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CVE-2026-43971— Link Header Directive Smuggling via Unescaped target/rel/Attribute Keys in cow_link:link/1

Quick assessment

Affected
ninenines cowlib
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.

Nine Nines Cowlib是Nine Nines组织的一款高并发Web服务器中间件。 Nine Nines Cowlib 2.9.0及之后版本存在输出处理不当漏洞，该漏洞源于输出编码或转义不当，cow_link:do_link/1函数在序列化Link头时未对目标URI、rel值和属性键进行转义或令牌语法验证，可能导致Link头指令走私，迫使受害者浏览器向攻击者控制的源发起带外连接。

CVSS 6.3 · Medium EPSS 0.52% · P43

Affected Version Matrix 2

VendorProduct Version RangeStatus
ninenines cowlib 2.9.0< 2.20.0 affected
485d58dfa91b91d98135dc95e5615f421715dae5< 89da27ee4c241f5d649ba7d9b7f2188918af6cea affected
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I. Basic Information for CVE-2026-43971

Vulnerability Information
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Vulnerability Title
Link Header Directive Smuggling via Unescaped target/rel/Attribute Keys in cow_link:link/1
Source: CVE Program / CVE List V5
Vulnerability Description
Improper Encoding or Escaping of Output vulnerability in ninenines cowlib allows Link header directive smuggling via unescaped special characters in cow_link:link/1. cow_link:do_link/1 in cowlib interpolates the target URI, rel value, and attribute keys directly into the serialized Link: header value without escaping or token-grammar validation. A > byte in target prematurely closes the URI slot, allowing an attacker to append additional link entries with attacker-chosen rel directives. A " or \ in rel escapes the quoted string and opens new parameters. Any byte — including whitespace, =, and " — in an attribute key is emitted verbatim. Because browsers act on Link: directives such as rel="preconnect", rel="preload", and rel="prerender", an attacker who can influence these fields in an application that round-trips parsed Link headers through cow_link:link/1 can force victim browsers to make out-of-band connections to attacker-controlled origins. This issue affects cowlib: from 2.9.0 before 2.20.0.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N
Source: CVE Program / CVE List V5
Vulnerability Type
对输出编码和转义不恰当
Source: CVE Program / CVE List V5
Vulnerability Title
Nine Nines Cowlib 输出处理不当漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Nine Nines Cowlib是Nine Nines组织的一款高并发Web服务器中间件。 Nine Nines Cowlib 2.9.0及之后版本存在输出处理不当漏洞，该漏洞源于输出编码或转义不当，cow_link:do_link/1函数在序列化Link头时未对目标URI、rel值和属性键进行转义或令牌语法验证，可能导致Link头指令走私，迫使受害者浏览器向攻击者控制的源发起带外连接。
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
ninenines cowlib 2.9.0 ~ 2.20.0 cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*
ninenines cowlib 485d58dfa91b91d98135dc95e5615f421715dae5 ~ 89da27ee4c241f5d649ba7d9b7f2188918af6cea cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*

II. Public POCs for CVE-2026-43971

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III. Intelligence Information for CVE-2026-43971

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Patches & Fixes for CVE-2026-43971 (2)

Vendor Advisories for CVE-2026-43971 (1)

News Coverage for CVE-2026-43971 (1)

IV. Related Vulnerabilities

Same product: cowlib
Same vendor: ninenines
Same weakness: CWE-116

V. Comments for CVE-2026-43971

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