CVE-2026-100804: Sandbox escape due to use-after-free in the Preferences: Backend component

Q: How do I fix CVE-2026-100804?

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

Q: Is CVE-2026-100804 being exploited?

CVE-2026-100804 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-2026-100804 affect?

CVE-2026-100804 affects firefox, thunderbird. Severity is critical (CVSS 9.6). Sandbox escape due to use-after-free in the Preferences: Backend component. This vulnerability was fixed in Thunderbird 157 and Firefox 157.

Q: How urgent is CVE-2026-100804?

Priority 39/100. Critical severity

This month

CRITICAL 9.6UPGRADEUPGRADE

CVE-2026-100804 is a critical-severity flaw (CVSS 9.6) affecting firefox, thunderbird. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Sandbox escape due to use-after-free in the Preferences: Backend component. This vulnerability was fixed in Thunderbird 157 and Firefox 157.

Why this priority

Critical severity

A fixed release or distro patch exists.

Risk facts

CVSS score	9.6 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.3% (top 79%)
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 17:38 UTCUpdated Record last modified

mozilla firefox UPGRADE

Affected: < 157.0.0

Fixed in: 157.0.0

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 157.0.0 (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.

mozilla thunderbird UPGRADE

Affected: < 157.0

Fixed in: 157.0

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 157.0 (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.

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-2026-100804?

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

Is CVE-2026-100804 being exploited?

CVE-2026-100804 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-2026-100804 affect?

CVE-2026-100804 affects firefox, thunderbird. Severity is critical (CVSS 9.6). Sandbox escape due to use-after-free in the Preferences: Backend component. This vulnerability was fixed in Thunderbird 157 and Firefox 157.

How urgent is CVE-2026-100804?

Priority 39/100. Critical severity

Software pages

Mozilla Firefox vulnerabilities · Mozilla Thunderbird vulnerabilities

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CVE-2026-6750 HIGH 8.8 UPGRADE UPGRADE Privilege escalation in the Graphics: WebRender component
CVE-2026-5733 HIGH 8.8 UPGRADE Incorrect boundary conditions in the Graphics: WebGPU component

References

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