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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.

Cowlib 库中存在“输出编码或转义不当”漏洞，攻击者可通过未转义的特殊字符进行 Link 头指令注入（Link header directive smuggling）。 在 cowlib 中， 函数直接将目标 URI、 值以及属性键插值到序列化的 头字段值中，而未进行任何转义或令牌语法验证。 若 参数中包含 字符，将提前关闭 URI 槽位，使攻击者能够追加额外的链接条目，并自定义 指令。 若 参数中包含 或 字符，可逃脱引号字符串的限制，从而注入新的参数。 属性键中的任何字节（包括空格、 和 ）均会按原样输出。

CVSS 6.3 · Medium EPSS 0.37% · P30

Affected Version Matrix 2

VendorProduct Version RangeStatus
ninenines cowlib 2.9.0< * 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 onward.
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

Affected Products

Vendor Product Affected Versions CPE Subscribe
ninenines cowlib 2.9.0 ~ * 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 (1)

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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