CVE-2026-103500: An attacker could cause a heap buffer overflow by getting a user to open an email that is greater than or equa

Routine

MEDIUM 6.1UPGRADE

CVE-2026-103500 is a medium-severity flaw (CVSS 6.1) affecting thunderbird. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

An attacker could cause a heap buffer overflow by getting a user to open an email that is greater than or equal to 2GB in size. This vulnerability was fixed in Thunderbird 157, Thunderbird 140.17, and Thunderbird 153.4.

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	6.1 (Medium)
Attack vector	Local
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.3% (top 80%)
CISA exploited list	No
Weakness	CWE-787, CWE-122
Published	2026-09-30
Last modified	2026-10-16

Publisher (CNA): Mozilla Corporation · Vendor: mozilla

Last updated 2026-10-16 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-09-30 18:18 UTCPublished CVE record published
2026-10-16 19:17 UTCUpdated Record last modified

mozilla thunderbird UPGRADE

Affected: < 140.17.0 · >= 141.0 and < 153.4.0 · >= 154.0 and < 157.0

Fixed in: 140.17.0, 153.4.0, 157.0

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 140.17.0 (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-2026-103500?

thunderbird: upgrade to 140.17.0, 153.4.0, 157.0. Exact commands for each OS are on this page.

Is CVE-2026-103500 being exploited?

CVE-2026-103500 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-2026-103500 affect?

CVE-2026-103500 affects thunderbird. Severity is medium (CVSS 6.1). An attacker could cause a heap buffer overflow by getting a user to open an email that is greater than or equal to 2GB in size. This vulnerability was fixed in Thunderbird 157, Thunderbird 140.17, and Thunderbird 153.4.

How urgent is CVE-2026-103500?

Priority 26/100. No elevated risk signals yet.

Software pages

Mozilla Thunderbird vulnerabilities

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References

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