CVE-2026-43966

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.

CVSS v3 5.3 MEDIUM

5.3^/10

CVSS v3 : MEDIUM

Vector :

Exploitability : 3.9 / Impact : 1.4

Attack Vector NETWORK

Attack Complexity LOW

Privileges Required NONE

User Interaction NONE

Confidentiality Impact NONE

Integrity Impact LOW

Availability Impact NONE

Scope UNCHANGED

References

Link	Resource
https://cna.erlef.org/cves/CVE-2026-43966.html	Third Party Advisory
https://github.com/ninenines/cowboy/commit/f77cb9b5e730e300fffb551db1ba5d1c4ed878ef	Patch
https://github.com/ninenines/gun/commit/4f35609eb37109b106a863fc9ba83d7ee64e3e42	Patch
https://osv.dev/vulnerability/EEF-CVE-2026-43966	Third Party Advisory

Configurations

Configuration 1 (hide)

cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*

History

No history.

Information

Published : 2026-06-08 17:16

Updated : 2026-08-17 17:56

NVD link : CVE-2026-43966

Mitre link : CVE-2026-43966

CVE.ORG link : CVE-2026-43966

JSON object : View

Products Affected

ninenines

cowlib

CWE

CWE-113

Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')

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This creates an encoder/decoder asymmetry: the matching parser accepts only printable ASCII (0x20\u20130x7E, 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.\n\nThis issue affects cowlib from 2.9.0."}, {"lang": "es", "value": "La vulnerabilidad de Neutralizaci\u00f3n Incorrecta de Secuencias CRLF en Encabezados HTTP ('Divisi\u00f3n de Solicitud/Respuesta HTTP') en ninenines cowlib permite la divisi\u00f3n de respuesta HTTP a trav\u00e9s de bytes no VCHAR en valores de cadena de campos estructurados.\n\ncow_http_struct_hd:escape_string/2 en cowlib solo escapa \\ y \", pasando todos los dem\u00e1s bytes textualmente. Esto crea una asimetr\u00eda de codificador/decodificador: el analizador coincidente acepta solo ASCII imprimible (0x20-0x7E, excluyendo \" y \\), pero el codificador emite cualquier byte, incluidos CR y LF. Una aplicaci\u00f3n que construye un encabezado HTTP estructurado a trav\u00e9s de cow_http_struct_hd:item/1 (o un envoltorio de nivel superior como cow_http_hd:wt_protocol/1) desde una entrada controlada por el atacante puede tener \\r\\n inyectado en el valor del encabezado serializado. Una vez en la red, el CRLF inyectado termina el encabezado actual y cualquier byte siguiente se interpreta como un nuevo encabezado, lo que permite la divisi\u00f3n de respuesta HTTP.\n\nEste problema afecta a cowlib desde la versi\u00f3n 2.9.0."}], "lastModified": "2026-08-17T17:56:14.473", "configurations": [{"nodes": [{"negate": false, "cpeMatch": [{"criteria": "cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*", "vulnerable": true, "matchCriteriaId": "A583B546-6841-4B45-9E4D-67B1926640AB", "versionEndExcluding": "2.16.0", "versionStartIncluding": "2.9.0"}], "operator": "OR"}]}], "sourceIdentifier": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db"}