Unbounded decoded HTTP/2 header-list size in cowlib cow_http2_machine enables HPACK amplification memory exhaustion
- Exploited
- No known exploitation
- Fix
- Fix or patch referenced
- Who is exposed
- Default setups
Do first: Check every install: default setups are affected.
- Last modified
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 41/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Default setups
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
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.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
Read the original 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.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure22/25Default setups, reachable over the network without a login
- Reach10/20Affects Hex packages
- Severity8.7/10CVSS 8.7
- No patch0/5No signal
What would change the score
- Would score about 81 if added to CISA KEV
- Would score about 76 if CISA's SSVC rates exploitation active
What to do
Check every install: default setups are affected.
Apply the patch listed under Evidence.
Workaround: Cowboy servers that do not need HTTP/2 can set protocols => [http] in the protocol options.
Timeline
- CVE published
- First seen by cve.now
Affected versions
- cowlibHex
From 2.7.0
- ninenines/cowlib
Version range not reported
- cowlibHex
From 2.7.0
Evidence 4
- Patch
- Repository
- Link
- Link
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-113431
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: cve.now feed data
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: cna.erlef.org
Which versions are affected by CVE-2026-113431?
cowlib (Hex): from 2.7.0. cowlib (Hex): from 2.7.0.Source: cve.now feed data
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: cna.erlef.org
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: cna.erlef.org