CVE-2023-4578

What it actually does

Mishandled out-of-memory condition during regex syntax checking in the JavaScript engine allows an allocation failure to be treated as a syntax error, resulting in memory corruption.

Why it matters to the business

Attackers can trigger browser or email client crashes, causing denial of service for users and potential disruption to web-dependent workflows; the issue has been public for over three years with low but persistent exploitability signals.

How an attack unfolds

An attacker crafts a JavaScript regular expression that forces the CheckRegExpSyntax path into an out-of-memory state; the subsequent exception is incorrectly converted, leading to a crash that an attacker can use for denial of service.

How to fix it

Upgrade Mozilla Firefox to version 117 or later, Firefox ESR to 115.2 or later, and Thunderbird to 115.2 or later. Apply updates through the product's built-in updater or package manager as soon as possible.

How to confirm the fix worked

Confirm the installed version meets or exceeds the fixed releases listed above; retest any internal JavaScript workloads that exercise regular-expression syntax checking to ensure no crashes occur under memory pressure.

Typical attack path

1. Deliver malicious JavaScript or HTML containing a specially crafted regular expression via a web page or email
2. Trigger JS::CheckRegExpSyntax in the affected JavaScript engine
3. Force an allocation failure inside the error-handling path
4. Cause the OOM exception to be mishandled as a syntax error, resulting in a crash

How to detect it

Review application crash logs and telemetry for unexpected terminations originating from the JavaScript engine during regular-expression processing; correlate with inbound web or email traffic containing unusual regex patterns.

What your security team should do

• Prioritize patching all Firefox, ESR, and Thunderbird instances within the next 30 days given the public disclosure age and availability impact
• Enable automatic updates for Mozilla products across the environment
• Monitor crash reports for regex-related JavaScript engine faults as a temporary detection measure