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Vulnerabilities

Vulnerability · CVE-2026-43966

HTTP Response Splitting via Non-VCHAR Bytes in cow_http_struct_hd:escape_string/2

Published Jun 8, 2026 · catalog updated Oct 14, 2026 · HTTP Response Splitting via

Check if you run it

No confirmed exploitation is on record for CVE-2026-43966. EPSS is 0.45%. Act only if a listed version of HTTP Response Splitting via is installed.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (HTTP Response Splitting via).
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
Medium6.3
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
0.45%
Remediation
Advisory link
Affected products
Hex cowlib · cpe ninenines/cowlib

## 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. 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 (or a higher-level wrapper such as ) from attacker-controlled input can have 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. ## Workarounds Validate all values passed into structured-fields header builders (directly via 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 ( ) or LF ( ) bytes. Applications using cowboy 2.16.0 or later are protected on the server side by the option (defaults to ), 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 request option (defaults to ), which raises an exception when an outgoing request header value contains CR or LF. ## Configurations The application must pass attacker-controlled data as a string value into (or a wrapper that delegates to it). Applications that construct structured-fields header values exclusively from trusted, application-controlled values are not affected.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • Hex cowlib2.10.0, 2.10.1, 2.11.0
  • Hex cowlibfrom 2.9.0
  • cpe ninenines/cowlibversion unspecified

How to fix CVE-2026-43966

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2026-43966. EPSS is 0.45%. Act only if a listed version of HTTP Response Splitting via is installed.

Which versions are affected?

Compare what you have installed with 2.10.0, 2.10.1, 2.11.0, from 2.9.0. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-113. EPSS 0.45%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

Timeline

Detection, updates, and resolution for this record.

  1. Jun 8, 2026 · NIST NVD

    Detected

    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

  2. Oct 14, 2026 · NIST NVD

    Updated

    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

Sources

  • CVE.orgVulnerability recordCVE-2026-43966

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2026-43966

    30-day exploitation probability 0.5%

    first.org
  • GitHub Advisory DatabasePackage advisoryGHSA-w4f7-4cxr-rv3c

    GitHub Advisory Database record mirrored by OSV.

    github.com
  • NIST NVDVulnerability recordCVE-2026-43966

    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

    nvd.nist.gov
  • OpenSSF OSV / OSV ScannerPackage advisoryEEF-CVE-2026-43966

    HTTP Response Splitting via Non-VCHAR Bytes in cow_http_struct_hd:escape_string/2

    osv.dev

References

Freshness

Last verified in this catalog Oct 18, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.