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Fencer Intel
medium 6.3

CVE-2026-43971 json

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

published 2mo ago modified 18d 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

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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
2mo ago

Exploit prediction

0.52% chance of exploitation in the next 30 days, higher than 44% of all scored CVEs (FIRST EPSS, 2026-11-09).

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 Intel 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.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.