Bug 2517738 (CVE-2026-43971)

Summary: CVE-2026-43971 cowlib: cowlib: Link header smuggling allows out-of-band connections to attacker-controlled origins
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: eglynn, jjoyce, jpretori, jschluet, lhh, mburns, mgarciac
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in ninenines cowlib. This vulnerability, due to improper encoding or escaping of output, allows an attacker to perform Link header directive smuggling. By manipulating the target URI, 'rel' value, or attribute keys in applications that process Link headers through cowlib, a remote attacker can inject malicious directives. This can force victim browsers to establish out-of-band connections to attacker-controlled servers, potentially leading to information disclosure or bypassing security controls.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Description OSIDB Bzimport 2026-08-18 09:11:24 UTC 
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.