Vulnerability in cowlib (CVE-2026-43966)
Summary
Vulnerability in cowlib (CVE-2026-43966). Risk of unauthorized operations or information disclosure. Mitigation: upgrade to 2.16.0 or later.
Original description
The description as published by the source.
Summary
Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting') vulnerability in ninenines cowlib allows HTTP response splitting via non-VCHAR bytes in structured-fields string values.
cow_http_struct_hd:escape_string/2 in cowlib only escapes \ and ", passing all other bytes through verbatim. This creates an encoder/decoder asymmetry: the matching parser accepts only printable ASCII (0x20–0x7E, excluding " and \), but the encoder emits any byte including CR and LF. An application that builds a structured HTTP header via cow_http_struct_hd:item/1 (or a higher-level wrapper such as cow_http_hd:wt_protocol/1) from attacker-controlled input can have \r\n injected into the serialized header value. Once on the wire, the injected CRLF terminates the current header and any following bytes are interpreted as a new header, enabling HTTP response splitting.
This issue affects cowlib: from 2.9.0 onward.
Workarounds
Validate all values passed into structured-fields header builders (directly via cow_http_struct_hd:item/1 or indirectly via higher-level wrappers) before calling the encoder. Reject any value that is not from a trusted, application-controlled source or that contains CR (\r) or LF (\n) bytes.
Applications using cowboy 2.16.0 or later are protected on the server side by the invalid_response_headers option (defaults to error_terminate), which rejects any outgoing response header value containing CR or LF…
Source: NIST National Vulnerability Database NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-43966 has been found in cowlib.
Risk of unauthorized operations or information disclosure. CVSS score: 5.3/10.
What to do: upgrade cowlib to 2.16.0 or later.
If unsure, ask your IT team or search "cowlib CVE-2026-43966" on the vendor's site.
CVE-2026-43966 (cowlib) — CWE-113 / CVSS v3 5.3
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Patched: 2.16.0 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
A vulnerability (CWE-113) exists in cowlib.
Check the vendor advisory for the affected versions of cowlib.
Severity: Medium (CVSS 5.3/10). Risk of unauthorized operations or information disclosure.
Update to 2.16.0.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'cowlib' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
-
1Identify exposure identify
grep -r 'cowlib' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
cowlibを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply patch patch
Upgrade cowlib to 2.16.0ステージング環境で 2.16.0 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
-
3Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
Fix status by distribution
Fixed packages published by each distribution for this CVE
- Debian DEBIAN-CVE-2026-43966 erlang-cowboy Fixed in: 2.16.1+dfsg-1
- Ubuntu UBUNTU-CVE-2026-43966 rabbitmq-server
References
- advisory osv.dev
- package hex.pm
- patch github.com
- web cna.erlef.org
- web github.com
- web github.com
- web nvd.nist.gov
- web github.com
- web github.com
- web github.com
- web github.com