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CVE-2026-16221— fast-uri vulnerable to host confusion via literal backslash authority delimiter

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.

fast-uri fast-uri是fast-uri的Web中间件。 fast-uri存在处理逻辑错误漏洞，该漏洞源于未将反斜杠字符（U+005C）视为权威分隔符，导致URL解析器与Node原生WHATWG URL解析器提取不同主机，可能导致应用被导向非预期目标，包括云元数据端点、回环地址或内部主机。以下版本受到影响：2.3.1至2.4.2版本、3.0.0至3.1.3版本和4.0.0至4.1.0版本。

CVSS 7.5 · High EPSS 0.44% · P38

Affected Version Matrix 6

VendorProduct Version RangeStatus
fast-uri fast-uri 2.3.1< 2.4.3 affected
2.4.3 unaffected
3.0.0< 3.1.4 affected
3.1.4 unaffected
4.0.0< 4.1.1 affected
4.1.1 unaffected
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I. Basic Information for CVE-2026-16221

Vulnerability Information
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Vulnerability Title
fast-uri vulnerable to host confusion via literal backslash authority delimiter
Source: CVE Program / CVE List V5
Vulnerability Description
Impact: fast-uri versions from 2.3.1 through 4.1.0 (including the 3.x line up to 3.1.3 and the 2.x line up to 2.4.2) do not treat a literal backslash character (U+005C) as an authority delimiter. Node's native WHATWG URL parser, used by fetch, undici, and Node's http and https clients, normalizes the backslash to a forward slash for special schemes such as http, https, ws, wss, ftp, and file. As a result, the two parsers extract different hosts from the same input string. Applications that use fast-uri to enforce host-based policy such as allowlists, denylists, loopback or SSRF filtering, redirect validation, or outbound proxy routing before passing the same URL into Node's URL or fetch consumers can be steered to an unintended destination, including cloud metadata endpoints, loopback, or internal hosts. Patches: upgrade to fast-uri 4.1.1, 3.1.4, or 2.4.3. Workarounds: none.
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
fast-uri 处理逻辑错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
fast-uri fast-uri是fast-uri的Web中间件。 fast-uri存在处理逻辑错误漏洞，该漏洞源于未将反斜杠字符（U+005C）视为权威分隔符，导致URL解析器与Node原生WHATWG URL解析器提取不同主机，可能导致应用被导向非预期目标，包括云元数据端点、回环地址或内部主机。以下版本受到影响：2.3.1至2.4.2版本、3.0.0至3.1.3版本和4.0.0至4.1.0版本。
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 ~ 2.4.3 -

II. Public POCs for CVE-2026-16221

# 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: SSRF bypass — fast-uri saw host=safe.example.com (allowlisted) but Node fetch hit 127.0.0.1:9999 and returned PROOF_70380efb786b3ec5
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 · 7375 chars
GitHub 复现环境（可 clone 直接跑）
Pro+ exclusive includes:
Vulnerability reproduction recording (real sandbox build + trigger, exclusive)
In-depth vulnerability mechanism
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Full executable POC code
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POC zip download
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III. Intelligence Information for CVE-2026-16221

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Vendor Advisories for CVE-2026-16221 (1)

IV. Related Vulnerabilities

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