# CVE-2026-43966: HTTP Response Splitting via Non-VCHAR Bytes in cow_http_struct_hd:escape_string/2

CVE-2026-43966 is fixed in erlang-cowboy on 1 platform by DEBIAN-CVE-2026-43966 (version 2.16.1+dfsg-1), released June 8, 2026. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2026-43966
- Priority: Routine
- CVSS: 5.3 (Low)
- EPSS: 0.0026
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jun 8, 2026
- Weakness: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-43966

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | erlang-cowboy Debian 14 | 2.16.1+dfsg-1 | [DEBIAN-CVE-2026-43966](https://patchblox.com/security/data/fix/linux/debian-cve-2026-43966.md) | Jun 8, 2026 |

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Data as of 2026-11-08T19:12:23Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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