CVE-2026-43966
Severity CVSS v4.0:
MEDIUM
Type:
CWE-113
Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Response Splitting')
Publication date:
08/06/2026
Last modified:
09/06/2026
Description
Improper Neutralization of CRLF Sequences in HTTP Headers (&#39;HTTP Request/Response Splitting&#39;) vulnerability in ninenines cowlib allows HTTP response splitting via non-VCHAR bytes in structured-fields string values.<br />
<br />
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.<br />
<br />
This issue affects cowlib from 2.9.0.
Impact
Base Score 4.0
6.30
Severity 4.0
MEDIUM