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Vulnerabilities

Security incident · CVE-2026-43971

Improper Encoding or Escaping of Output vulnerability in ninenines cowlib allows Link header directive smuggling via unescaped special characters in cow_link:link/1.

Published Aug 18, 2026 · catalog updated Oct 22, 2026 · Improper Encoding or Escaping

Last checked Oct 26, 2026. A connected source was read within the last 7 days.

Check if you run it

No confirmed exploitation is on record for CVE-2026-43971. EPSS is 0.52%. Act only if a listed version of Improper Encoding or Escaping is installed.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (Improper Encoding or Escaping).
  • No version range is stored. A customer cannot check an installed version against this record.
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
Medium6.3
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
0.52%
Remediation
Not established
Affected products
See sources for product scope

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.

No affected version ranges are stored yet. Use the vendor advisory before you decide this record does not apply.

How to fix CVE-2026-43971

  • No stored fix

    No vendor fix text is stored. Use the linked advisory before you change production.

  • Version check

    No version range is stored, so an installed version cannot be checked on this page.

  • Advisory

    Open the cve_org record and follow its fix.

  • Official advisories

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2026-43971. EPSS is 0.52%. Act only if a listed version of Improper Encoding or Escaping is installed.

Which versions are affected?

Affected version ranges are not on this record. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-116. EPSS 0.52%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

Timeline

Detection, updates, and resolution for this record.

  1. Aug 18, 2026 · NIST NVD

    Detected

    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,

  2. Oct 22, 2026 · NIST NVD

    Updated

    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,

Sources

  • CVE.orgVulnerability recordCVE-2026-43971

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2026-43971

    30-day exploitation probability 0.52%

    first.org
  • NIST NVDVulnerability recordCVE-2026-43971

    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 seriali

    nvd.nist.gov

References

Freshness

Last verified in this catalog Oct 26, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.

  • CVE.orgOct 23, 2026
  • FIRST EPSSOct 23, 2026
  • NIST NVDOct 23, 2026