CVE-2026-4692: Sandbox escape in the Responsive Design Mode component

Q: How do I fix CVE-2026-4692?

firefox: upgrade to 115.34.0, 140.9.0, 149.0; thunderbird: upgrade to 140.9.0, 149.0. Exact commands for each OS are on this page.

Q: Is CVE-2026-4692 being exploited?

CVE-2026-4692 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-2026-4692 affect?

CVE-2026-4692 affects firefox, thunderbird. Severity is critical (CVSS 10.0). Sandbox escape in the Responsive Design Mode component. This vulnerability was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

Q: How urgent is CVE-2026-4692?

Priority 51/100. Critical severity; Remotely exploitable, no login, no user action

This month

CRITICAL 10.0REMOTEUPGRADEUPGRADE

CVE-2026-4692 is a critical-severity flaw (CVSS 10.0) affecting firefox, thunderbird. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Sandbox escape in the Responsive Design Mode component. This vulnerability was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

Why this priority

Critical severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	10.0 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.5% (top 57%)
CISA exploited list	No
Weakness	CWE-653
Published	2026-03-24
Last modified	2026-07-15

Publisher (CNA): Mozilla Corporation · Vendor: mozilla

Last updated 2026-07-15 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-03-24 13:16 UTCPublished CVE record published
2026-07-15 02:22 UTCUpdated Record last modified

mozilla firefox UPGRADE

Affected: < 115.34.0 · < 149.0 · >= 128.0 and < 140.9.0

Fixed in: 115.34.0, 140.9.0, 149.0

Safe to downgrade to: a release before 128.0

1. Check installed version

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

2. UPGRADE to 115.34.0 (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 128.0 (Debian / Ubuntu)

apt-cache madison firefox sudo apt install firefox=<VERSION_BEFORE_128.0> 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_128.0> 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: < 140.9.0 · < 149.0

Fixed in: 140.9.0, 149.0

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 140.9.0 (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-2026-4692?

firefox: upgrade to 115.34.0, 140.9.0, 149.0; thunderbird: upgrade to 140.9.0, 149.0. Exact commands for each OS are on this page.

Is CVE-2026-4692 being exploited?

CVE-2026-4692 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-2026-4692 affect?

CVE-2026-4692 affects firefox, thunderbird. Severity is critical (CVSS 10.0). Sandbox escape in the Responsive Design Mode component. This vulnerability was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

How urgent is CVE-2026-4692?

Priority 51/100. Critical severity; Remotely exploitable, no login, no user action

Software pages

Mozilla Firefox vulnerabilities · Mozilla Thunderbird vulnerabilities

Related CVEs

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CVE-2026-6758 HIGH 7.5 UPGRADE UPGRADE Use-after-free in the JavaScript: WebAssembly component
CVE-2026-6771 CRITICAL 9.8 UPGRADE UPGRADE Mitigation bypass in the DOM: Security component
CVE-2026-6747 HIGH 7.5 UPGRADE UPGRADE Use-after-free in the WebRTC component

References

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