CVE-2026-100791: Use-after-free in the DOM: Core & HTML component

Routine

HIGH 8.8UPGRADEUPGRADE

CVE-2026-100791 is a high-severity flaw (CVSS 8.8) affecting firefox, thunderbird. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Use-after-free in the DOM: Core & HTML component. This vulnerability was fixed in Firefox ESR 153.4, Thunderbird 157, Thunderbird 140.17, Thunderbird 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17.

Why this priority

High severity

A fixed release or distro patch exists.

Risk facts

CVSS score	8.8 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.4% (top 66%)
CISA exploited list	No
Weakness	CWE-416
Published	2026-09-29
Last modified	2026-10-19

Publisher (CNA): Mozilla Corporation · Vendor: mozilla

Last updated 2026-10-19 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-09-29 13:17 UTCPublished CVE record published
2026-10-19 15:31 UTCUpdated Record last modified

mozilla firefox UPGRADE

Affected: < 115.42.0 · >= 116.0 and < 140.17.0 · >= 141.0 and < 153.4.0 · >= 154.0.0 and < 157.0.0

Fixed in: 115.42.0, 140.17.0, 153.4.0, 157.0.0

Safe to downgrade to: a release before 116.0

1. Check installed version

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

2. UPGRADE to 115.42.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade firefox

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade firefox

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 116.0 (Debian / Ubuntu)

apt-cache madison firefox sudo apt install firefox=<VERSION_BEFORE_116.0> sudo apt-mark hold firefox

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list firefox sudo dnf downgrade firefox-<VERSION_BEFORE_116.0> sudo dnf versionlock add firefox

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.

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-2026-100791?

firefox: upgrade to 115.42.0, 140.17.0, 153.4.0, 157.0.0; thunderbird: upgrade to 140.17.0, 153.4.0, 157.0. Exact commands for each OS are on this page.

Is CVE-2026-100791 being exploited?

CVE-2026-100791 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-2026-100791 affect?

CVE-2026-100791 affects firefox, thunderbird. Severity is high (CVSS 8.8). Use-after-free in the DOM: Core & HTML component. This vulnerability was fixed in Firefox ESR 153.4, Thunderbird 157, Thunderbird 140.17, Thunderbird 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17.

How urgent is CVE-2026-100791?

Priority 39/100. High severity

Software pages

Mozilla Firefox vulnerabilities · Mozilla Thunderbird vulnerabilities

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References

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