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 onward.
INFO
Published Date :
Aug. 18, 2026, 9:17 a.m.
Last Modified :
Aug. 18, 2026, 9:17 a.m.
Remotely Exploit :
Yes !
Source :
6b3ad84c-e1a6-4bf7-a703-f496b71e49db
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| 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.
| URL | Resource |
|---|---|
| https://cna.erlef.org/cves/CVE-2026-43971.html | |
| https://osv.dev/vulnerability/EEF-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.
-
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