CVE-2026-16368: Firefox WebAssembly Buffer Overflow Flaw

CVE-2026-16368 Overview

CVE-2026-16368 is a memory safety vulnerability in the JavaScript WebAssembly component used by Mozilla Firefox and Thunderbird. The flaw stems from incorrect boundary conditions [CWE-119] in how the engine validates WebAssembly operations. An attacker who convinces a user to visit a crafted web page can trigger the condition without authentication or user interaction beyond navigation. Mozilla addressed the issue in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13.

Critical Impact
Remote attackers can exploit this vulnerability over the network to compromise confidentiality, integrity, and availability without privileges or user interaction.

Affected Products

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

Discovery Timeline

2026-07-21 - CVE-2026-16368 published to NVD
2026-07-22 - Last updated in NVD database

Technical Details for CVE-2026-16368

Vulnerability Analysis

The vulnerability resides in the JavaScript WebAssembly (Wasm) component of the SpiderMonkey engine. Incorrect boundary conditions allow operations to reference memory outside intended limits during Wasm execution. This class of flaw, categorized under [CWE-119] Improper Restriction of Operations within the Bounds of a Memory Buffer, typically enables adversaries to read or corrupt adjacent memory. Attackers can leverage such primitives to achieve arbitrary code execution inside the content process.

Because WebAssembly executes with high performance and reduced runtime checks by design, boundary validation errors carry direct exploitation potential. The attack surface is broad since any site or embedded content that loads a Wasm module can trigger the vulnerable code path.

Root Cause

The root cause is faulty boundary condition logic within the WebAssembly implementation. Bounds checks either failed to cover edge cases or produced incorrect results, allowing operations that should have been rejected. Refer to Mozilla Security Advisory MFSA-2026-68 and Mozilla Bug Report #2051015 for the technical fix details.

Attack Vector

Exploitation occurs remotely over the network. An attacker hosts a malicious page containing a crafted WebAssembly module. When a victim loads the page in an unpatched Firefox or Thunderbird build, the module triggers the boundary violation. No authentication or user interaction beyond opening the content is required. Thunderbird is affected when it processes email content that renders scripting, though scripting is disabled by default in mail.

See Mozilla Security Advisory MFSA-2026-70, MFSA-2026-71, and MFSA-2026-72 for product-specific advisories.

Detection Methods for CVE-2026-16368

Indicators of Compromise

Unexpected child process spawns from firefox.exe, thunderbird.exe, or their Linux and macOS equivalents following browsing sessions.
Renderer or content process crashes referencing WebAssembly modules or SpiderMonkey js::wasm symbols in crash reports.
Outbound network connections from browser content processes to previously unseen domains hosting .wasm payloads.

Detection Strategies

Inventory installed Firefox and Thunderbird versions across the fleet and flag any host running builds earlier than Firefox 153, ESR 140.13, or Thunderbird 153/140.13.
Hunt for anomalous behavior originating from browser processes, including memory allocation spikes, unexpected shellcode-style regions, and code execution outside signed modules.
Correlate web proxy logs with endpoint telemetry to identify sessions where WebAssembly content preceded browser instability.

Monitoring Recommendations

Enable browser telemetry and crash reporting to central logging for anomaly review.
Alert on process lineage where browser processes launch scripting hosts, cmd.exe, powershell.exe, or shell interpreters.
Track EDR detections tied to browser exploitation chains, including in-memory injection and unsigned module loads.

How to Mitigate CVE-2026-16368

Immediate Actions Required

Update Firefox to version 153 or later on all endpoints.
Update Firefox ESR deployments to 140.13 or later.
Update Thunderbird to 153 or Thunderbird ESR to 140.13 or later.
Restart affected applications after patching to ensure the vulnerable code is unloaded from memory.

Patch Information

Mozilla published fixes in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13. Complete advisory details are available in MFSA-2026-68, MFSA-2026-70, MFSA-2026-71, and MFSA-2026-72.

Workarounds

Disable JavaScript execution for untrusted sites using enterprise policy javascript.enabled where operationally feasible.
Restrict WebAssembly execution through the javascript.options.wasm preference in about:config on managed builds until patches are applied.
Enforce web filtering to block access to unknown or high-risk domains that could serve crafted Wasm modules.
Confirm Thunderbird continues to render remote content and scripting disabled by default for message bodies.

bash

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

# Verify Thunderbird version
thunderbird --version

# Example enterprise policy to enforce updates (policies.json)
{
  "policies": {
    "DisableAppUpdate": false,
    "AppAutoUpdate": true
  }
}