# CVE-2026-113431: Unbounded decoded HTTP/2 header-list size in cowlib cow_http2_machine enables HPACK amplification memory exhaustion

Status: published. Published: 2026-11-06. Updated: 2026-11-06.

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.

## Remediation

- workaround: 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. (source: CNA: EEF)
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: EEF) — https://github.com/ninenines/cowlib/commit/a3b15e29b3067644bfd580246c87f7af7a90b3b8

## Affected products and versions

- ninenines / cowlib (cowlib): ≥ 2.7.0; < * (affected, semver). Source: CNA: EEF
- ninenines / cowlib (ninenines/cowlib): ≥ b461b119e78e4e09bb28b186b09da7ed4a86a0dd; < a3b15e29b3067644bfd580246c87f7af7a90b3b8 (affected, git). Source: CNA: EEF

## Severity and prioritization

- CVSS 4.0: 8.7 (high); 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; source: CNA: EEF

- CVSS 4.0: 8.7 (high); 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/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; source: NVD: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-770

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://cna.erlef.org/cves/CVE-2026-113431.html (CNA: EEF; related, third-party-advisory)

- https://osv.dev/vulnerability/EEF-CVE-2026-113431 (CNA: EEF; related)

- https://github.com/ninenines/cowlib/commit/b461b119e78e4e09bb28b186b09da7ed4a86a0dd (CNA: EEF; related)

- https://github.com/ninenines/cowlib/commit/a3b15e29b3067644bfd580246c87f7af7a90b3b8 (CNA: EEF; patch)

- https://cna.erlef.org/cves/CVE-2026-113431.html (6b3ad84c-e1a6-4bf7-a703-f496b71e49db)

- https://github.com/ninenines/cowlib/commit/a3b15e29b3067644bfd580246c87f7af7a90b3b8 (6b3ad84c-e1a6-4bf7-a703-f496b71e49db)

- https://github.com/ninenines/cowlib/commit/b461b119e78e4e09bb28b186b09da7ed4a86a0dd (6b3ad84c-e1a6-4bf7-a703-f496b71e49db)

- https://osv.dev/vulnerability/EEF-CVE-2026-113431 (6b3ad84c-e1a6-4bf7-a703-f496b71e49db)

## Provenance

- CNA: EEF: https://www.cve.org/CVERecord?id=CVE-2026-113431; retrieved 2026-11-06; source updated 2026-11-06

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-113431; retrieved 2026-11-06; source updated 2026-11-06

Revision: 4f818a725dbe1a636e6ee9ee1500a72bddd1391cec59abc42b59b25e025b7862

Canonical: https://y2.dev/cve/CVE-2026-113431

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