CVE-2026-43974: Unexpected Status Code or Return Value vulnerability in ninenines gun (gun_http module) allows a malicious HTT

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HIGH 7.5REMOTEUPGRADE

CVE-2026-43974 is a high-severity flaw (CVSS 7.5) affecting gun. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Unexpected Status Code or Return Value vulnerability in ninenines gun (gun_http module) allows a malicious HTTP server to force the client into raw protocol mode via an unsolicited 101 Switching Protocols response. In gun_http:handle_inform/8, when a 101 Switching Protocols response is received over HTTP/1.1, the function verifies only that the Upgrade header is syntactically valid and that the stream reference is a plain reference(). It does no

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.5% (top 59%)
CISA exploited list	No
Weakness	CWE-841
Published	2026-06-08
Last modified	2026-08-18

Publisher (CNA): EEF · Vendor: ninenines

Last updated 2026-08-18 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-06-08 15:16 UTCPublished CVE record published
2026-08-18 15:44 UTCUpdated Record last modified

ninenines gun UPGRADE

Affected: >= 2.0.0 and < 2.4.0

Fixed in: 2.4.0

Safe to downgrade to: a release before 2.0.0

1. Check installed version

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

2. UPGRADE to 2.4.0 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade gun

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 2.0.0 (Debian / Ubuntu)

apt-cache madison gun sudo apt install gun=<VERSION_BEFORE_2.0.0> sudo apt-mark hold gun

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

dnf --showduplicates list gun sudo dnf downgrade gun-<VERSION_BEFORE_2.0.0> sudo dnf versionlock add gun

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-2026-43974?

gun: upgrade to 2.4.0. Exact commands for each OS are on this page.

Is CVE-2026-43974 being exploited?

CVE-2026-43974 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-2026-43974 affect?

CVE-2026-43974 affects gun. Severity is high (CVSS 7.5). Unexpected Status Code or Return Value vulnerability in ninenines gun (gun_http module) allows a malicious HTTP server to force the client into raw protocol mode via an unsolicited 101 Switching Protocols response. In gun_http:handle_infor

How urgent is CVE-2026-43974?

Priority 42/100. High severity; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-43973 HIGH 7.5 UPGRADE Uncontrolled Resource Consumption vulnerability in ninenines gun (gun_http module) allows a malicious server t
CVE-2026-43972 HIGH 7.2 UPGRADE Origin Validation Error vulnerability in ninenines gun (gun_http2 module) allows cross-origin cookie injection

References

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