CVE-2026-16369: Firefox WebAssembly Integer Overflow Flaw

CVE-2026-16369 Overview

CVE-2026-16369 is an integer overflow vulnerability [CWE-190] in the JavaScript WebAssembly component used by Mozilla Firefox and Thunderbird. Mozilla addressed the flaw in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13. The vulnerability is exploitable over the network without authentication or user interaction, according to the published CVSS vector. A remote attacker can trigger the overflow by delivering crafted WebAssembly content to a target browser or mail client, potentially compromising confidentiality, integrity, and availability of the affected process.

Critical Impact
A network-reachable attacker can trigger an integer overflow in the WebAssembly engine, enabling memory corruption that may lead to arbitrary code execution within the browser or mail client process.

Affected Products

Mozilla Firefox versions prior to 153
Mozilla Firefox ESR versions prior to 140.13
Mozilla Thunderbird versions prior to 153 and 140.13

Discovery Timeline

2026-07-21 - CVE-2026-16369 published to NVD
2026-07-22 - Last updated in NVD database

Technical Details for CVE-2026-16369

Vulnerability Analysis

The vulnerability resides in the WebAssembly (Wasm) subsystem of the SpiderMonkey JavaScript engine shared by Firefox and Thunderbird. An integer overflow occurs when arithmetic on untrusted size or index values wraps past the maximum representable value. When the resulting undersized value is used to allocate a buffer or bound a memory operation, subsequent reads or writes fall outside the intended allocation. The condition is classified under [CWE-190] Integer Overflow or Wraparound.

Mozilla tracked the issue in Bugzilla entry 2051854 and shipped fixes across four advisories: MFSA-2026-68, MFSA-2026-70, MFSA-2026-71, and MFSA-2026-72. Thunderbird inherits the flaw because it embeds the same rendering and scripting engine as Firefox.

Root Cause

The root cause is unchecked or improperly checked arithmetic on 32-bit size values inside the WebAssembly component. Attacker-controlled inputs, such as module section sizes, table lengths, or memory growth requests, are combined with other integer values without validating that the sum stays within representable bounds. The wrapped result then flows into allocation or bounds-check logic, producing an inconsistency between what the engine believes it allocated and what it actually accesses.

Attack Vector

Exploitation requires a victim to load attacker-controlled WebAssembly. The most direct path is a malicious or compromised website that ships a crafted .wasm module executed by the JavaScript engine. In Thunderbird, remote HTML content rendered in messages can reach the same code paths. No privileges and no user interaction beyond visiting the content are required, according to the published CVSS metrics. Successful exploitation typically leads to out-of-bounds memory access, which attackers chain with heap-shaping techniques to achieve arbitrary code execution inside the content process.

Mozilla has not published exploitation details, and no public proof-of-concept is currently listed. The EPSS score at publication is 0.414%. See the Mozilla Bug Report #2051854 for tracking information.

Detection Methods for CVE-2026-16369

Indicators of Compromise

Firefox or Thunderbird content process crashes with signatures pointing to js::wasm frames, particularly around allocation or bounds-check functions.
Unexpected child processes spawned by firefox.exe, firefox-bin, or thunderbird.exe shortly after loading web content.
Outbound network connections initiated by browser child processes to unfamiliar hosts following delivery of .wasm content.

Detection Strategies

Inventory endpoints running Firefox below 153, Firefox ESR below 140.13, or Thunderbird below 153 and 140.13 using software asset management or EDR telemetry.
Alert on repeated browser tab or content process crashes, which are a common precursor to memory corruption exploitation attempts.
Inspect proxy or DNS logs for delivery of application/wasm content from newly registered or low-reputation domains.

Monitoring Recommendations

Enable process creation, module load, and crash telemetry on endpoints running Mozilla products and forward events to a central analytics pipeline.
Correlate browser crash events with subsequent process execution or persistence activity from the same user session.
Track patch compliance for Firefox and Thunderbird versions across the fleet on a daily cadence until coverage is complete.

How to Mitigate CVE-2026-16369

Immediate Actions Required

Upgrade Firefox to version 153 or later on all managed endpoints.
Upgrade Firefox ESR to 140.13 or later where extended support builds are deployed.
Upgrade Thunderbird to 153 or 140.13, matching the ESR track in use.
Restart browser and mail client processes after patching to ensure the updated binaries are loaded.

Patch Information

Mozilla published fixes in the following advisories: MFSA-2026-68, MFSA-2026-70, MFSA-2026-71, and MFSA-2026-72. Administrators should distribute the fixed builds through their standard software deployment tooling and verify version reporting from endpoints after rollout.

Workarounds

Disable JavaScript execution for untrusted origins using enterprise browser policies where feasible until patches are applied.
Configure Thunderbird to render messages as plain text, reducing exposure to remote HTML and embedded scripting.
Restrict outbound access from user endpoints to categorized business sites through a web proxy to limit exposure to attacker-controlled WebAssembly payloads.

bash

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

# Windows: query installed version via registry
reg query "HKLM\SOFTWARE\Mozilla\Mozilla Firefox" /v CurrentVersion

# macOS: read the application bundle version
defaults read /Applications/Firefox.app/Contents/Info CFBundleShortVersionString