CVE-2026-8466: Cowboy Denial of Service Vulnerability

HIGH
Advisory Identifier: GHSA-jfc2-q6qh-g5x8
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CWE-770
Published
Advisory Summary

CVE-2026-8466 is a HIGH denial of service vulnerability (CWE-770: Allocation of Resources Without Limits or Throttling) affecting Cowboy libraries in the Hex ecosystem. The issue involves unbounded buffer accumulation in multipart header parsing causes denial of service in cowboy. Remediation is available by upgrading affected packages to version 2.15.0.

CVE-2026-8466 at a Glance

CVE Identifier CVE-2026-8466
GHSA Identifier GHSA-jfc2-q6qh-g5x8
Severity HIGH
CVSS v3.1 Base Score N/A
CWE Classification CWE-770: Allocation of Resources Without Limits or Throttling
Affected Product Cowboy
Package Ecosystem Hex
Published Date
Last Updated

How to Fix CVE-2026-8466

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: 2.15.0 
Upgrade cowboy to 2.15.0
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-2026-8466, 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 2.15.0), 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-770: Allocation of Resources Without Limits or Throttling, where the product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated. 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

Allocation of Resources Without Limits or Throttling vulnerability in ninenines cowboy allows denial of service via unbounded buffer accumulation in multipart header parsing. cowboy_req:read_part/3 in src/cowboy_req.erl accumulates incoming request bytes into a Buffer binary with no upper-bound check. When cow_multipart:parse_headers/2 returns more or {more, Buffer2}, the function reads up to Length bytes (default 64 KB) from the request body and recurses with the enlarged buffer. There is no equivalent of the byte_size(Acc) > Length guard present in the sibling function read_part_body/4. An unauthenticated attacker can send a multipart/form-data request whose body never yields a complete header section — for example, a body that never contains the advertised boundary delimiter, or one whose header lines never contain \r\n\r\n — and force the server process to accumulate memory linearly with the bytes the protocol layer is willing to deliver. A handful of concurrent such uploads is sufficient to exhaust BEAM memory. This issue affects cowboy from 2.0.0 before 2.15.0.

CVE-2026-8466 Affected Cowboy Versions

1 component(s)
Fixed Versions:
2.15.0
Product / Package Ecosystem Affected Versions Fixed Version
cowboy
 Hex
≥ 2.0.0
< 2.15.0
2.15.0

References

NVD — CVE-2026-8466 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-8466
Security Fix Commit FIX COMMIT
https://github.com/ninenines/cowboy/commit/5c6a2061b41bb5771c4659fac7d5a822dca5bafb
cna.erlef.org/cves/cve-2026-8466.... WEB
https://cna.erlef.org/cves/CVE-2026-8466.html
Cowboy Source Repository REPOSITORY
https://github.com/ninenines/cowboy
osv.dev/vulnerability/eef-c... WEB
https://osv.dev/vulnerability/EEF-CVE-2026-8466

Frequently Asked Questions

What is CVE-2026-8466?
CVE-2026-8466 is a security vulnerability in Cowboy involving Unbounded buffer accumulation in multipart header parsing causes denial of service in cowboy under CWE-770: Allocation of Resources Without Limits or Throttling.
Which Cowboy versions are affected?
Affected versions include releases prior to 2.15.0 across affected packages: cowboy.
What version fixes CVE-2026-8466?
Upgrading to 2.15.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is cowboy affected?
Yes, cowboy is affected in releases prior to 2.15.0. Upgrade your dependency to 2.15.0 or later.
What CWE is associated with CVE-2026-8466?
CVE-2026-8466 is classified under CWE-770: Allocation of Resources Without Limits or Throttling (The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.).
How can I check Hex dependencies for CVE-2026-8466?
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-jfc2-q6qh-g5x8",
  "modified" : "2026-05-19T20:12:33Z",
  "published" : "2026-05-13T21:32:06Z",
  "aliases" : [ "CVE-2026-8466" ],
  "summary" : "Cowboy: Unbounded buffer accumulation in multipart header parsing causes denial of service in cowboy",
  "details" : "Allocation of Resources Without Limits or Throttling vulnerability in ninenines cowboy allows denial of service via unbounded buffer accumulation in multipart header parsing.\n\ncowboy_req:read_part/3 in src/cowboy_req.erl accumulates incoming request bytes into a Buffer binary with no upper-bound check. When cow_multipart:parse_headers/2 returns more or {more, Buffer2}, the function reads up to Length bytes (default 64 KB) from the request body and recurses with the enlarged buffer. There is no equivalent of the byte_size(Acc) > Length guard present in the sibling function read_part_body/4. An unauthenticated attacker can send a multipart/form-data request whose body never yields a complete header section — for example, a body that never contains the advertised boundary delimiter, or one whose header lines never contain \\r\\n\\r\\n — and force the server process to accumulate memory linearly with the bytes the protocol layer is willing to deliver. A handful of concurrent such uploads is sufficient to exhaust BEAM memory.\n\nThis issue affects cowboy from 2.0.0 before 2.15.0.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Hex",
      "name" : "cowboy"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.0.0"
      }, {
        "fixed" : "2.15.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-8466"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ninenines/cowboy/commit/5c6a2061b41bb5771c4659fac7d5a822dca5bafb"
  }, {
    "type" : "WEB",
    "url" : "https://cna.erlef.org/cves/CVE-2026-8466.html"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/ninenines/cowboy"
  }, {
    "type" : "WEB",
    "url" : "https://osv.dev/vulnerability/EEF-CVE-2026-8466"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-770" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-19T20:12:33Z",
    "nvd_published_at" : "2026-05-13T19:17:30Z"
  }
}