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Fencer Vulns
medium 6.3

CVE-2026-43971 json

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

published 2026-08-18 2 mo ago modified 2026-10-22 17 d ago also EEF-CVE-2026-43971 , EUVD-2026-60573

cvss
6.3
version 4.0, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.5%
EPSS 0.5% chance of exploitation in 30 days, percentile 44%
public exploits
0
none known

Summary

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.

Patch / fix

  • hex cowlib 2.20.0
  • ninenines cowlib 2.20.0
  • erlang-cowlib Debian 14 2.20.0-1

debian advisories DEBIAN-CVE-2026-43971

  • rabbitmq-server4.3 hummingbird 1 4.3.5-1.1.hum1

red hat advisories RHSA-2026:66210

  • rabbitmq-server Azure Linux 3.0 3.13.7-9

azure linux advisories AZL-96873

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-43971 updated cvss 4.3

cowlib: cowlib: Link header smuggling allows out-of-band connections to attacker-controlled origins

  • Red Hat Hardened Images rabbitmq-server4.2 4.2.9-0.2.hum1
    affected: workaround, no fix planned “Will not fix”
  • Red Hat OpenStack Platform 17.1 rabbitmq-server Red Hat OpenStack Platform 18.0 rabbitmq-server
    affected: workaround, no fix available “Fix deferred”
  • Red Hat Hardened Images rabbitmq-server4.3 4.3.5-1.1.hum1
    fixed: vendor fix RHSA-2026:66210
  • Red Hat OpenStack Platform 16.2 rabbitmq-server
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
2 mo ago

Exploit prediction

0.52% chance of exploitation in the next 30 days, higher than 44% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 4.0 6.3 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
CNA 4.0 6.3 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/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

Affected packages

ecosystem package affected fixed in
hex cowlib
>=2.9.0, <2.20.0
2.20.0

Affected products

  • ninenines cowlib

Classification

Weakness
  • CWE-116 Improper Encoding or Escaping of Output

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.