CVE-2026-14899: The code to parse MIME headers for display when forwarding a message (if the setting to view all headers was e

Q: How do I fix CVE-2026-14899?

thunderbird: upgrade to 140.13.0. Exact commands for each OS are on this page.

Q: Is CVE-2026-14899 being exploited?

CVE-2026-14899 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-2026-14899 affect?

CVE-2026-14899 affects thunderbird. Severity is high (CVSS 7.5). The code to parse MIME headers for display when forwarding a message (if the setting to view all headers was enabled) had an off-by-one error, allowing a single byte to be read from the memory after the buffer for the headers, and potential

Q: How urgent is CVE-2026-14899?

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

Routine

HIGH 7.5REMOTEUPGRADE

CVE-2026-14899 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 code to parse MIME headers for display when forwarding a message (if the setting to view all headers was enabled) had an off-by-one error, allowing a single byte to be read from the memory after the buffer for the headers, and potentially crashing Thunderbird. This vulnerability was fixed in Thunderbird 153 and Thunderbird 140.13.

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-193
Published	2026-07-22
Last modified	2026-10-24

Publisher (CNA): Mozilla Corporation · Vendor: mozilla

Last updated 2026-10-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-22 20:16 UTCPublished CVE record published
2026-10-24 13:10 UTCUpdated Record last modified

mozilla thunderbird UPGRADE

Affected: < 140.13.0

Fixed in: 140.13.0

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 140.13.0 (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.

Exact fixes by distribution

Fedora 46 · sugar-toolkit-gtk3 · FIXED · 0.122-2.fc46 · FEDORA-2026-d11eb2faf0 · source

sudo dnf upgrade --advisory=FEDORA-2026-d11eb2faf0

Fedora ELN · ntpd-rs · FIXED · 1.7.2-3.eln159 · FEDORA-2026-57e4a99310 · source

sudo dnf upgrade --advisory=FEDORA-2026-57e4a99310

Debian 12 · thunderbird · FIXED · 1:140.13.0esr-2~deb12u1

sudo apt update && sudo apt install --only-upgrade thunderbird dpkg --compare-versions "$(dpkg-query -W -f='${Version}' thunderbird)" ge "1:140.13.0esr-2~deb12u1" && echo PATCHED || echo STILL-VULNERABLE

Debian 13 · thunderbird · FIXED · 1:140.13.0esr-2~deb13u1

sudo apt update && sudo apt install --only-upgrade thunderbird dpkg --compare-versions "$(dpkg-query -W -f='${Version}' thunderbird)" ge "1:140.13.0esr-2~deb13u1" && echo PATCHED || echo STILL-VULNERABLE

RHEL 10 · thunderbird · FIXED · thunderbird-140.13.0-1.el10_2

sudo dnf upgrade --advisory=RHSA-2026:49621 rpm -q thunderbird

RHEL 10.0 Extended Update Support · thunderbird · FIXED · thunderbird-140.13.0-1.el10_0

sudo dnf upgrade --advisory=RHSA-2026:53445 rpm -q thunderbird

RHEL 8 · thunderbird · FIXED · thunderbird-140.13.0-1.el8_10

sudo dnf upgrade --advisory=RHSA-2026:49922 rpm -q thunderbird

RHEL 8.4 Advanced Mission Critical Update Support · thunderbird · FIXED · thunderbird-140.13.0-1.el8_4

sudo dnf upgrade --advisory=RHSA-2026:53455 rpm -q thunderbird

RHEL 8.4 Extended Update Support Long-Life Add-On · thunderbird · FIXED · thunderbird-140.13.0-1.el8_4

sudo dnf upgrade --advisory=RHSA-2026:53455 rpm -q thunderbird

Amazon Linux 2 · see advisory · FIXED · ALAS2-2026-3868 · source

sudo yum update --advisory=ALAS2-2026-3868

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-2026-14899?

thunderbird: upgrade to 140.13.0. Exact commands for each OS are on this page.

Is CVE-2026-14899 being exploited?

CVE-2026-14899 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-2026-14899 affect?

CVE-2026-14899 affects thunderbird. Severity is high (CVSS 7.5). The code to parse MIME headers for display when forwarding a message (if the setting to view all headers was enabled) had an off-by-one error, allowing a single byte to be read from the memory after the buffer for the headers, and potential

How urgent is CVE-2026-14899?

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

Software pages

Mozilla Thunderbird vulnerabilities

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References

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