---
id: CVE-2026-113431
title: >-
  Unbounded decoded HTTP/2 header-list size in cowlib cow_http2_machine enables
  HPACK amplification memory exhaustion
summary: >-
  Allocation of Resources Without Limits or Throttling vulnerability in
  ninenines cowlib allows an unauthenticated remote peer to exhaust the memory
  of an HTTP/2 endpoint via HPACK-indexed header fields.


  cow_http2_machine:headers_decode/4…
severity: high
cvss: 8.7
cvssVector: 'CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N'
cvssSource: cna
cwe:
  - CWE-770
vendor: ninenines
product: cowlib
affected:
  - cowlib >= 2.7.0 < *
  - >-
    ninenines/cowlib >= b461b119e78e4e09bb28b186b09da7ed4a86a0dd <
    a3b15e29b3067644bfd580246c87f7af7a90b3b8
published: '2026-11-06'
updated: '2026-11-06'
sourceUpdated: '2026-11-06T10:28:15.837Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-113431'
references:
  - url: 'https://cna.erlef.org/cves/CVE-2026-113431.html'
    label: EEF CNA record for CVE-2026-113431
  - url: 'https://osv.dev/vulnerability/EEF-CVE-2026-113431'
    label: OSV record EEF-CVE-2026-113431
  - url: >-
      https://github.com/ninenines/cowlib/commit/b461b119e78e4e09bb28b186b09da7ed4a86a0dd
    label: Introducing commit b461b11 in ninenines/cowlib
  - url: >-
      https://github.com/ninenines/cowlib/commit/a3b15e29b3067644bfd580246c87f7af7a90b3b8
    label: Fix commit a3b15e2 in ninenines/cowlib
tags:
  - cve.org
ingestedAt: '2026-11-06T10:49:35.342Z'
---

## Overview

Allocation of Resources Without Limits or Throttling vulnerability in ninenines cowlib allows an unauthenticated remote peer to exhaust the memory of an HTTP/2 endpoint via HPACK-indexed header fields.

cow_http2_machine:headers_decode/4 in src/cow_http2_machine.erl decodes every HEADERS block with cow_hpack:decode/3. It bounds the compressed block size and the number of fields, but not the size of the decoded header list that RFC 9113 section 6.5.2 places under SETTINGS_MAX_HEADER_LIST_SIZE. A peer stores one header value of up to the dynamic table size (4096 bytes by default) and then references it with one-byte indexed fields, so about 100 bytes on the wire decode to a header list of about 390 KB. A consumer that copies the decoded values, as Cowboy does when cowboy_http2:headers_to_map/2 joins repeated cookie fields into one binary, allocates that amount for every stream. The total across streams and connections is not bounded.

This issue affects cowlib: from 2.7.0 onward.

## Affected

- `cowlib >= 2.7.0 < *`
- `ninenines/cowlib >= b461b119e78e4e09bb28b186b09da7ed4a86a0dd < a3b15e29b3067644bfd580246c87f7af7a90b3b8`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

### Workarounds

Cowboy servers that do not need HTTP/2 can set protocols => [http] in the protocol options. The listener then answers the HTTP/2 preface with 501 Not Implemented and the decoder is never reached.

Lowering max_headers in the HTTP/2 protocol options shrinks the per-stream allocation in proportion, to about max_headers times the 4096-byte table entry. This reduces the amplification but does not remove it.