CVE-2026-84642: The values of the mail.allowed_attachment_hostnames advanced config setting were used in a regular expression

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HIGH 7.5REMOTEUPGRADE

CVE-2026-84642 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.

The values of the mail.allowed_attachment_hostnames advanced config setting were used in a regular expression without escaping. For some possible valid hostnames, this could allow certain unintended hostnames to also match and serve remote attachments. This vulnerability was fixed in Thunderbird 155 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	0.4% (top 64%)
CISA exploited list	No
Weakness	CWE-1333
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 19:00 UTCUpdated Record last modified

mozilla thunderbird UPGRADE

Affected: < 153.2.0 · >= 154.0 and < 155.0

Fixed in: 153.2.0, 155.0

Safe to downgrade to: a release before 154.0

1. Check installed version

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

2. UPGRADE to 153.2.0 (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 154.0 (Debian / Ubuntu)

apt-cache madison thunderbird sudo apt install thunderbird=<VERSION_BEFORE_154.0> 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_154.0> 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-2026-84642?

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

Is CVE-2026-84642 being exploited?

CVE-2026-84642 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-2026-84642 affect?

CVE-2026-84642 affects thunderbird. Severity is high (CVSS 7.5). The values of the mail.allowed_attachment_hostnames advanced config setting were used in a regular expression without escaping. For some possible valid hostnames, this could allow certain unintended hostnames to also match and serve remote

How urgent is CVE-2026-84642?

Priority 40/100. High severity; Remotely exploitable, no login, no user action

Software pages

Mozilla Thunderbird vulnerabilities

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References

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