CVE-2026-43966Network attack vector

HTTP response splitting via unescaped CRLF bytes

Published Jun 8, 2026 · Updated Jun 8, 2026

HTTP response splitting in Nine Nines Cowlib 2.9.0 allows remote attackers to inject headers through structured-field string values. The cow_http_struct_hd:escape_string/2 routine escapes only backslashes and quotes, so CR and LF bytes pass unchanged into serialized headers. Exploitation requires an application to pass attacker-controlled strings into the structured-field builder; injected bytes can terminate a header and create a new one.

CVSS severity6.3
Medium
EPSS probability0.25%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP response splitting in Nine Nines Cowlib 2.9.0 allows remote attackers to inject headers through structured-field string values. The cow_http_struct_hd:escape_string/2 routine escapes only backslashes and quotes, so CR and LF bytes pass unchanged into serialized headers. Exploitation requires an application to pass attacker-controlled strings into the structured-field builder; injected bytes can terminate a header and create a new one.

The record

CVE
CVE-2026-43966
Published
Jun 8, 2026
Updated
Jun 8, 2026
Vendor
Nine Nines
Product
Cowlib
Classifications
CWE-113, CAPEC-34, T1565.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 8, 2026CVE publishedPublication date reported by the CVE source.
  2. Jun 8, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=2.9.0
  2. Affected versionversion=a8b793db3d6ffe91d62f81baf41b1dab4cd78fb6

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Nine Nines · Cowlibversion=2.9.0; version=a8b793db3d6ffe91d62f81baf41b1dab4cd78fb6

Published CVSS scores

5.3NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
6.3Erlang Ecosystem FoundationCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-113Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')
CAPEC-34HTTP Response Splitting
T1565.002Transmitted Data Manipulation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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