CVE-2026-43969: cowlib Cookie Request Header Injection Vulnerability

LOW
Advisory Identifier: GHSA-g2wm-735q-3f56
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CVSS 3.2
CWE-93
Published
Advisory Summary

CVE-2026-43969 is a LOW (CVSS 3.2) cookie request header injection vulnerability (CWE-93: Improper Neutralization of CRLF Sequences ('CRLF Injection')) affecting cowlib libraries in the Hex ecosystem. The issue involves cookie request header injection via unvalidated encoder in cow_cookie:cookie/1. Remediation is available in the latest vendor maintenance releases.

CVE-2026-43969 at a Glance

CVE Identifier CVE-2026-43969
GHSA Identifier GHSA-g2wm-735q-3f56
Severity LOW
CVSS v3.1 Base Score 3.2
CWE Classification CWE-93: Improper Neutralization of CRLF Sequences ('CRLF Injection')
Affected Product cowlib
Package Ecosystem Hex
Published Date
Last Updated

Security Impact

CVSS: 3.2

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N).

Attack Vector Local (AV:L)
Attack Complexity High (AC:H)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Changed (S:C)
Confidentiality None (C:N)
Integrity Low (I:L)
Availability None (A:N)

How to Fix CVE-2026-43969

To remediate this vulnerability, upgrade all affected cowlib dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

cowlib fixed: Latest secure release 
Upgrade cowlib to Latest secure release
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for cowlib

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-43969, inspect your project dependencies for cowlib:

inspect dependencies for cowlib

Analyze the terminal output to locate any version of cowlib in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than the patched release), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-93: Improper Neutralization of CRLF Sequences ('CRLF Injection'), where the product uses crlf (carriage return line feeds) as a special element, e.g. to separate lines or records, but it does not neutralize or incorrectly neutralizes crlf sequences from inputs. In cowlib, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability in ninenines cowlib allows HTTP request splitting and cookie smuggling via unvalidated cookie name and value fields. cow_cookie:cookie/1 in cowlib builds a client-side Cookie: request header from a list of name-value pairs without validating either field. An attacker who controls the cookie names or values passed to this function can inject ;, ,, CR, LF, or TAB characters into the serialized header. This enables two classes of attack: cookie smuggling within a single header (e.g. injecting "; admin=1" to introduce a phantom cookie that the receiving server treats as authentic) and HTTP request header splitting (injecting CRLF to append arbitrary headers or smuggle a complete second request against a shared upstream proxy). The decoder side (parse_cookie_name/1, parse_cookie_value/1) and setcookie/3 already validate and reject these characters; the encoder alone is missing the check. This issue affects cowlib from 2.9.0.

CVE-2026-43969 Affected cowlib Versions

1 component(s)
Product / Package Ecosystem Affected Versions Fixed Version
cowlib
 Hex
≥ 2.9.0
≤ 2.16.1

References

NVD — CVE-2026-43969 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-43969
Security Fix Commit FIX COMMIT
https://github.com/erlef/cowlib/commit/177953dd51540da11090666c1f007214127a1144
cna.erlef.org/cves/cve-2026-43969... WEB
https://cna.erlef.org/cves/CVE-2026-43969.html
cowlib Source Repository REPOSITORY
https://github.com/ninenines/cowlib
osv.dev/vulnerability/eef-c... WEB
https://osv.dev/vulnerability/EEF-CVE-2026-43969

Frequently Asked Questions

What is CVE-2026-43969?
CVE-2026-43969 is a security vulnerability in cowlib involving Cookie Request Header Injection via Unvalidated Encoder in cow_cookie:cookie/1 under CWE-93: Improper Neutralization of CRLF Sequences ('CRLF Injection').
Which cowlib versions are affected?
Affected versions include releases prior to current release patches across affected packages: cowlib.
What version fixes CVE-2026-43969?
Upgrading to current release patches (or the latest patched release for your respective branch) resolves the vulnerability.
Is cowlib affected?
Yes, cowlib is affected in releases prior to Latest secure release. Upgrade your dependency to Latest secure release or later.
What is the CVSS score of CVE-2026-43969?
CVE-2026-43969 has a CVSS v3.1 base score of 3.2 (LOW) with vector string CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N.
What CWE is associated with CVE-2026-43969?
CVE-2026-43969 is classified under CWE-93: Improper Neutralization of CRLF Sequences ('CRLF Injection') (The product uses CRLF (carriage return line feeds) as a special element, e.g. to separate lines or records, but it does not neutralize or incorrectly neutralizes CRLF sequences from inputs.).
How can I check Hex dependencies for CVE-2026-43969?
Run the dependency tree command: 'inspect dependencies for cowlib'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-g2wm-735q-3f56",
  "modified" : "2026-06-08T20:17:02Z",
  "published" : "2026-05-11T21:31:34Z",
  "aliases" : [ "CVE-2026-43969" ],
  "summary" : "cowlib: Cookie Request Header Injection via Unvalidated Encoder in cow_cookie:cookie/1",
  "details" : "Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability in ninenines cowlib allows HTTP request splitting and cookie smuggling via unvalidated cookie name and value fields.\n\ncow_cookie:cookie/1 in cowlib builds a client-side Cookie: request header from a list of name-value pairs without validating either field. An attacker who controls the cookie names or values passed to this function can inject ;, ,, CR, LF, or TAB characters into the serialized header. This enables two classes of attack: cookie smuggling within a single header (e.g. injecting \"; admin=1\" to introduce a phantom cookie that the receiving server treats as authentic) and HTTP request header splitting (injecting CRLF to append arbitrary headers or smuggle a complete second request against a shared upstream proxy). The decoder side (parse_cookie_name/1, parse_cookie_value/1) and setcookie/3 already validate and reject these characters; the encoder alone is missing the check.\n\nThis issue affects cowlib from 2.9.0.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Hex",
      "name" : "cowlib"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.9.0"
      }, {
        "last_affected" : "2.16.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-43969"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/erlef/cowlib/commit/177953dd51540da11090666c1f007214127a1144"
  }, {
    "type" : "WEB",
    "url" : "https://cna.erlef.org/cves/CVE-2026-43969.html"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/ninenines/cowlib"
  }, {
    "type" : "WEB",
    "url" : "https://osv.dev/vulnerability/EEF-CVE-2026-43969"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-93" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-18T17:48:35Z",
    "nvd_published_at" : "2026-05-11T19:16:25Z"
  }
}