931

# 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.
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claude_code · 9807 chars
GitHub 复现环境（可 clone 直接跑）
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III. Intelligence Information for CVE-2026-75931

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Vendor Advisories for CVE-2026-75931 (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-2026-75975 7.5 HIGH fast-uri vulnerable to server-side request forgery via malformed IPv6 normalization
CVE-2026-76172 7.5 HIGH fast-uri vulnerable to host confusion via percent-encoded scheme normalization

IV. Related Vulnerabilities

Same product: fast-uri
Same vendor: fast-uri
Same weakness: CWE-436

V. Comments for CVE-2026-75931

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