Critical

Sandbox escape in Firefox/Thunderbird Responsive Design Mode

IdentifiersCVE-2026-4692CWE-693

CVE-2026-4692 is a sandbox escape vulnerability in the Responsive Design Mode component of Mozilla Firefox and Thunderbird. The provided content identifies the issue only at a high level as a sandbox escape and states that it was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9. No additional technical details about the root cause, vulnerable function, or exploit path are available in the provided material.

For your environment

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation could allow an attacker to escape the intended sandbox restrictions associated with the Responsive Design Mode component. A sandbox escape can reduce or bypass isolation boundaries and may enable follow-on compromise beyond the confined context. The provided content does not specify whether this leads directly to arbitrary code execution, privilege escalation, or data access, so the precise downstream impact is currently not available.

Mitigation

If you can’t patch tonight, do this now.

If immediate patching is not possible, reduce exposure by avoiding use of the Responsive Design Mode feature until updates can be applied. Standard hardening measures such as limiting untrusted content handling and prioritizing rapid deployment of Mozilla security updates may reduce risk, but no vendor-specific mitigation beyond upgrading is provided in the available content.

Remediation

Patch, then assume compromise.

Upgrade affected products to a fixed release: Firefox 149 or later, Firefox ESR 115.34 or later, Firefox ESR 140.9 or later, Thunderbird 149 or later, or Thunderbird 140.9 or later.

PUBLIC EXPLOITS

Exploits

No public exploits tracked yet. Mallory keeps watching.

VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

EXPOSURE SURFACE

Affected products & vendors

Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.

VendorProductType

MozillaFirefoxapplication

MozillaFirefox Esrapplication

MozillaThunderbirdapplication

Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.

ACTIVITY FEED

Recent activity

9 sources tracked across advisories and community write-ups. News coverage will land here when it surfaces.

9 SOURCESView more in app

No news coverage yet. Advisories and community discussion only.

What this page doesn’t show

The version that knows your environment.

This page is what’s public. Mallory adds the parts that aren’t: which of your assets are affected, which adversaries are exploiting it right now, which detections to deploy, and what to do tonight.

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Exposure mapping

Query your assets running an affected version, and investigate the blast radius.

Threat actor evidence

Every observed campaign linking this CVE to a named adversary.

Associated malware

Malware families riding this exploit, with evidence and IOCs.

Detection signatures

YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.

Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity2