Vulnerability advisory

CVE-2026-43966

HTTP Response Splitting via Non-VCHAR Bytes in cow_http_struct_hd:escape_string/2

Published Jun 8, 2026Updated Oct 28, 2026OSV record ↗

Also known as EEF-CVE-2026-43966GHSA-w4f7-4cxr-rv3c

Severity

CVSS 4.0CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N

Affected versions

Repository

Affected commits	Fixed by commit
GIT github.com/ninenines/cowboy ↗
All commits before f77cb9b5e730e300fffb551db1ba5d1c4ed878ef	`f77cb9b5e730e300fffb551db1ba5d1c4ed878ef`
GIT github.com/ninenines/cowlib ↗
b34155f725245848dca3a505398e262b1d50f25b to before aca0ad953417b29bab2c41eeb4c37c98606c848b	`aca0ad953417b29bab2c41eeb4c37c98606c848b`
Not specified	`4401b858f2229abb3ea115c60924f6c77e5f8aa3`
Not specified	`a8b793db3d6ffe91d62f81baf41b1dab4cd78fb6`
GIT github.com/ninenines/gun ↗
All commits before 4f35609eb37109b106a863fc9ba83d7ee64e3e42	`4f35609eb37109b106a863fc9ba83d7ee64e3e42`
Also listed as affected (64) `2.15.0` `2.14.2` `2.14.1` `2.14.0` `2.13.0` `2.12.0` `2.11.0` `2.10.0` `2.9.0` `2.8.0` `2.7.0` `2.6.3` `2.6.2` `2.6.1` `2.6.0` `2.5.0` `2.4.0` `2.3.0` `2.2.2` `2.2.1` `2.2.0` `2.1.0` `2.0.0` `2.0.0-rc.4` `2.0.0-rc.3` `2.0.0-rc.2` `2.0.0-rc.1` `2.0.0-pre.10` `2.0.0-pre.9` `2.0.0-pre.8` `2.0.0-pre.7` `2.0.0-pre.6` `2.0.0-pre.5` `2.0.0-pre.4` `2.0.0-pre.3` `2.0.0-pre.2` `2.0.0-pre.1` `1.0.0` `0.10.0` `0.9.0` `0.8.6` `0.8.5` `0.8.4` `0.8.3` `0.8.2` `0.8.1` `0.8.0` `0.6.1` `0.6.0` `0.4.0` `0.2.0` `2.12.1` `2.10.1` `2.9.1` `2.0.1` `1.3.0` `1.2.0` `1.1.0` `1.0.0-rc.1` `1.0.0-pre.5` `1.0.0-pre.4` `1.0.0-pre.3` `1.0.0-pre.2` `1.0.0-pre.1`

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.

CVE-2026-43966

Affected versions

Repository

Description

References