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CVE-2026-16387: Site isolation issue in the Networking component

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CRITICAL 9.8REMOTEUPGRADEUPGRADE

CVE-2026-16387 is a critical-severity flaw (CVSS 9.8) affecting firefox, thunderbird. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Site isolation issue in the Networking component. This vulnerability was fixed in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13.

Why this priority

  • Critical severity
  • Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score9.8 (Critical)
Attack vectorNetwork (remote)
Attack complexityLow
Privileges neededNone
User interactionNone
EPSS exploit chance0.2% (top 90%)
CISA exploited listNo
WeaknessCWE-200, CWE-284, CWE-346
Published2026-07-21
Last modified2026-10-24
Publisher (CNA): Mozilla Corporation · Vendor: mozilla

Share: LinkedIn · X · Reddit

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

Am I affected?

Timeline

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-2026-16387?

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-2026-16387 being exploited?

CVE-2026-16387 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-2026-16387 affect?

CVE-2026-16387 affects firefox, thunderbird. Severity is critical (CVSS 9.8). Site isolation issue in the Networking component. This vulnerability was fixed in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13.

How urgent is CVE-2026-16387?

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

Software pages

Mozilla Firefox vulnerabilities · Mozilla Thunderbird vulnerabilities

Related CVEs

References

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