CVE-2024-3854 Overview
CVE-2024-3854 is an out-of-bounds read vulnerability in the Just-In-Time (JIT) compiler used by Mozilla Firefox, Firefox Extended Support Release (ESR), and Thunderbird. The JIT incorrectly optimized certain switch statement code patterns and emitted machine code that read memory outside the intended bounds. An attacker who convinces a user to load a crafted web page can trigger the flaw and potentially read sensitive memory from the renderer process. Mozilla classified the issue as high severity and fixed it in Firefox 125, Firefox ESR 115.10, and Thunderbird 115.10. The weakness is tracked under [CWE-125] (Out-of-Bounds Read).
Critical Impact
Remote attackers can exploit malicious web content to trigger out-of-bounds memory reads in the browser process, leading to information disclosure or memory corruption that may aid further exploitation.
Affected Products
- Mozilla Firefox versions earlier than 125
- Mozilla Firefox ESR versions earlier than 115.10
- Mozilla Thunderbird versions earlier than 115.10
Discovery Timeline
- 2024-04-16 - CVE-2024-3854 published to NVD
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2024-3854
Vulnerability Analysis
The vulnerability resides in the JavaScript JIT compiler shared by Firefox and Thunderbird. The JIT applies optimizations when compiling JavaScript switch statements into native machine code. For specific code patterns, the optimizer generates instructions that index past the end of an internal data structure, producing an out-of-bounds read at runtime.
Because the flaw lives in compiled native code rather than the interpreter, the bounds violation occurs without triggering normal runtime safety checks. The read accesses adjacent process memory, which may contain pointers, JavaScript object data, or other content useful to an attacker. Mozilla rates the issue as high severity in advisories MFSA 2024-18, MFSA 2024-19, and MFSA 2024-20.
Root Cause
The root cause is an incorrect optimization path inside the JIT compiler for switch statement code generation. The optimizer fails to maintain accurate bounds for jump tables or backing storage during the emission of native instructions. The resulting code performs reads using an unchecked or improperly clamped index value. Additional technical detail is tracked in Mozilla Bug 1884552.
Attack Vector
Exploitation requires user interaction. A victim must visit a web page or open HTML email content that delivers JavaScript designed to trigger the vulnerable JIT code path. Once the JIT compiles the malicious script, the generated machine code performs the out-of-bounds read inside the content process. Attackers can pair the leak with other browser flaws to escape the sandbox or achieve code execution. No authentication is required, and the attack is reachable over the network through any site loaded in the browser.
No public proof-of-concept exploit code is available, and no verified code examples are provided. Refer to the linked Mozilla advisories and Bugzilla entry for the authoritative technical analysis.
Detection Methods for CVE-2024-3854
Indicators of Compromise
- Renderer or content process crashes referencing IonMonkey, WarpMonkey, or other Firefox JIT components after loading untrusted web content
- Unexpected firefox.exe or thunderbird.exe child process behavior, such as spawning shells or writing executables to disk after browsing activity
- Outbound connections from the browser process to attacker-controlled hosts that immediately precede or follow a crash event
Detection Strategies
- Inventory installed Firefox, Firefox ESR, and Thunderbird versions across endpoints and flag any host running a version below Firefox 125, ESR 115.10, or Thunderbird 115.10
- Monitor browser telemetry and crash dumps for repeated faults in JavaScript engine modules, which can indicate exploitation attempts against JIT flaws
- Correlate web proxy logs with endpoint process telemetry to surface users who loaded suspicious script-heavy pages immediately before a browser crash
Monitoring Recommendations
- Centralize endpoint and browser telemetry so analysts can pivot from a crash event to the originating URL, parent process, and downstream child processes
- Alert on any post-exploitation behavior originating from browser processes, including credential access, LSASS interaction on Windows, or persistence creation
- Track patch compliance for Mozilla products as part of routine vulnerability management reporting
How to Mitigate CVE-2024-3854
Immediate Actions Required
- Upgrade Firefox to version 125 or later on all managed endpoints
- Upgrade Firefox ESR deployments to 115.10 or later, prioritizing kiosks and shared workstations
- Upgrade Thunderbird to 115.10 or later, and disable remote content in email where business requirements allow
- Apply Debian and other distribution security updates referenced in Debian LTS announcement msg00012 and Debian LTS announcement msg00013
Patch Information
Mozilla shipped fixes in Firefox 125, Firefox ESR 115.10, and Thunderbird 115.10. Details are published in the Mozilla Foundation Security Advisory MFSA 2024-18 for Firefox, MFSA 2024-19 for Firefox ESR, and MFSA 2024-20 for Thunderbird. Linux distributions including Debian have released corresponding backported packages.
Workarounds
- Restrict JavaScript execution on untrusted sites using browser policies or extensions such as NoScript until patches are deployed
- Enforce strict allow-listing of permitted sites in high-risk environments and block uncategorized or newly registered domains at the web proxy
- Configure Thunderbird to render messages as plain text and block remote content to reduce exposure to malicious HTML payloads
# Verify installed Mozilla product versions on Linux endpoints
firefox --version
thunderbird --version
# Example Debian/Ubuntu patch deployment
sudo apt-get update
sudo apt-get install --only-upgrade firefox-esr thunderbird
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
