CVE-2025-13016: Mozilla Firefox Buffer Overflow Vulnerability

CVE-2025-13016 Overview

CVE-2025-13016 is a boundary condition vulnerability in the JavaScript WebAssembly component of Mozilla Firefox and Thunderbird. Incorrect boundary checks in the WebAssembly engine can allow specially crafted content to violate memory safety assumptions. Mozilla addressed the issue in Firefox 145, Firefox ESR 140.5, Thunderbird 145, and Thunderbird 140.5. The flaw is reachable through normal web browsing when a user loads attacker-controlled content. The vulnerability is tracked under [CWE-703] (Improper Check or Handling of Exceptional Conditions) per the NVD record.

Critical Impact

A remote attacker can deliver malicious web content that triggers a boundary violation in the WebAssembly engine, potentially compromising confidentiality, integrity, and availability of the browser process.

Affected Products

• Mozilla Firefox prior to version 145
• Mozilla Firefox ESR prior to version 140.5
• Mozilla Thunderbird prior to version 145 and 140.5

Discovery Timeline

• 2025-11-11 - CVE-2025-13016 published to NVD
• 2026-04-13 - Last updated in NVD database

Technical Details for CVE-2025-13016

Vulnerability Analysis

The vulnerability resides in the WebAssembly (Wasm) component of Mozilla's SpiderMonkey JavaScript engine. Incorrect boundary conditions in Wasm code generation or runtime checks can allow memory accesses outside the validated range. WebAssembly modules execute in a sandboxed linear memory model, and the engine enforces bounds on every memory access. When those checks are computed incorrectly, the resulting access can read or write outside expected limits. Mozilla classifies the issue with high impact across confidentiality, integrity, and availability. Successful exploitation requires user interaction, such as visiting a page that loads a malicious Wasm module. The advisory notes the bug was fixed in coordinated releases for Firefox and Thunderbird.

Root Cause

The root cause is an incorrect boundary condition in the WebAssembly subsystem ([CWE-703]). Boundary computation errors in just-in-time compiled or interpreted Wasm code paths break the memory safety guarantees that the Wasm sandbox depends on. See Mozilla Bug Report #1992130 for technical context.

Attack Vector

The attack vector is network-based. An attacker hosts a page containing a crafted WebAssembly module and induces a victim to visit it, for example through phishing, malvertising, or a compromised site. In Thunderbird, scripting is disabled in mail by default, which limits exposure to browser-like contexts. The high attack complexity reflects the conditions required to convert the boundary error into reliable exploitation. No public proof-of-concept or in-the-wild exploitation is reported in the available data.

No verified exploit code is available. Refer to Mozilla Security Advisory MFSA-2025-87 and Mozilla Security Advisory MFSA-2025-88 for vendor technical details.

Detection Methods for CVE-2025-13016

Indicators of Compromise

• Firefox or Thunderbird process crashes referencing the WebAssembly or js::wasm subsystem in crash reports.
• Unexpected child process spawns from firefox.exe or thunderbird.exe following browsing activity.
• Outbound network connections from the browser process to uncategorized or newly registered domains hosting .wasm content.

Detection Strategies

• Inventory installed Firefox and Thunderbird versions across the fleet and flag instances below the fixed versions (145, ESR 140.5).
• Monitor browser telemetry and crash dumps for repeated faults inside the WebAssembly engine.
• Correlate web proxy logs for downloads of WebAssembly modules combined with browser crashes on the same endpoint.

Monitoring Recommendations

• Ingest endpoint process, file, and network telemetry into a central analytics platform to baseline browser behavior.
• Alert on browser processes performing post-exploitation behaviors such as credential file access, LSASS access, or remote shell spawning.
• Track Mozilla advisories MFSA-2025-87 and MFSA-2025-88 for updates and additional related CVEs.

How to Mitigate CVE-2025-13016

Immediate Actions Required

• Upgrade Firefox to version 145 or later, and Firefox ESR to 140.5 or later.
• Upgrade Thunderbird to version 145 or 140.5 or later on all endpoints that have it installed.
• Enforce browser auto-update policies through Group Policy, MDM, or configuration management tooling.
• Validate patch deployment by querying installed application versions across managed endpoints.

Patch Information

Mozilla released fixes in Firefox 145, Firefox ESR 140.5, Thunderbird 145, and Thunderbird 140.5. Patch details are documented in Mozilla Security Advisory MFSA-2025-87, Mozilla Security Advisory MFSA-2025-88, Mozilla Security Advisory MFSA-2025-90, and Mozilla Security Advisory MFSA-2025-91.

Workarounds

• Where immediate patching is not possible, restrict execution of WebAssembly by setting javascript.options.wasm to false in about:config as a temporary measure. This will break sites that rely on Wasm.
• Apply network-level controls to block known malicious domains and limit access to untrusted web content.
• Use enterprise policies to disable WebAssembly through the Firefox policies JSON until patches are deployed.

# Configuration example: Firefox enterprise policy to disable WebAssembly
# Path: /etc/firefox/policies/policies.json  (Linux)
#       %ProgramFiles%\Mozilla Firefox\distribution\policies.json  (Windows)
{
  "policies": {
    "Preferences": {
      "javascript.options.wasm": {
        "Value": false,
        "Status": "locked"
      }
    }
  }
}