CVE-2026-109720: Firefox WebAssembly JIT RCE Vulnerability

CVE-2026-109720 Overview

CVE-2026-109720 is a Just-In-Time (JIT) compiler miscompilation flaw in the JavaScript WebAssembly component used by Mozilla Firefox and Thunderbird. The flaw causes the JIT engine to emit incorrect machine code when compiling specific WebAssembly inputs. Mozilla fixed the issue in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5. The vulnerability is documented across Mozilla Security Advisories MFSA-2026-106, MFSA-2026-108, MFSA-2026-109, and MFSA-2026-110. Additional technical context is available in Mozilla Bug Report #2074160.

Critical Impact

Miscompilation inside a browser JIT can corrupt program state, break type and memory safety invariants, and provide a primitive that attackers may escalate to arbitrary code execution within the content process.

Affected Products

• Mozilla Firefox versions prior to 158
• Mozilla Firefox ESR versions prior to 153.5
• Mozilla Thunderbird versions prior to 158 and prior to 153.5

Discovery Timeline

• 2026-10-13 - CVE-2026-109720 published to NVD
• 2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-109720

Vulnerability Analysis

The flaw resides in the WebAssembly tier of the SpiderMonkey JavaScript engine. During JIT compilation, the optimizer produces incorrect machine code for certain WebAssembly inputs. The resulting code violates assumptions that the runtime relies on for type and memory safety.

Miscompilation bugs in a browser JIT typically translate into exploitable primitives. An attacker who controls the input can steer the compiler into emitting code paths that read or write out of bounds, confuse value types, or skip required safety checks. Those primitives can then be chained toward sandboxed code execution in the renderer process.

The EPSS exploitation probability is low at publication, but JIT bugs in Firefox have historically been weaponized by exploit developers and commercial offensive vendors. Treat the issue as a browser renderer compromise risk until patches are deployed.

Root Cause

The root cause is an incorrect code generation decision inside the WebAssembly JIT pipeline. The emitted instructions do not match the semantics of the source WebAssembly module under specific conditions. Mozilla has not published the precise optimizer pass or instruction pattern at fault; see Mozilla Bug Report #2074160 for upstream details.

Attack Vector

Exploitation requires a victim to load attacker-controlled web content that delivers a crafted WebAssembly module. Standard drive-by browsing, malicious advertising, and compromised sites are viable delivery channels. Thunderbird is also affected because it renders web content using the same engine, although scripting is disabled in mail by default.

No verified public proof-of-concept code is available. Refer to Mozilla Security Advisory MFSA-2026-109 for vendor-authoritative impact information.

Detection Methods for CVE-2026-109720

Indicators of Compromise

• Firefox or Thunderbird plugin-container or content process crashes with signatures referencing the WebAssembly baseline or optimizing compiler.
• Unexpected child process spawns from firefox.exe or thunderbird.exe, such as command shells or script interpreters.
• Outbound connections from the browser process to newly registered or low-reputation domains shortly after visiting an untrusted site.

Detection Strategies

• Inventory installed Firefox, Firefox ESR, and Thunderbird versions across managed endpoints and flag any build older than the fixed releases listed by Mozilla.
• Hunt for anomalous process lineage where the browser renderer process executes LOLBins such as powershell.exe, cmd.exe, or rundll32.exe.
• Correlate browser crash telemetry with web navigation logs to surface sites that reliably trigger WebAssembly-related faults.

Monitoring Recommendations

• Forward endpoint process, module load, and network telemetry to a centralized analytics platform and retain long enough to support retrospective hunts.
• Monitor web proxy logs for WebAssembly content (application/wasm) delivered from uncategorized or newly observed domains.
• Track Mozilla security advisories and align patch SLAs with the browser risk profile of the environment.

How to Mitigate CVE-2026-109720

Immediate Actions Required

• Upgrade Firefox to 158 or later, Firefox ESR to 153.5 or later, Thunderbird to 158 or later, and Thunderbird ESR to 153.5 or later.
• Push the update through enterprise management tooling and validate the deployed build on representative endpoints.
• Restart browser and mail client sessions after patching to ensure the vulnerable JIT code is unloaded from memory.

Patch Information

Mozilla addressed the issue in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5. Fix details and affected component information are published in Mozilla Security Advisory MFSA-2026-106, MFSA-2026-108, MFSA-2026-109, and MFSA-2026-110.

Workarounds

• Where patching is delayed, disable WebAssembly in Firefox by setting javascript.options.wasm to false in about:config. Validate business application compatibility before deploying this change broadly.
• Restrict browsing to trusted sites using enterprise web filtering until patches are rolled out.
• Keep Thunderbird's default setting that disables remote content and JavaScript in messages to reduce exposure through mail.

# Enterprise policy snippet to disable WebAssembly in Firefox via policies.json
{
  "policies": {
    "Preferences": {
      "javascript.options.wasm": {
        "Value": false,
        "Status": "locked"
      }
    }
  }
}