External risk intelligence

Mozilla JavaScript WebAssembly Invalid Pointer Vulnerability

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2026-100788

The vulnerability exists within the WebAssembly component of web browsers (Firefox and Thunderbird). While these are internet-connected applications, they are client-side software. Vulnerabilities in browser internals require the user to navigate to malicious content, and the component is not a public-facing server, gateway, or service exposed to the internet in a typical deployment sense.

Halo Surface Signal: 1 out of 5 — much less likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A critical vulnerability has been identified in the JavaScript WebAssembly component affecting certain Mozilla products. This flaw could allow for significant compromise of confidentiality, integrity, and availability if exploited. The main concern is confirming relevance and exposure to our specific technology stack.

Flaw in browser's WebAssembly component.
High impact if exploited, requires confirmation.
Assess relevance and potential exposure.

Attack Path

How an attacker could exploit the issue

An attacker could exploit an invalid pointer issue within the JavaScript WebAssembly component of affected browsers. This could occur if a user visits a malicious website or opens a specially crafted document, leading to the execution of arbitrary code.

No authentication or user interaction needed.
Malicious content triggers invalid pointer.
Allows arbitrary code execution.

Live Threat

Current exploitation, exposure, and threat context

When supported by the advisory, an invalid pointer in the WebAssembly component could allow an attacker to execute arbitrary code. This could affect system data and service behavior.

System data and service behavior at risk.
Arbitrary code execution when supported.
Potential for widespread system compromise.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability impacts the WebAssembly component within Firefox and Thunderbird. Ownership will likely fall to teams managing end-user computing, browser deployment, or application support. The initial practical step is to identify all instances of the affected browsers, confirm their reachability and criticality to business operations, and then engage the accountable owner to plan remediation during a maintenance window.

End-user computing or application teams own.
Verify browser reachability and criticality.
Plan remediation during maintenance windows.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

Free EASM Trial See How Signal Works

Frequently asked questions

What is the JavaScript WebAssembly component in Firefox and Thunderbird?

WebAssembly is a technology that allows high-performance code to run in a browser. It enables web applications to execute complex tasks, like video editing or games, at near-native speeds. In Firefox and Thunderbird, this component handles the execution of such code, essentially acting as a specialized engine within the software to bridge the gap between web content and your computer's hardware.

What does an invalid pointer vulnerability mean for CVE-2026-100788?

This vulnerability is classified as CWE-824, which refers to accessing an uninitialized pointer. Think of a pointer as a digital map that tells the software exactly where to find data in your computer's memory. Because this pointer is invalid, the software might mistakenly read from or write to the wrong memory location. This memory error can lead to system instability or allow an attacker to take control of the application.

How is the invalid pointer triggered in this CVE?

The flaw is triggered when the browser processes specific, maliciously crafted WebAssembly content. An attacker would typically need to lure a user to a compromised website or convince them to open a specially prepared document. Simply having the browser installed or running it in a safe environment without interacting with untrusted web content does not trigger this specific memory error.

Is my system at risk according to Halo Surface Signal?

Halo Surface Signal notes this vulnerability is unlikely to be triggered like a traditional server-side flaw. Because this is client-side software, it is not a service exposed to the internet. While your browser is internet-connected, the risk depends on your browsing habits, as you must navigate to malicious content for the vulnerability to be activated.

What steps should I take if I use affected Mozilla software?

Your first step is to verify the version of Firefox or Thunderbird currently running on your systems. Check for official updates released by Mozilla, as they have provided fixes in newer versions such as Firefox 157 and Thunderbird 157. Once identified, prioritize applying these updates to ensure your browser environment is protected against this memory-related vulnerability.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.