CVE-2024-3853 Overview
CVE-2024-3853 is a use-after-free vulnerability in Mozilla Firefox affecting versions prior to 125. The flaw occurs when a JavaScript realm is in the process of being initialized while garbage collection starts concurrently. This race condition between realm initialization and the garbage collector can leave dangling references to freed memory. Attackers who trigger the condition through crafted JavaScript can achieve memory corruption in the browser process. The vulnerability is tracked under CWE-416: Use After Free and was addressed in Mozilla Security Advisory MFSA 2024-18.
Critical Impact
Successful exploitation can lead to memory corruption, potential arbitrary code execution within the Firefox content process, and compromise of confidentiality, integrity, and availability.
Affected Products
- Mozilla Firefox versions prior to 125
- Builds using the affected SpiderMonkey JavaScript engine realm initialization path
- Downstream distributions bundling Firefox < 125
Discovery Timeline
- 2024-04-16 - CVE-2024-3853 published to the National Vulnerability Database (NVD)
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2024-3853
Vulnerability Analysis
The vulnerability resides in SpiderMonkey, the JavaScript engine used by Firefox. A JavaScript realm represents an isolated execution environment with its own set of intrinsic objects and global scope. When Firefox creates a new realm, several allocation and initialization steps run before the realm reaches a consistent state. If the garbage collector runs during this partially initialized window, it can free objects that the initialization path still references. The engine then continues to operate on a pointer to freed memory, producing a classic use-after-free condition. Detailed engine-level notes are available in Mozilla Bugzilla Report #1884427.
Root Cause
The root cause is missing synchronization between realm initialization and garbage collection triggers. The initialization code does not fully register or root the intermediate objects with the garbage collector before yielding control. As a result, the collector treats those objects as unreachable and reclaims their memory. Subsequent access through the still-valid pointer accesses freed heap memory.
Attack Vector
Exploitation requires an attacker to lure a user to a page hosting malicious JavaScript that forces realm creation and simultaneous memory pressure to trigger garbage collection. The attacker can achieve this by allocating and releasing large object graphs while invoking APIs that create new realms, such as iframe insertion or Worker and Realm constructions. Once the use-after-free is triggered, an attacker can groom the heap to place attacker-controlled data at the freed location and hijack control flow within the content process. No prior authentication is required, and no user interaction beyond visiting the page is needed.
No public proof-of-concept or exploit tooling is currently listed for this issue. Technical specifics are referenced in Mozilla Bugzilla Report #1884427.
Detection Methods for CVE-2024-3853
Indicators of Compromise
- Firefox content process crashes with MOZ_CRASH or segmentation faults inside SpiderMonkey realm or GC routines
- Unexpected child process termination correlated with visits to untrusted or newly registered domains
- Anomalous JavaScript workloads that repeatedly instantiate realms while forcing large allocations
Detection Strategies
- Inventory installed Firefox versions across managed endpoints and flag any build below 125
- Monitor browser telemetry and crash reports for repeated SpiderMonkey faults on the same host
- Correlate browser process crashes with outbound network activity to identify potential exploitation attempts
Monitoring Recommendations
- Enable EDR visibility into browser child process creation, memory allocation anomalies, and crash events
- Track process lineage from firefox.exe and its content processes to detect follow-on activity such as shell spawning
- Ingest Firefox crash reports and browser telemetry into the SIEM for centralized correlation
How to Mitigate CVE-2024-3853
Immediate Actions Required
- Upgrade Firefox to version 125 or later on all managed endpoints and servers
- Prioritize workstations that regularly browse untrusted or high-risk web content
- Verify Firefox ESR and downstream Linux distribution builds have received backported fixes
Patch Information
Mozilla released the fix in Firefox 125. Administrators should reference Mozilla Security Advisory MFSA 2024-18 for the complete list of resolved issues in that release. Enterprise deployments using Firefox ESR should apply the corresponding ESR update that includes the SpiderMonkey realm initialization fix.
Workarounds
- Disable JavaScript on untrusted origins using about:config (javascript.enabled) where feasible
- Enforce strict content policies and site allow-lists through browser management tooling
- Route web traffic through a secure web gateway that blocks known malicious domains until patching is complete
# Verify installed Firefox version on Linux
firefox --version
# Windows: query installed version via registry
reg query "HKLM\SOFTWARE\Mozilla\Mozilla Firefox" /v CurrentVersion
# macOS: check application bundle version
defaults read /Applications/Firefox.app/Contents/Info CFBundleShortVersionString
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
