CVE-2026-43971
Link Header Directive Smuggling via Unescaped target/rel/Attribute Keys in cow_link:link/1
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.
INFO
Published Date :
Aug. 18, 2026, 9:17 a.m.
Last Modified :
Oct. 22, 2026, 10:10 p.m.
Remotely Exploit :
Yes !
Source :
6b3ad84c-e1a6-4bf7-a703-f496b71e49db
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 4.0 | MEDIUM | 6b3ad84c-e1a6-4bf7-a703-f496b71e49db | ||||
| CVSS 4.0 | MEDIUM | 6b3ad84c-e1a6-4bf7-a703-f496b71e49db |
Solution
- Update cowlib to a patched version.
- Sanitize user-supplied URI and attribute data.
- Validate Link header directives before use.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-43971.
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-43971 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-43971
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-43971 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-43971 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
CVE Translated by [email protected]
Oct. 22, 2026
Action Type Old Value New Value Added Translation Title: cowlib de ninenines, Description: La vulnerabilidad de codificación o escape de salida incorrectos en ninenines cowlib permite el contrabando de directivas de encabezado Link a través de caracteres especiales sin escape en cow_link:link/1. cow_link:do_link/1 en cowlib interpola la URI de destino, el valor rel y las claves de atributo directamente en el valor del encabezado Link: serializado sin escape ni validación de gramática de tokens. Un byte > en el destino cierra prematuramente la ranura URI, permitiendo a un atacante añadir entradas de enlace adicionales con directivas rel elegidas por el atacante. Un ’ o \ en rel escapa la cadena entre comillas y abre nuevos parámetros. Cualquier byte -incluyendo espacios en blanco, =, y ’- en una clave de atributo se emite textualmente. Debido a que los navegadores actúan sobre directivas Link: como rel=’preconnect’, rel=’preload’ y rel=’prerender’, un atacante que puede influir en estos campos en una aplicación que procesa encabezados Link analizados a través de cow_link:link/1 puede forzar a los navegadores de las víctimas a realizar conexiones fuera de banda a orígenes controlados por el atacante. Este problema afecta a cowlib: a partir de la versión 2.9.0. -
CVE Modified by 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
Sep. 16, 2026
Action Type Old Value New Value Changed 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 onward. 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. Added Affected Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/43xxx/CVE-2026-43971.json">CVE-2026-43971</a> Added Reference https://cna.erlef.org/cves/CVE-2026-43971.html Added Reference https://github.com/ninenines/cowlib/commit/485d58dfa91b91d98135dc95e5615f421715dae5 Added Reference https://github.com/ninenines/cowlib/commit/89da27ee4c241f5d649ba7d9b7f2188918af6cea Added Reference https://osv.dev/vulnerability/EEF-CVE-2026-43971 Removed Reference https://cna.erlef.org/cves/CVE-2026-43971.html Removed Reference https://github.com/ninenines/cowlib/commit/89da27ee4c241f5d649ba7d9b7f2188918af6cea Removed Reference https://osv.dev/vulnerability/EEF-CVE-2026-43971 -
CVE Modified by 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
Aug. 18, 2026
