CVE-2026-84640: A maliciously constructed mail header could lead to a one byte read past the end of a buffer

Routine

HIGH 7.5REMOTEUPGRADE

CVE-2026-84640 is a high-severity flaw (CVSS 7.5) affecting thunderbird. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A maliciously constructed mail header could lead to a one byte read past the end of a buffer. This vulnerability was fixed in Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.3% (top 82%)
CISA exploited list	No
Weakness	CWE-126
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 22:17 UTCPublished CVE record published
2026-09-03 19:01 UTCUpdated Record last modified

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.

Advisories and coverage

Official advisories

Amazon Linux 2 ALAS: ALAS2-2026-3952 (important): thunderbird 2026-09-28

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-2026-84640?

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

Is CVE-2026-84640 being exploited?

CVE-2026-84640 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-2026-84640 affect?

CVE-2026-84640 affects thunderbird. Severity is high (CVSS 7.5). A maliciously constructed mail header could lead to a one byte read past the end of a buffer. This vulnerability was fixed in Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.

How urgent is CVE-2026-84640?

Priority 36/100. High severity; Remotely exploitable, no login, no user action

Software pages

Mozilla Thunderbird vulnerabilities

Related CVEs

CVE-2026-6746 HIGH 7.5 UPGRADE UPGRADE Use-after-free in the DOM: Core & HTML component
CVE-2026-6758 HIGH 7.5 UPGRADE UPGRADE Use-after-free in the JavaScript: WebAssembly component
CVE-2026-6771 CRITICAL 9.8 UPGRADE UPGRADE Mitigation bypass in the DOM: Security component
CVE-2026-6747 HIGH 7.5 UPGRADE UPGRADE Use-after-free in the WebRTC component
CVE-2026-8093 HIGH 8.1 UPGRADE UPGRADE Memory safety bugs present in Thunderbird 150.0.1

References

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