CVE-2026-43971

Summary

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.

Affected Software

VendorProductVersion RangeStatus
nineninescowlib2.9.0 < *affected
nineninescowlib485d58dfa91b91d98135dc95e5615f421715dae5 < 89da27ee4c241f5d649ba7d9b7f2188918af6ceaaffected

Weaknesses

  • CWE-116: CWE-116 Improper Encoding or Escaping of Output

Workarounds

Sanitize values before passing them to cow_link:link/1: reject or strip any target value containing >, any rel value containing " or , and any attribute key containing characters outside the HTTP token grammar. Alternatively, ensure all Link entry fields are derived exclusively from trusted, application-controlled values rather than untrusted input.

ADP Enrichment

CISA ADP Vulnrichment

  • SSVC:
  • Exploitation: none
    • Automatable: no
    • Technical Impact: partial

References