# CVE-2026-43966

## Summary

- **CVE ID:** CVE-2026-43966
- **Severity:** MEDIUM
- **CVSS Score:** 6.3 (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)
- **CWE:** CWE-113
- **Published:** Jun 8, 2026
- **Last Modified:** Jun 9, 2026

## 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.

## Affected Products

- ninenines — cowlib (2.9.0)
- ninenines — cowlib (a8b793db3d6ffe91d62f81baf41b1dab4cd78fb6)

## References

- [CNA](https://cna.erlef.org/cves/CVE-2026-43966.html)
- [CNA](https://osv.dev/vulnerability/EEF-CVE-2026-43966)
- [CNA](https://github.com/ninenines/cowboy/commit/f77cb9b5e730e300fffb551db1ba5d1c4ed878ef)
- [CNA](https://github.com/ninenines/gun/commit/4f35609eb37109b106a863fc9ba83d7ee64e3e42)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.25%
- **EPSS Percentile:** 16.3

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-10._