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CVE-2026-43971· NVD / CVE Program· CNA EEF

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

Noroxi assessment

Priority: Monitor (action score 25/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 0.52% (FIRST EPSS).
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
  • Fixed version: ninenines cowlib 2.20.0, 89da27ee4c241f5d649ba7d9b7f2188918af6cea; azl3 rabbitmq-server 3.13.7-9 on Azure Linux 3.0 3.13.7-9.

Low priority for now. Upgrade to ninenines cowlib 2.20.0, 89da27ee4c241f5d649ba7d9b7f2188918af6cea; azl3 rabbitmq-server 3.13.7-9 on Azure Linux 3.0 3.13.7-9 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

MediumCVSS 6.3 · v4.0—No exploit Fix available
Published
Aug 18, 2026
Updated
Oct 22, 2026
EPSS
0.5% · 43th percentile

Report tools

JSON

Action score

25

Monitor

Low priority for now.

CVSS
25 / 40 · 6.3 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.5%

CISA SSVC decision

Exploitation
none
Automatable
no
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · EEF
6.3
NVD has not scored this yet; the score shown is the CNA’s.

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

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2026-43971

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

—

Versions reported by the vendor

Affected version ranges reported by the assigning authority (EEF). Independent of NVD's CPE analysis and usually ahead of it.

  • ninenines cowlib

    • 2.9.0 and later · before 2.20.0affected · semver
    • 485d58dfa91b91d98135dc95e5615f421715dae5 and later · before 89da27ee4c241f5d649ba7d9b7f2188918af6ceaaffected · git

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
Debian:14erlang-cowlibbefore 2.20.0-12.20.0-1
Hexcowlibfrom 2.9.0 · before 2.20.02.20.0
Red Hat:hummingbird:1rabbitmq-server4.3before 0:4.3.5-1.1.hum10:4.3.5-1.1.hum1

Same product

ninenines: all records

Other highest-scoring records for the same primary product.

  • CVE-2026-59248Unbounded HPACK/QPACK prefixed-integer decoding in Cowlib causes memory-exhaustion DoS
    34Monitor
  • CVE-2026-7790Unbounded chunk-size hex digits in cowlib cause quadratic CPU and memory DoS
    34Monitor
  • CVE-2026-43970Decompression Bomb in cow_spdy:inflate/2 Allows Memory Exhaustion via Crafted SPDY Frame
    32Monitor

Published before and after this one in the same product

  • ← EarlierCVE-2026-43968May 11, 2026 · CR Injection in SSE Encoder Enables Event Splitting via cow_sse:event/1
    25Monitor
  • ← EarlierCVE-2026-43969May 11, 2026 · Cookie Request Header Injection via Unvalidated Encoder in cow_cookie:cookie/1
    8Monitor
  • ← EarlierCVE-2026-43966Jun 8, 2026 · HTTP Response Splitting via Non-VCHAR Bytes in cow_http_struct_hd:escape_string/2
    25Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
ninenines cowlib2.20.0Vendor (CNA)
ninenines cowlib89da27ee4c241f5d649ba7d9b7f2188918af6ceaVendor (CNA)
azl3 rabbitmq-server 3.13.7-9 on Azure Linux 3.03.13.7-9Microsoft (MSRC)
debian:erlang-cowlib2.20.0-1 · Debian:14Package registry (OSV)
Hex:cowlib2.20.0Package registry (OSV)
red hat:rabbitmq-server4.30:4.3.5-1.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
Integrity
Availability
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
X

Weakness class (CWE)

CWE-116 · Improper Encoding or Escaping of Output

CVSS vector

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

nvd-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

ATT&CK techniques

—

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (3 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records