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CVE-2026-6754: Use-after-free in the JavaScript Engine component

39
Routine
HIGH 7.5REMOTEUPGRADEUPGRADE

CVE-2026-6754 is a high-severity flaw (CVSS 7.5) affecting firefox, thunderbird. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Use-after-free in the JavaScript Engine component. This vulnerability was fixed in Firefox 150, Firefox ESR 115.35, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10.

Why this priority

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

A fixed release or distro patch exists.

Risk facts

CVSS score7.5 (High)
Attack vectorNetwork (remote)
Attack complexityLow
Privileges neededNone
User interactionNone
EPSS exploit chance0.4% (top 67%)
CISA exploited listNo
WeaknessCWE-416, CWE-825
Published2026-04-21
Last modified2026-10-25
Publisher (CNA): Mozilla Corporation · Vendor: mozilla

Share: LinkedIn · X · Reddit

Last updated 2026-10-25 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

mozilla firefox UPGRADE
Affected: < 115.35.0 · < 150.0 · >= 140.0 and < 140.10.0
Fixed in: 115.35.0, 140.10.0, 150.0
Safe to downgrade to: a release before 140.0
1. Check installed version
apt-cache policy firefox || rpm -q firefox || firefox --version
2. UPGRADE to 115.35.0 (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 140.0 (Debian / Ubuntu)
apt-cache madison firefox sudo apt install firefox=<VERSION_BEFORE_140.0> 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_140.0> 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.
mozilla thunderbird UPGRADE
Affected: < 140.10.0
Fixed in: 140.10.0
Safe to downgrade to: no unaffected older release
1. Check installed version
apt-cache policy thunderbird || rpm -q thunderbird || thunderbird --version
2. UPGRADE to 140.10.0 (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.
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-2026-6754?

firefox: upgrade to 115.35.0, 140.10.0, 150.0; thunderbird: upgrade to 140.10.0. Exact commands for each OS are on this page.

Is CVE-2026-6754 being exploited?

CVE-2026-6754 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-2026-6754 affect?

CVE-2026-6754 affects firefox, thunderbird. Severity is high (CVSS 7.5). Use-after-free in the JavaScript Engine component. This vulnerability was fixed in Firefox 150, Firefox ESR 115.35, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10.

How urgent is CVE-2026-6754?

Priority 39/100. High severity; Remotely exploitable, no login, no user action

Software pages

Mozilla Firefox vulnerabilities · Mozilla Thunderbird vulnerabilities

Related CVEs

References

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