CVE-2026-16363: Firefox WebAssembly JIT RCE Vulnerability

CVE-2026-16363 Overview

CVE-2026-16363 is a Just-In-Time (JIT) miscompilation vulnerability in the JavaScript WebAssembly component used by Mozilla Firefox and Thunderbird. The flaw affects the JIT compiler's handling of WebAssembly code, resulting in incorrect calculations [CWE-682] during code generation. Mozilla fixed the issue in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13. The vulnerability is reachable over the network through crafted web content, requires no privileges, and needs no user interaction beyond visiting a malicious page.

Critical Impact
A remote attacker can trigger memory corruption through a malicious WebAssembly module, potentially leading to arbitrary code execution within the browser process.

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-2026-16363 published to NVD
2026-07-22 - Last updated in NVD database

Technical Details for CVE-2026-16363

Vulnerability Analysis

The vulnerability resides in the JIT compiler that transforms WebAssembly bytecode into native machine code inside the SpiderMonkey JavaScript engine. A miscompilation occurs when the compiler produces machine code that does not correctly represent the semantics of the source WebAssembly instructions. This category of flaw is classified as an incorrect calculation weakness [CWE-682].

When the JIT emits faulty native code, downstream operations can access memory outside intended bounds, misinterpret typed values, or bypass runtime checks. Attackers who control the WebAssembly module input can shape the miscompiled output to achieve exploitable primitives such as type confusion or out-of-bounds memory access inside the content process.

Root Cause

The root cause is an error in the WebAssembly JIT compilation pipeline that produces incorrect native code for specific instruction sequences or optimization paths. Because WebAssembly modules are loaded and compiled from untrusted web origins, attacker-controlled inputs reach the vulnerable compilation logic without prior sanitization by the user.

Attack Vector

Exploitation requires a victim to load attacker-controlled web content that delivers a malicious WebAssembly module. The module triggers the miscompilation path, and subsequent execution of the flawed native code yields memory corruption. See Mozilla Bug Report #2047689 and Mozilla Security Advisory MFSA-2026-68 for vendor technical details.

// No verified public proof-of-concept is available for CVE-2026-16363.
// Refer to Mozilla Bugzilla #2047689 for reproducer details when released.

Detection Methods for CVE-2026-16363

Indicators of Compromise

Unexpected child process spawns from firefox.exe, firefox-bin, or thunderbird binaries following browsing activity.
Renderer or content process crashes referencing WebAssembly frames or JIT-generated code regions in crash reports.
Outbound network connections to unknown hosts initiated by browser processes shortly after loading untrusted pages.

Detection Strategies

Monitor endpoints for Firefox and Thunderbird versions below the patched builds using software inventory tooling.
Alert on browser processes writing executable files, loading unsigned modules, or performing lateral movement behaviors.
Correlate browser crash telemetry with WebAssembly-heavy domains to surface potential exploitation attempts.

Monitoring Recommendations

Ingest browser crash and telemetry logs into a centralized SIEM for anomaly analysis.
Track process lineage from browser processes to detect post-exploitation payload execution.
Review web proxy logs for requests to .wasm resources served from low-reputation domains.

How to Mitigate CVE-2026-16363

Immediate Actions Required

Upgrade Firefox to version 153 or later and Firefox ESR to 140.13 or later on all managed endpoints.
Upgrade Thunderbird to version 153 or later, or to 140.13 or later on the ESR branch.
Prioritize patching for users who browse untrusted sites or handle high-risk email content.

Patch Information

Mozilla addressed CVE-2026-16363 in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13. Consult Mozilla Security Advisory MFSA-2026-68, MFSA-2026-70, MFSA-2026-71, and MFSA-2026-72 for full advisory details.

Workarounds

Where patching is delayed, disable WebAssembly by setting javascript.options.wasm to false in about:config.
Enforce strict site isolation and block execution of .wasm content from untrusted origins via web filtering.
Restrict browser use to trusted sites through enterprise policy until patches are deployed.

bash

# Firefox enterprise policy example to disable WebAssembly as a temporary workaround
# policies.json
{
  "policies": {
    "Preferences": {
      "javascript.options.wasm": {
        "Value": false,
        "Status": "locked"
      }
    }
  }
}