{"id":"CVE-2026-43966","aliases":["EEF-CVE-2026-43966","GHSA-w4f7-4cxr-rv3c"],"url":"https://o3.security/vulnerability/CVE-2026-43966","summary":"HTTP Response Splitting via Non-VCHAR Bytes in cow_http_struct_hd:escape_string/2","details":"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.\n\ncow_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.\n\nThis issue affects cowlib from 2.9.0.","published":"2026-06-08T16:34:33.364Z","modified":"2026-08-19T03:48:17.217003105Z","cvss":null,"epss":{"score":0.0025,"percentile":0.16561,"asOf":"2026-09-17"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Hex","name":"cowboy","fixedVersion":"2.16.0"},{"ecosystem":"Hex","name":"gun","fixedVersion":"2.16.0"}],"fix":{"url":"https://github.com/ninenines/cowboy/commit/f77cb9b5e730e300fffb551db1ba5d1c4ed878ef","label":"ninenines/cowboy@f77cb9b"},"references":[{"type":"WEB","url":"https://github.com"},{"type":"WEB","url":"https://osv.dev/vulnerability/EEF-CVE-2026-43966"},{"type":"WEB","url":"https://repo.hex.pm"},{"type":"ADVISORY","url":"https://cna.erlef.org/cves/CVE-2026-43966.html"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/43xxx/CVE-2026-43966.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-43966"},{"type":"FIX","url":"https://github.com/ninenines/cowboy/commit/f77cb9b5e730e300fffb551db1ba5d1c4ed878ef"},{"type":"FIX","url":"https://github.com/ninenines/gun/commit/4f35609eb37109b106a863fc9ba83d7ee64e3e42"},{"type":"PACKAGE","url":"https://github.com/ninenines/cowlib"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-19T03:48:17.217003105Z"}}