CVE-2026-43966
HTTP Response Splitting via Non-VCHAR Bytes in cow_http_struct_hd:escape_string/2
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 before 2.22.0.
Fixes and workarounds
Workaround
Validate all values passed into structured-fields header builders (directly via cow_http_struct_hd:item/1 or indirectly via higher-level wrappers) before calling the encoder. Reject any value that is not from a trusted, application-controlled source or that contains CR (\r) or LF (\n) bytes. Applications using cowboy 2.16.0 or later are protected on the server side by the invalid_response_headers option (defaults to error_terminate), which rejects any outgoing response header value containing CR or LF before it reaches the wire. Applications using gun 2.4.0 or later are protected on the client side by the invalid_request_headers request option (defaults to raise), which raises an exception when an outgoing request header value contains CR or LF.
Source: CNA: EEFFix
The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches.
Source: CNA: EEFAffected products and versions
| Product | Versions | Authority |
|---|---|---|
| cowlib ninenines cowlib
hex |
≥ 2.9.0; < 2.22.0 (affected, semver)
Other versions: unaffected |
CNA: EEF |
| cowlib ninenines ninenines/cowlib
github |
≥ a8b793db3d6ffe91d62f81baf41b1dab4cd78fb6; < 4401b858f2229abb3ea115c60924f6c77e5f8aa3 (affected, git)
Other versions: unaffected |
CNA: EEF |
Version ranges are source assertions. Check the vendor advisory and your installed configuration before determining exposure.
CVSS assessments
| Version / score | Vector | Authority |
|---|---|---|
| 4.0 / 6.3 medium |
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 | CNA: EEF |
Exploit evidence
No exploit evidence in collected sources. This is not proof of no exploitation.
References
- https://github.com/ninenines/cowboy/commit/f77cb9b5e730e300fffb551db1ba5d1c4ed878efCNA: EEF · mitigation
- https://cna.erlef.org/cves/CVE-2026-43966.htmlCNA: EEF · related, third-party-advisory
- https://osv.dev/vulnerability/EEF-CVE-2026-43966CNA: EEF · related
- https://github.com/ninenines/gun/commit/4f35609eb37109b106a863fc9ba83d7ee64e3e42CNA: EEF · mitigation
- https://github.com/ninenines/cowlib/commit/a8b793db3d6ffe91d62f81baf41b1dab4cd78fb6CNA: EEF · related
- https://github.com/ninenines/cowlib/commit/4401b858f2229abb3ea115c60924f6c77e5f8aa3CNA: EEF · patch
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