CVE-2026-43966 (GCVE-0-2026-43966)
Vulnerability from cvelistv5 – Published: 2026-06-08 16:34 – Updated: 2026-06-09 04:38
VLAI
Title
HTTP Response Splitting via Non-VCHAR Bytes in cow_http_struct_hd:escape_string/2
Summary
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.
Severity
SSVC
Exploitation: none
Automatable: no
Technical Impact: partial
CISA Coordinator (v2.0.3)
CWE
- CWE-113 - Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')
Assigner
References
4 references
| URL | Tags |
|---|---|
| https://cna.erlef.org/cves/CVE-2026-43966.html | relatedthird-party-advisory |
| https://osv.dev/vulnerability/EEF-CVE-2026-43966 | related |
| https://github.com/ninenines/cowboy/commit/f77cb9… | mitigation |
| https://github.com/ninenines/gun/commit/4f35609eb… | mitigation |
Impacted products
Credits
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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.\", \"supportingMedia\": [{\"type\": \"text/html\", \"value\": \"\u003cp\u003eImproper Neutralization of CRLF Sequences in HTTP Headers (\u0027HTTP Request/Response Splitting\u0027) vulnerability in ninenines cowlib allows HTTP response splitting via non-VCHAR bytes in structured-fields string values.\u003c/p\u003e\u003cp\u003e\u003ctt\u003ecow_http_struct_hd:escape_string/2\u003c/tt\u003e in cowlib only escapes \u003ctt\u003e\\\\\u003c/tt\u003e and \u003ctt\u003e\\\"\u003c/tt\u003e, passing all other bytes through verbatim. This creates an encoder/decoder asymmetry: the matching parser accepts only printable ASCII (0x20\\u20130x7E, excluding \u003ctt\u003e\\\"\u003c/tt\u003e and \u003ctt\u003e\\\\\u003c/tt\u003e), but the encoder emits any byte including CR and LF. An application that builds a structured HTTP header via \u003ctt\u003ecow_http_struct_hd:item/1\u003c/tt\u003e (or a higher-level wrapper such as \u003ctt\u003ecow_http_hd:wt_protocol/1\u003c/tt\u003e) from attacker-controlled input can have \u003ctt\u003e\\\\r\\\\n\u003c/tt\u003e 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.\u003c/p\u003e\u003cp\u003eThis issue affects cowlib from 2.9.0.\u003c/p\u003e\", \"base64\": false}]}], \"problemTypes\": [{\"descriptions\": [{\"lang\": \"en\", \"type\": \"CWE\", \"cweId\": \"CWE-113\", \"description\": \"CWE-113 Improper Neutralization of CRLF Sequences in HTTP Headers (\u0027HTTP Request/Response Splitting\u0027)\"}]}], \"configurations\": [{\"lang\": \"en\", \"value\": \"The application must pass attacker-controlled data as a string value into cow_http_struct_hd:item/1 (or a wrapper that delegates to it). Applications that construct structured-fields header values exclusively from trusted, application-controlled values are not affected.\", \"supportingMedia\": [{\"type\": \"text/html\", \"value\": \"\u003cp\u003eThe application must pass attacker-controlled data as a string value into \u003ctt\u003ecow_http_struct_hd:item/1\u003c/tt\u003e (or a wrapper that delegates to it). Applications that construct structured-fields header values exclusively from trusted, application-controlled values are not affected.\u003c/p\u003e\", \"base64\": false}]}], \"cpeApplicability\": [{\"nodes\": [{\"negate\": false, \"cpeMatch\": [{\"criteria\": \"cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionStartIncluding\": \"2.9.0\"}], \"operator\": \"OR\"}]}], \"providerMetadata\": {\"orgId\": \"6b3ad84c-e1a6-4bf7-a703-f496b71e49db\", \"shortName\": \"EEF\", \"dateUpdated\": \"2026-06-08T16:38:17.405Z\"}}}",
"cveMetadata": "{\"cveId\": \"CVE-2026-43966\", \"state\": \"PUBLISHED\", \"dateUpdated\": \"2026-06-08T18:38:08.160Z\", \"dateReserved\": \"2026-05-04T18:23:25.573Z\", \"assignerOrgId\": \"6b3ad84c-e1a6-4bf7-a703-f496b71e49db\", \"datePublished\": \"2026-06-08T16:34:33.364Z\", \"assignerShortName\": \"EEF\"}",
"dataType": "CVE_RECORD",
"dataVersion": "5.2"
}
}
}
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Experimental. This forecast is provided for visualization only and may change without notice. Do not use it for operational decisions.
Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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