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CVE-2026-43966— HTTP Response Splitting via Non-VCHAR Bytes in cow_http_struct_hd:escape_string/2

Quick assessment

Affected
ninenines cowlib
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Cowlib是Nine Nines开源的一个Web协议消息解析与构建库。 Cowlib 2.9.0版本存在注入漏洞，该漏洞源于CRLF序列中和不当，可能导致HTTP响应拆分，允许攻击者通过结构化字段字符串值中的非VCHAR字节注入CRLF。

CVSS 6.3 · Medium EPSS 0.45% · P38

Affected Version Matrix 2

VendorProduct Version RangeStatus
ninenines cowlib 2.9.0< * affected
a8b793db3d6ffe91d62f81baf41b1dab4cd78fb6< 4401b858f2229abb3ea115c60924f6c77e5f8aa3 affected
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I. Basic Information for CVE-2026-43966

Vulnerability Information
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Vulnerability Title
HTTP Response Splitting via Non-VCHAR Bytes in cow_http_struct_hd:escape_string/2
Source: CVE Program / CVE List V5
Vulnerability Description
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 onward.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
HTTP头部中CRLF序列转义处理不恰当(HTTP响应分割)
Source: CVE Program / CVE List V5
Vulnerability Title
Cowlib 注入漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Cowlib是Nine Nines开源的一个Web协议消息解析与构建库。 Cowlib 2.9.0版本存在注入漏洞，该漏洞源于CRLF序列中和不当，可能导致HTTP响应拆分，允许攻击者通过结构化字段字符串值中的非VCHAR字节注入CRLF。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
ninenines cowlib 2.9.0 ~ * cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*
ninenines cowlib a8b793db3d6ffe91d62f81baf41b1dab4cd78fb6 ~ 4401b858f2229abb3ea115c60924f6c77e5f8aa3 cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*

II. Public POCs for CVE-2026-43966

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III. Intelligence Information for CVE-2026-43966

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Patches & Fixes for CVE-2026-43966 (3)

Vendor Advisories for CVE-2026-43966 (2)

Same Patch Batch · ninenines · 2026-06-08 · 4 CVEs total

CVE-2026-43972 gun HTTP/2 PUSH_PROMISE authority not validated against connection origin allows cross-ori
CVE-2026-43973 gun HTTP/1.1 response buffer has no size limit allowing server-controlled memory exhaustio
CVE-2026-43974 gun HTTP/1.1 client accepts unsolicited 101 Switching Protocols response allowing server-d

IV. Related Vulnerabilities

Same product: cowlib
Same vendor: ninenines
Same weakness: CWE-113

V. Comments for CVE-2026-43966

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