CVE-2023-37202: Mozilla Firefox Use-After-Free Vulnerability

CVE-2023-37202 Overview

CVE-2023-37202 is a use-after-free vulnerability [CWE-416] in Mozilla Firefox, Firefox ESR, and Thunderbird. Cross-compartment wrappers wrapping a scripted proxy could cause objects from other compartments to be stored in the main compartment, leading to memory corruption. An attacker who convinces a user to load a malicious page can trigger the condition and potentially achieve arbitrary code execution within the browser process.

Critical Impact
Successful exploitation can lead to arbitrary code execution in the context of the browser, compromising confidentiality, integrity, and availability of the affected system.

Affected Products

Mozilla Firefox versions prior to 115
Mozilla Firefox ESR versions prior to 102.13
Mozilla Thunderbird versions prior to 102.13
Debian Linux 10, 11, and 12 (packaged Firefox ESR and Thunderbird)

Discovery Timeline

2023-07-05 - CVE-2023-37202 published to NVD
2023-07-05 - Mozilla releases security advisories MFSA-2023-22, MFSA-2023-23, and MFSA-2023-24
2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-37202

Vulnerability Analysis

The vulnerability resides in SpiderMonkey, Mozilla's JavaScript engine, specifically within the cross-compartment wrapper (CCW) mechanism. Compartments are isolation boundaries used to separate JavaScript objects belonging to different origins or privilege levels. When a CCW wraps a scripted JavaScript Proxy, the proxy's traps can execute attacker-controlled code during routine wrapper operations.

The flaw allows objects from another compartment to be stored inside the main compartment without proper wrapping. When the original compartment is later garbage collected, the dangling reference held by the main compartment becomes a use-after-free condition. Triggering the freed memory through subsequent JavaScript operations can produce memory corruption suitable for control-flow hijacking.

Details are tracked in Mozilla Bug Report #1834711.

Root Cause

The root cause is improper handling of scripted Proxy objects in the cross-compartment wrapping logic. The engine assumes wrapper operations are side-effect free, but proxy traps invoke user-defined JavaScript that can store references across compartment boundaries before lifetime guarantees are enforced.

Attack Vector

Exploitation requires the victim to load attacker-controlled web content in Firefox, or to render an HTML email in Thunderbird with remote content enabled. The attack is network-reachable and requires user interaction in the form of visiting a page. No privileges or credentials are required on the target system.

No public proof-of-concept code is available, and the issue is not listed in the CISA Known Exploited Vulnerabilities catalog. Refer to the Mozilla advisories for technical context.

Detection Methods for CVE-2023-37202

Indicators of Compromise

Firefox or Thunderbird processes spawning unexpected child processes such as cmd.exe, powershell.exe, bash, or sh
Browser crash reports referencing SpiderMonkey, JSObject, or Wrapper modules following visits to untrusted pages
Outbound network connections from firefox.exe or thunderbird.exe to low-reputation domains shortly after content rendering

Detection Strategies

Inventory installed Firefox, Firefox ESR, and Thunderbird versions across the fleet and flag any builds below Firefox 115, Firefox ESR 102.13, or Thunderbird 102.13
Monitor endpoint telemetry for anomalous memory access patterns, exception handlers, or shellcode-style behavior originating from browser processes
Correlate web proxy logs with browser crash events to identify pages that trigger renderer failures

Monitoring Recommendations

Enable browser crash reporting and forward reports to a centralized logging system for analysis
Apply EDR rules that detect process injection, suspicious memory allocations, and unauthorized child process creation from browser binaries
Track Mozilla security advisories and Debian DSA notices (DSA-5450, DSA-5451) for follow-on patches

How to Mitigate CVE-2023-37202

Immediate Actions Required

Update Mozilla Firefox to version 115 or later on all endpoints
Update Firefox ESR to 102.13 or later for environments requiring the extended support release
Update Thunderbird to 102.13 or later, prioritizing systems that render HTML email with remote content
On Debian systems, apply the updates from Debian LTS Announcement July 2023 #6 and Debian LTS Announcement July 2023 #15

Patch Information

Mozilla addressed the use-after-free in the cross-compartment wrapper code path. Fixed builds are Firefox 115, Firefox ESR 102.13, and Thunderbird 102.13. Refer to MFSA-2023-22, MFSA-2023-23, and MFSA-2023-24 for vendor guidance. Debian shipped corresponding packages in DSA-5450 and DSA-5451.

Workarounds

Disable JavaScript on untrusted sites using the javascript.enabled preference or a content-blocking extension until patches are applied
Configure Thunderbird to render messages as plain text and block remote content to reduce exposure from HTML email
Restrict browsing on high-value hosts to vetted internal sites using web proxy allow-lists until updates are deployed

bash

# Verify installed Firefox version on Debian/Ubuntu
firefox --version

# Apply package updates
sudo apt update && sudo apt install --only-upgrade firefox-esr thunderbird

# Verify post-patch versions meet the fixed baseline
firefox-esr --version   # Expect 102.13 or later
thunderbird --version   # Expect 102.13 or later