Action Type Old Value New Value Added Reference https://github.com/ninenines/cowlib/commit/89da27ee4c241f5d649ba7d9b7f2188918af6cea Changed Affected [{'cpes': ['cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*'], 'repo': 'https://github.com/ninenines/cowlib', 'vendor': 'ninenines', 'modules': ['cow_link'], 'product': 'cowlib', 'versions': [{'status': 'affected', 'version': '2.9.0', 'lessThan': '*', 'versionType': 'semver'}], 'packageURL': 'pkg:hex/cowlib', 'packageName': 'cowlib', 'programFiles': ['src/cow_link.erl'], 'collectionURL': 'https://repo.hex.pm', 'defaultStatus': 'unaffected', 'programRoutines': [{'name': 'cow_link:link/1'}, {'name': 'cow_link:do_link/1'}, {'name': 'cow_link:escape/2'}]}, {'cpes': ['cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*'], 'repo': 'https://github.com/ninenines/cowlib', 'vendor': 'ninenines', 'modules': ['cow_link'], 'product': 'cowlib', 'versions': [{'status': 'affected', 'version': '485d58dfa91b91d98135dc95e5615f421715dae5', 'lessThan': '*', 'versionType': 'git'}], 'packageURL': 'pkg:github/ninenines/cowlib', 'packageName': 'ninenines/cowlib', 'programFiles': ['src/cow_link.erl'], 'collectionURL': 'https://github.com', 'defaultStatus': 'unaffected', 'programRoutines': [{'name': 'cow_link:link/1'}, {'name': 'cow_link:do_link/1'}, {'name': 'cow_link:escape/2'}]}] [{'cpes': ['cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*'], 'repo': 'https://github.com/ninenines/cowlib', 'vendor': 'ninenines', 'modules': ['cow_link'], 'product': 'cowlib', 'versions': [{'status': 'affected', 'version': '2.9.0', 'lessThan': '*', 'versionType': 'semver'}], 'packageURL': 'pkg:hex/cowlib', 'packageName': 'cowlib', 'programFiles': ['src/cow_link.erl'], 'collectionURL': 'https://repo.hex.pm', 'defaultStatus': 'unaffected', 'programRoutines': [{'name': 'cow_link:link/1'}, {'name': 'cow_link:do_link/1'}, {'name': 'cow_link:escape/2'}]}, {'cpes': ['cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*'], 'repo': 'https://github.com/ninenines/cowlib', 'vendor': 'ninenines', 'modules': ['cow_link'], 'product': 'cowlib', 'versions': [{'status': 'affected', 'version': '485d58dfa91b91d98135dc95e5615f421715dae5', 'lessThan': '89da27ee4c241f5d649ba7d9b7f2188918af6cea', 'versionType': 'git'}], 'packageURL': 'pkg:github/ninenines/cowlib', 'packageName': 'ninenines/cowlib', 'programFiles': ['src/cow_link.erl'], 'collectionURL': 'https://github.com', 'defaultStatus': 'unaffected', 'programRoutines': [{'name': 'cow_link:link/1'}, {'name': 'cow_link:do_link/1'}, {'name': 'cow_link:escape/2'}]}] -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Aug. 18, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-43971', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'none'}, {'automatable': 'no'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-08-18T14:11:55.645955Z'} -
New CVE Received by 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
Aug. 18, 2026
Action Type Old Value New Value Added Affected [{'cpes': ['cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*'], 'repo': 'https://github.com/ninenines/cowlib', 'vendor': 'ninenines', 'modules': ['cow_link'], 'product': 'cowlib', 'versions': [{'status': 'affected', 'version': '2.9.0', 'lessThan': '*', 'versionType': 'semver'}], 'packageURL': 'pkg:hex/cowlib', 'packageName': 'cowlib', 'programFiles': ['src/cow_link.erl'], 'collectionURL': 'https://repo.hex.pm', 'defaultStatus': 'unaffected', 'programRoutines': [{'name': 'cow_link:link/1'}, {'name': 'cow_link:do_link/1'}, {'name': 'cow_link:escape/2'}]}, {'cpes': ['cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*'], 'repo': 'https://github.com/ninenines/cowlib', 'vendor': 'ninenines', 'modules': ['cow_link'], 'product': 'cowlib', 'versions': [{'status': 'affected', 'version': '485d58dfa91b91d98135dc95e5615f421715dae5', 'lessThan': '*', 'versionType': 'git'}], 'packageURL': 'pkg:github/ninenines/cowlib', 'packageName': 'ninenines/cowlib', 'programFiles': ['src/cow_link.erl'], 'collectionURL': 'https://github.com', 'defaultStatus': 'unaffected', 'programRoutines': [{'name': 'cow_link:link/1'}, {'name': 'cow_link:do_link/1'}, {'name': 'cow_link:escape/2'}]}] Added 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 onward. Added CVSS V4.0 AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X Added CWE CWE-116 Added Reference https://cna.erlef.org/cves/CVE-2026-43971.html Added Reference https://osv.dev/vulnerability/EEF-CVE-2026-43971