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

Official 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.

Affected packages & fixed versions

Affected products (as reported)

References

• EEF CNA record for CVE-2026-43971 related, third-party-advisory
• OSV record EEF-CVE-2026-43971 related
• Introducing commit 485d58d in ninenines/cowlib related
• Fix commit 89da27e in ninenines/cowlib patch