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

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.

CVSS

6.3 MEDIUM (v4.0, CNA)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

Weakness (CWE)

CWE-116

Known exploited

Not in CISA's KEV catalog

EPSS (exploitation in the next 30 days)

Not loaded for this CVE

Assigned by

EEF

NVD analysis

Awaiting Analysis

Record last updated

Sep 16, 2026

First seen by InfoSec Signals

Oct 9, 2026

The CVE record on cve.org · NVD's entry

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