CVE-2026-84641: A malicious IMAP server can trigger use-after-free and heap-memory disclosure by sending a crafted ID response

Routine

HIGH 7.5REMOTEUPGRADE

CVE-2026-84641 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 malicious IMAP server can trigger use-after-free and heap-memory disclosure by sending a crafted ID response. Heap contents can ultimately be persisted to prefs.js. 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 81%)
CISA exploited list	No
Weakness	CWE-416
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-2026-84641?

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

Is CVE-2026-84641 being exploited?

CVE-2026-84641 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-2026-84641 affect?

CVE-2026-84641 affects thunderbird. Severity is high (CVSS 7.5). A malicious IMAP server can trigger use-after-free and heap-memory disclosure by sending a crafted ID response. Heap contents can ultimately be persisted to prefs.js. This vulnerability was fixed in Thunderbird 155, Thunderbird 140.15, and

How urgent is CVE-2026-84641?

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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