CVE-2026-43966: Cowboy Cowboy And Gun Affected By Vulnerability

MODERATE
Advisory Identifier: GHSA-w4f7-4cxr-rv3c
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CWE-113
Published
Advisory Summary

CVE-2026-43966 is a MODERATE cowboy and gun affected by vulnerability (CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')) affecting Cowboy libraries in the Hex ecosystem. The issue involves cowboy and gun affected by an http request/response splitting vulnerability. Remediation is available by upgrading affected packages to version 2.16.0.

CVE-2026-43966 at a Glance

CVE Identifier CVE-2026-43966
GHSA Identifier GHSA-w4f7-4cxr-rv3c
Severity MODERATE
CVSS v3.1 Base Score N/A
CWE Classification CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')
Affected Product Cowboy
Package Ecosystem Hex
Published Date
Last Updated

How to Fix CVE-2026-43966

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

cowboy fixed: 2.16.0 
Upgrade cowboy to 2.16.0
gun fixed: 2.16.0 
Upgrade gun to 2.16.0
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 cowboy

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-43966, inspect your project dependencies for Cowboy:

inspect dependencies for cowboy

Analyze the terminal output to locate any version of Cowboy in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 2.16.0), 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-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting'), where the product receives data from an http agent/component (e.g., web server, proxy, browser, etc.), but it does not neutralize or incorrectly neutralizes cr and lf characters before the data is included in outgoing http headers. In Cowboy, 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 in HTTP Headers ('HTTP Request/Response Splitting') vulnerability in ninenines cowlib allows HTTP response splitting via non-VCHAR bytes in structured-fields string values. cow_http_struct_hd:escape_string/2 in cowlib only escapes \ and ", passing all other bytes through verbatim. This creates an encoder/decoder asymmetry: the matching parser accepts only printable ASCII (0x20–0x7E, excluding " and \), but the encoder emits any byte including CR and LF. An application that builds a structured HTTP header via cow_http_struct_hd:item/1 (or a higher-level wrapper such as cow_http_hd:wt_protocol/1) from attacker-controlled input can have \r\n injected into the serialized header value. Once on the wire, the injected CRLF terminates the current header and any following bytes are interpreted as a new header, enabling HTTP response splitting. This issue affects cowlib from 2.9.0.

CVE-2026-43966 Affected Cowboy Versions

2 component(s)
Fixed Versions:
2.16.0
Product / Package Ecosystem Affected Versions Fixed Version
cowboy
 Hex
< 2.16.0
2.16.0
gun
 Hex
< 2.16.0
2.16.0

References

NVD — CVE-2026-43966 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-43966
Cowboy Source Repository REPOSITORY
https://github.com/ninenines/cowlib/pull/163#issuecomment-4952645232
Cowboy Source Repository REPOSITORY
https://github.com/ninenines/cowlib/pull/166#issuecomment-5067554701
Security Fix Commit FIX COMMIT
https://github.com/ninenines/cowboy/commit/f77cb9b5e730e300fffb551db1ba5d1c4ed878ef
Security Fix Commit FIX COMMIT
https://github.com/ninenines/gun/commit/4f35609eb37109b106a863fc9ba83d7ee64e3e42
cna.erlef.org/cves/cve-2026-43966... WEB
https://cna.erlef.org/cves/CVE-2026-43966.html
Cowboy Source Repository REPOSITORY
https://github.com/ninenines/cowlib
osv.dev/vulnerability/eef-c... WEB
https://osv.dev/vulnerability/EEF-CVE-2026-43966

Frequently Asked Questions

What is CVE-2026-43966?
CVE-2026-43966 is a security vulnerability in Cowboy involving cowboy and gun affected by an HTTP Request/Response Splitting vulnerability under CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting').
Which Cowboy versions are affected?
Affected versions include releases prior to 2.16.0 across affected packages: cowboy, gun.
What version fixes CVE-2026-43966?
Upgrading to 2.16.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is cowboy affected?
Yes, cowboy is affected in releases prior to 2.16.0. Upgrade your dependency to 2.16.0 or later.
Is gun affected?
Yes, gun is affected in releases prior to 2.16.0. Upgrade your dependency to 2.16.0 or later.
What CWE is associated with CVE-2026-43966?
CVE-2026-43966 is classified under CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting') (The product receives data from an HTTP agent/component (e.g., web server, proxy, browser, etc.), but it does not neutralize or incorrectly neutralizes CR and LF characters before the data is included in outgoing HTTP headers.).
How can I check Hex dependencies for CVE-2026-43966?
Run the dependency tree command: 'inspect dependencies for cowboy'. 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-w4f7-4cxr-rv3c",
  "modified" : "2026-07-29T15:48:39Z",
  "published" : "2026-06-08T18:31:51Z",
  "aliases" : [ "CVE-2026-43966" ],
  "summary" : "cowboy and gun affected by an HTTP Request/Response Splitting vulnerability",
  "details" : "Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting') vulnerability in ninenines cowlib allows HTTP response splitting via non-VCHAR bytes in structured-fields string values.\n\ncow_http_struct_hd:escape_string/2 in cowlib only escapes \\ and \", passing all other bytes through verbatim. This creates an encoder/decoder asymmetry: the matching parser accepts only printable ASCII (0x20–0x7E, excluding \" and \\), but the encoder emits any byte including CR and LF. An application that builds a structured HTTP header via cow_http_struct_hd:item/1 (or a higher-level wrapper such as cow_http_hd:wt_protocol/1) from attacker-controlled input can have \\r\\n injected into the serialized header value. Once on the wire, the injected CRLF terminates the current header and any following bytes are interpreted as a new header, enabling HTTP response splitting.\n\nThis issue affects cowlib from 2.9.0.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Hex",
      "name" : "cowboy"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.16.0"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Hex",
      "name" : "gun"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
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      }, {
        "fixed" : "2.16.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "< 2.4.0"
    }
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-43966"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ninenines/cowlib/pull/163#issuecomment-4952645232"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ninenines/cowlib/pull/166#issuecomment-5067554701"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ninenines/cowboy/commit/f77cb9b5e730e300fffb551db1ba5d1c4ed878ef"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ninenines/gun/commit/4f35609eb37109b106a863fc9ba83d7ee64e3e42"
  }, {
    "type" : "WEB",
    "url" : "https://cna.erlef.org/cves/CVE-2026-43966.html"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/ninenines/cowlib"
  }, {
    "type" : "WEB",
    "url" : "https://osv.dev/vulnerability/EEF-CVE-2026-43966"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-113" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-29T15:48:39Z",
    "nvd_published_at" : "2026-06-08T17:16:43Z"
  }
}