CVE-2025-4919: Mozilla Firefox Buffer Overflow Vulnerability

CVE-2025-4919 Overview

CVE-2025-4919 is an out-of-bounds read and write vulnerability affecting Mozilla Firefox and Thunderbird. The flaw resides in the JavaScript engine, where attackers can confuse array index sizes to access memory outside intended boundaries. Successful exploitation allows reading or writing arbitrary data on a JavaScript object, which can lead to remote code execution within the browser process. Mozilla addressed the issue across Firefox 138.0.4, Firefox ESR 128.10.1, Firefox ESR 115.23.1, Thunderbird 128.10.2, and Thunderbird 138.0.2. The vulnerability is classified under [CWE-125] (Out-of-Bounds Read) and requires user interaction, typically by loading attacker-controlled web content.

Critical Impact

Remote attackers can trigger out-of-bounds memory access in the JavaScript engine via crafted web content, enabling potential code execution with confidentiality, integrity, and availability impact.

Affected Products

• Mozilla Firefox (versions prior to 138.0.4)
• Mozilla Firefox ESR (versions prior to 128.10.1 and 115.23.1)
• Mozilla Thunderbird (versions prior to 128.10.2 and 138.0.2)

Discovery Timeline

• 2025-05-17 - CVE-2025-4919 published to NVD
• 2026-04-13 - Last updated in NVD database

Technical Details for CVE-2025-4919

Vulnerability Analysis

The vulnerability stems from inconsistent handling of array index sizes within Mozilla's SpiderMonkey JavaScript engine. When the engine processes JavaScript objects, it relies on internal assumptions about index size representations to compute memory offsets. An attacker can manipulate these assumptions to confuse the engine into using a mismatched index size during array element access.

This confusion produces an incorrect offset calculation, allowing the engine to read from or write to memory addresses outside the bounds of the target JavaScript object. The mismatched index sizes break the type and bounds invariants the engine relies on for memory safety. Because JavaScript objects are heap-allocated, the out-of-bounds access can corrupt adjacent heap structures, including function pointers and object metadata.

For full technical context, consult the Mozilla Bug Report #1966614 and the Mozilla Security Advisory MFSA-2025-36.

Root Cause

The root cause is an index size confusion in the JavaScript engine's array access path. The engine fails to consistently validate the type or width of an index value before computing the memory location of the target element. This breaks the bounds check, producing an out-of-bounds read or write classified as [CWE-125].

Attack Vector

Exploitation requires the victim to load attacker-controlled JavaScript, typically by visiting a malicious web page in Firefox or rendering crafted HTML email content in Thunderbird. No privileges are required, but user interaction is necessary. After triggering the out-of-bounds access, an attacker can shape the heap to gain arbitrary read/write primitives and pivot toward code execution within the renderer process sandbox.

No public proof-of-concept exploit code has been verified for this CVE. Refer to the Mozilla Security Advisory MFSA-2025-37 for vendor-confirmed exploitation conditions.

Detection Methods for CVE-2025-4919

Indicators of Compromise

• Firefox or Thunderbird process crashes with signatures consistent with heap corruption during JavaScript execution.
• Unexpected child process spawns from firefox.exe or thunderbird.exe, especially shells or scripting interpreters.
• Outbound network connections from browser processes to unfamiliar domains immediately after loading new content.
• Browser telemetry showing repeated tab crashes on specific URLs prior to compromise.

Detection Strategies

• Inventory all Firefox and Thunderbird installations and compare versions against the fixed releases (138.0.4, 128.10.1, 115.23.1, 128.10.2, 138.0.2).
• Monitor endpoint telemetry for anomalous behavior originating from browser processes, including memory access violations and unexpected module loads.
• Correlate web proxy logs with browser crash events to identify candidate exploit URLs.

Monitoring Recommendations

• Enable crash reporting and forward Firefox/Thunderbird crash dumps to a central location for review.
• Alert on browser processes performing actions outside expected behavior, such as writing to autorun locations or modifying scheduled tasks.
• Track DNS and HTTP traffic from browser processes to high-risk or newly registered domains.

How to Mitigate CVE-2025-4919

Immediate Actions Required

• Update Mozilla Firefox to version 138.0.4 or later on all endpoints.
• Update Firefox ESR deployments to 128.10.1 or 115.23.1 depending on the ESR branch in use.
• Update Thunderbird to 128.10.2 or 138.0.2 on all systems where mail clients are deployed.
• Validate patch deployment through software inventory tools and remove or isolate non-compliant hosts.

Patch Information

Mozilla published fixes in Firefox 138.0.4, Firefox ESR 128.10.1, Firefox ESR 115.23.1, Thunderbird 128.10.2, and Thunderbird 138.0.2. Details are available in the Mozilla Security Advisory MFSA-2025-38, MFSA-2025-40, and MFSA-2025-41. Debian users should apply the updates referenced in the Debian LTS Announcement.

Workarounds

• Disable JavaScript execution in Thunderbird for HTML-rendered messages where business workflows permit.
• Restrict browsing to trusted domains via web filtering until patching is complete.
• Apply application allow-listing to prevent browser processes from spawning unauthorized child processes.

# Verify installed Firefox version on Linux endpoints
firefox --version

# Verify installed Thunderbird version on Linux endpoints
thunderbird --version

# Debian/Ubuntu: apply security updates
sudo apt update && sudo apt install --only-upgrade firefox-esr thunderbird