PT-2026-47347 · Ninenines · Cowlib

Loïc Hoguin + 1 · Published 2026-06-08 · Updated 2026-06-08 · CVE-2026-43966

Mitre

CVSS v4.0

6.3

Medium

Base vector

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

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 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.

Fix

Weakness Enumeration

CWE-113

Related Identifiers

CVE-2026-43966

Affected Products

Cowlib

References · 5

• https://nvd.nist.gov/vuln/detail/CVE-2026-43966 · Security Note
• https://github.com/ninenines/gun/commit/4f35609eb37109b106a863fc9ba83d7ee64e3e42 · Note
• https://github.com/ninenines/cowboy/commit/f77cb9b5e730e300fffb551db1ba5d1c4ed878ef · Note
• https://cna.erlef.org/cves/CVE-2026-43966.html · Note
• https://osv.dev/vulnerability/EEF-CVE-2026-43966 · Note