CVE-2024-3855

What it actually does

The JIT compiler performed incorrect optimizations on MSubstr operations, resulting in out-of-bounds memory reads during script execution.

Why it matters to the business

Attackers could potentially read sensitive data from browser memory or cause crashes, increasing risk of information disclosure in affected browser sessions across user devices.

How an attack unfolds

An attacker delivers malicious JavaScript that triggers the misoptimized MSubstr path in the JIT, allowing controlled out-of-bounds reads to leak memory contents for further exploitation or data theft.

How to fix it

Upgrade Firefox to version 125 or later to receive the corrected JIT handling for MSubstr operations.

How to confirm the fix worked

Confirm browser version is 125 or higher; test with known triggering JavaScript patterns in a controlled environment to verify absence of out-of-bounds behavior.

Typical attack path

1. Host a webpage containing specially crafted JavaScript targeting MSubstr operations
2. Victim browser loads and JIT-compiles the script
3. JIT produces incorrect code leading to out-of-bounds read
4. Attacker exfiltrates leaked memory contents via script-controlled channels

How to detect it

Review browser process crash logs and memory dumps for patterns consistent with MSubstr-related out-of-bounds access; correlate with unusual JavaScript execution artifacts in web activity logs.

What your security team should do

• Prioritize browser updates to Firefox 125+ across all managed endpoints
• Monitor for new exploit maturity indicators given the memory corruption nature
• Enforce automatic update policies to reduce exposure window