CVE-2026-16405: Information disclosure in the Networking: WebSockets component

Routine

HIGH 7.5REMOTEUPGRADE

CVE-2026-16405 is a high-severity flaw (CVSS 7.5) affecting firefox. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Information disclosure in the Networking: WebSockets component. This vulnerability was fixed in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13.

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	0.3% (top 84%)
CISA exploited list	No
Weakness	CWE-200
Published	2026-07-21
Last modified	2026-10-24

Publisher (CNA): Mozilla Corporation · Vendor: mozilla

Last updated 2026-10-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-21 13:17 UTCPublished CVE record published
2026-10-24 20:10 UTCUpdated Record last modified

mozilla firefox UPGRADE

Affected: < 153.0.0 · >= 140.1.0 and < 140.13.0

Fixed in: 140.13.0, 153.0.0

Safe to downgrade to: a release before 140.1.0

1. Check installed version

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

2. UPGRADE to 140.13.0 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade firefox

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 140.1.0 (Debian / Ubuntu)

apt-cache madison firefox sudo apt install firefox=<VERSION_BEFORE_140.1.0> sudo apt-mark hold firefox

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

dnf --showduplicates list firefox sudo dnf downgrade firefox-<VERSION_BEFORE_140.1.0> sudo dnf versionlock add firefox

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-2026-16405?

firefox: upgrade to 140.13.0, 153.0.0. Exact commands for each OS are on this page.

Is CVE-2026-16405 being exploited?

CVE-2026-16405 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-2026-16405 affect?

CVE-2026-16405 affects firefox. Severity is high (CVSS 7.5). Information disclosure in the Networking: WebSockets component. This vulnerability was fixed in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13.

How urgent is CVE-2026-16405?

Priority 35/100. High severity; Remotely exploitable, no login, no user action

Software pages

Mozilla Firefox vulnerabilities · Mozilla Thunderbird vulnerabilities

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References

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