CVE-2026-84142: Internally found bugs present in Thunderbird 154

Q: How do I fix CVE-2026-84142?

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

Q: Is CVE-2026-84142 being exploited?

CVE-2026-84142 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-84142 affect?

CVE-2026-84142 affects firefox, thunderbird. Severity is critical (CVSS 9.8). Internally found bugs present in Thunderbird 154. 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

Q: How urgent is CVE-2026-84142?

Priority 51/100. Critical severity; Remotely exploitable, no login, no user action

This month

CRITICAL 9.8REMOTEUPGRADEUPGRADE

CVE-2026-84142 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. 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 and Thunderbird 155.

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	0.6% (top 54%)
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: < 155.0.0

Fixed in: 155.0.0

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 155.0.0 (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: < 155.0

Fixed in: 155.0

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 155.0 (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-2026-84142?

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

Is CVE-2026-84142 being exploited?

CVE-2026-84142 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-2026-84142 affect?

CVE-2026-84142 affects firefox, thunderbird. Severity is critical (CVSS 9.8). Internally found bugs present in Thunderbird 154. 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

How urgent is CVE-2026-84142?

Priority 51/100. 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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