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CVE-2026-75899— fast-uri vulnerable to server-side request forgery via repeated hostname percent-decoding

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 2.4.1至2.4.5之前版本、3.1.2至3.1.6之前版本和4.0.0至4.1.3之前版本存在服务端请求伪造漏洞，该漏洞源于解析主机名时对百分号转义进行重复解码，导致嵌套的百分号编码输入被多次解码，可能将请求路由至不同网络目的地，从而导致服务端请求伪造和主机策略绕过。

CVSS 7.5 · High EPSS 0.22% · P12

Possible ATT&CK Techniques 1 AI

T1190 · Exploit Public-Facing Application

Affected Version Matrix 6

VendorProduct Version RangeStatus
fast-uri fast-uri 2.4.1< 2.4.5 affected
2.4.5 unaffected
3.1.2< 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-2026-75899

Vulnerability Information
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Vulnerability Title
fast-uri vulnerable to server-side request forgery via repeated hostname percent-decoding
Source: CVE Program / CVE List V5
Vulnerability Description
fast-uri is a URI parser for Node.js. It decodes percent escapes in a hostname during parsing and then decodes the parsed hostname a second time during authority recomposition, so a single call to normalize or resolve can turn nested percent-encoded input into a different network destination such as a loopback hostname or address. For example, a doubly encoded host that spells out a loopback name decodes to that live host in one operation, which contradicts RFC 3986 section 2.4 that an implementation must not decode the same string more than once. An application that normalizes or resolves an untrusted HTTP-family URI before outbound routing, redirect validation, or a host-policy check can receive a destination different from the one the original encoded host represented, giving a server-side request forgery and host-policy bypass primitive. This is an incomplete-fix variant of CVE-2026-6322. The affected versions are 2.4.1 up to but not including 2.4.5, 3.1.2 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 normalize percent escapes once and preserve encoded percent signs. 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
对同一数据的双重编码
Source: CVE Program / CVE List V5
Vulnerability Title
Fastify fast-uri 服务端请求伪造漏洞
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 2.4.1至2.4.5之前版本、3.1.2至3.1.6之前版本和4.0.0至4.1.3之前版本存在服务端请求伪造漏洞，该漏洞源于解析主机名时对百分号转义进行重复解码，导致嵌套的百分号编码输入被多次解码，可能将请求路由至不同网络目的地，从而导致服务端请求伪造和主机策略绕过。
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 2.4.1 ~ 2.4.5 -

II. Public POCs for CVE-2026-75899

# 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: fast-uri double-decoded the host to loopback localhost; exfiltrated proof token PROOF_95d19be5086ed57a via forged request to 127.0.0.1:8081
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 · 9500 chars
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III. Intelligence Information for CVE-2026-75899

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Vendor Advisories for CVE-2026-75899 (1)

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

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
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-174

V. Comments for CVE-2026-75899

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