CVE-2026-43966: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting') vulnerability in

Routine

MEDIUM 5.3REMOTEUPGRADE

CVE-2026-43966 is a medium-severity flaw (CVSS 5.3) affecting cowlib. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 \), b

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.3% (top 83%)
CISA exploited list	No
Weakness	CWE-113
Published	2026-06-08
Last modified	2026-10-14

Publisher (CNA): EEF · Vendor: ninenines

Last updated 2026-10-14 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-06-08 17:16 UTCPublished CVE record published
2026-10-14 10:17 UTCUpdated Record last modified

ninenines cowlib UPGRADE

Affected: >= 2.9.0 and < 2.16.0

Fixed in: 2.16.0

Safe to downgrade to: a release before 2.9.0

1. Check installed version

apt-cache policy cowlib || rpm -q cowlib || cowlib --version

2. UPGRADE to 2.16.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade cowlib

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade cowlib

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 2.9.0 (Debian / Ubuntu)

apt-cache madison cowlib sudo apt install cowlib=<VERSION_BEFORE_2.9.0> sudo apt-mark hold cowlib

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list cowlib sudo dnf downgrade cowlib-<VERSION_BEFORE_2.9.0> sudo dnf versionlock add cowlib

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-43966?

cowlib: upgrade to 2.16.0. Exact commands for each OS are on this page.

Is CVE-2026-43966 being exploited?

CVE-2026-43966 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-43966 affect?

CVE-2026-43966 affects cowlib. Severity is medium (CVSS 5.3). 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:escap

How urgent is CVE-2026-43966?

Priority 28/100. Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-43969 LOW 3.2 UPGRADE Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability in ninenines cowlib allows HTTP req
CVE-2026-7790 HIGH 7.5 UPGRADE Uncontrolled Resource Consumption vulnerability in ninenines cowlib (cow_http_te module) allows Excessive Allo
CVE-2026-43968 MEDIUM 4.0 UPGRADE Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability in ninenines cowlib allows SSE even

References

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