CVE-2026-16396: Privilege escalation in WebExtensions

Routine

HIGH 8.8UPGRADEUPGRADE

CVE-2026-16396 is a high-severity flaw (CVSS 8.8) affecting firefox, thunderbird. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Privilege escalation in WebExtensions. This vulnerability was fixed in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13.

Why this priority

High severity

A fixed release or distro patch exists.

Risk facts

CVSS score	8.8 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.2% (top 86%)
CISA exploited list	No
Weakness	CWE-269
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: < 140.13.0 · < 153.0.0

Fixed in: 140.13.0, 153.0.0

Safe to downgrade to: no unaffected older release

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.

mozilla thunderbird UPGRADE

Affected: < 140.13.0 · >= 141.0 and < 153.0

Fixed in: 140.13.0, 153.0

Safe to downgrade to: a release before 141.0

1. Check installed version

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

2. UPGRADE to 140.13.0 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade thunderbird

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 141.0 (Debian / Ubuntu)

apt-cache madison thunderbird sudo apt install thunderbird=<VERSION_BEFORE_141.0> sudo apt-mark hold thunderbird

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

dnf --showduplicates list thunderbird sudo dnf downgrade thunderbird-<VERSION_BEFORE_141.0> sudo dnf versionlock add thunderbird

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-2026-16396?

firefox: upgrade to 140.13.0, 153.0.0; thunderbird: upgrade to 140.13.0, 153.0. Exact commands for each OS are on this page.

Is CVE-2026-16396 being exploited?

CVE-2026-16396 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-2026-16396 affect?

CVE-2026-16396 affects firefox, thunderbird. Severity is high (CVSS 8.8). Privilege escalation in WebExtensions. This vulnerability was fixed in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13.

How urgent is CVE-2026-16396?

Priority 34/100. High severity

Software pages

Mozilla Firefox vulnerabilities · Mozilla Thunderbird vulnerabilities

Related CVEs

CVE-2026-16389 CRITICAL 9.8 UPGRADE UPGRADE Incorrect boundary conditions, integer overflow in the Libraries component in NSS
CVE-2026-16353 CRITICAL 9.8 UPGRADE UPGRADE Invalid pointer in the DOM: Bindings (WebIDL) component
CVE-2026-16360 CRITICAL 9.8 UPGRADE UPGRADE Memory safety bugs present in Thunderbird ESR 140.12 and Thunderbird 152
CVE-2026-16363 CRITICAL 9.8 UPGRADE UPGRADE JIT miscompilation in the JavaScript: WebAssembly component
CVE-2026-16369 CRITICAL 9.8 UPGRADE UPGRADE Integer overflow in the JavaScript: WebAssembly component

References

← Back to latest CVEs