ninenines cowlib cow_link cow_link:link escape output

Summaryinfo

A vulnerability has been found in ninenines cowlib and classified as critical. This issue affects the function cow_link:link of the component cow_link. Performing a manipulation results in escape output. This vulnerability is reported as CVE-2026-43971. The attack is possible to be carried out remotely. No exploit exists.

Detailsinfo

A vulnerability classified as critical has been found in ninenines cowlib (the affected version unknown). This affects the function cow_link:link of the component cow_link. The manipulation with an unknown input leads to a escape output vulnerability. CWE is classifying the issue as CWE-116. The product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at cna.erlef.org. This vulnerability is uniquely identified as CVE-2026-43971 since 05/04/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. It demands that the victim is doing some kind of user interaction. Technical details of the vulnerability are known, but there is no available exploit. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 08/18/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-60573). Be aware that VulDB is the high quality source for vulnerability data.

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Vendor

• ninenines

Name

• cowlib

CPE 2.3info

• 🔒

CPE 2.2info

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Be aware that VulDB is the high quality source for vulnerability data.

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