Critical

Type Confusion RCE in Firefox IonMonkey switch statement optimization

IdentifiersCVE-2026-4698CWE-843· Access of Resource Using…

CVE-2026-4698 is a remote code execution vulnerability in Mozilla's JavaScript JIT engine, specifically the IonMonkey component used by Firefox and Thunderbird. The flaw is described by Mozilla as a JIT miscompilation issue and, per the supporting context, occurs during optimization of JavaScript switch statements. Improper validation of user-supplied data can lead to a type confusion condition, causing the JIT compiler to make incorrect assumptions about object or value types during optimized execution. An attacker can trigger the vulnerable condition by supplying crafted JavaScript in a malicious web page or file, potentially causing arbitrary code execution in the context of the affected application process. The issue was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

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 can allow a remote attacker to achieve arbitrary code execution in the context of the current Firefox or Thunderbird process. The provided context also indicates high impact to confidentiality, integrity, and availability, consistent with the cited CVSS vector AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H. Because exploitation is triggered through malicious JavaScript content, the attack can be delivered remotely, but requires user interaction such as visiting a malicious page or opening a malicious file.

Mitigation

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

If immediate patching is not possible, reduce exposure by limiting access to untrusted web content and untrusted HTML or script-bearing files in affected Firefox and Thunderbird installations. Enterprise defenders can temporarily restrict or isolate browser use for high-risk users and workflows until patched versions are deployed. No specific vendor-supplied workaround beyond upgrading is provided in the available context.

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. Apply Mozilla's April 2026 security updates referenced in the advisory for CVE-2026-4698.

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

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

ACTIVITY FEED

Recent activity

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

12 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 activity4