CVE-2026-84143: Internally found bugs present in Thunderbird 154, Thunderbird ESR 153.1 and Thunderbird ESR 140.14

This month

CRITICAL 9.8REMOTEUPGRADEUPGRADE

CVE-2026-84143 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.

Internally found bugs present in Thunderbird 154, Thunderbird ESR 153.1 and Thunderbird ESR 140.14. Some of these bugs showed evidence of memory corruption or another security-relevant defect and we presume that with enough effort some of these could have been exploited. This vulnerability was fixed in Firefox 155, Firefox ESR 140.15, Firefox ESR 153.2, Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.

Why this priority

Critical severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 69%)
CISA exploited list	No
Weakness	CWE-119, CWE-200
Published	2026-09-01
Last modified	2026-09-03

Publisher (CNA): Mozilla Corporation · Vendor: mozilla

Last updated 2026-09-03 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-09-01 13:20 UTCPublished CVE record published
2026-09-03 18:17 UTCUpdated Record last modified

mozilla firefox UPGRADE

Affected: < 140.15.0 · >= 141.0.0 and < 153.2.0 · >= 154.0.0 and < 155.0.0

Fixed in: 140.15.0, 153.2.0, 155.0.0

Safe to downgrade to: a release before 141.0.0

1. Check installed version

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

2. UPGRADE to 140.15.0 (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 141.0.0 (Debian / Ubuntu)

apt-cache madison firefox sudo apt install firefox=<VERSION_BEFORE_141.0.0> 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_141.0.0> 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: < 140.15.0 · >= 141.0 and < 153.2.0 · >= 154.0 and < 155.0

Fixed in: 140.15.0, 153.2.0, 155.0

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 140.15.0 (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-2026-84143?

firefox: upgrade to 140.15.0, 153.2.0, 155.0.0; thunderbird: upgrade to 140.15.0, 153.2.0, 155.0. Exact commands for each OS are on this page.

Is CVE-2026-84143 being exploited?

CVE-2026-84143 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-2026-84143 affect?

CVE-2026-84143 affects firefox, thunderbird. Severity is critical (CVSS 9.8). Internally found bugs present in Thunderbird 154, Thunderbird ESR 153.1 and Thunderbird ESR 140.14. Some of these bugs showed evidence of memory corruption or another security-relevant defect and we presume that with enough effort some of t

How urgent is CVE-2026-84143?

Priority 47/100. Critical 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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