External risk intelligence

Sandbox Escape in Security Process Sandboxing Component

CVE advisorySeverity: CRITICAL (CVSS 9.6)

CVE-2026-100760

This vulnerability affects a process sandboxing component within a web browser and email client. Sandbox escapes require the attacker to have already compromised the application's initial process, which occurs on the client-side. The vulnerability is not an internet-facing service or listener, and is not designed for external network exposure.

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 security component responsible for process sandboxing within certain browser and email applications. This issue could potentially allow an attacker to break out of a restricted environment, which might lead to broader system compromise. The primary concern at this stage is to determine if our organization utilizes the affected software and, if so, to what extent.

Unrestricted access from a protected environment.
High impact if exploited, needs verification.
Confirm relevance and potential exposure.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this by tricking a user into visiting a malicious website or opening a crafted email. This interaction would involve the browser's security sandbox, a component designed to isolate potentially harmful code. If successful, the vulnerability could allow the attacker to break out of this isolated environment, leading to significant compromise.

No authentication required to trigger.
User interaction with malicious content.
Sandbox escape leading to system compromise.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability in the process sandboxing component could allow an attacker to break out of the isolated environment when a user interacts with a malicious site or email. When supported by the advisory, this could potentially affect system data and user data.

System and user data could be compromised.
Malicious content could trigger a sandbox escape.
Sensitive information exposure is possible.

Operational Fix

Recommended remediation, mitigation, and detection steps

Real-world action for this vulnerability requires identifying where Firefox and Thunderbird are deployed, assessing their exposure and criticality, and then coordinating remediation with the relevant platform or application owners. The primary step is to locate all instances, determine their business impact and reachability, and then assign ownership for the fix.

Platform and application teams own remediation.
Verify user exposure and business criticality first.
Plan and coordinate updates 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.

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Frequently asked questions

What is the Process Sandboxing component in Firefox and Thunderbird?

This component acts as a security boundary that isolates web content or email processing from the underlying operating system. It creates a restricted environment, often called a sandbox, designed to contain potentially malicious code. By limiting what a browser or email client can access on your computer, it prevents a single site or message from gaining full control over your machine if it contains harmful scripts.

What does CWE-693 mean for CVE-2026-100760?

CWE-693 refers to protection mechanism failures. In the context of this CVE, it means the sandbox's wall failed to hold. A sandbox escape is a serious weakness where code that should have been trapped inside the browser or email client manages to bypass those restrictions, effectively breaking the security barrier intended to keep your system and its data isolated from untrusted activity.

How does an attacker trigger this sandbox escape?

The attacker requires you to perform a specific action, such as clicking a link to a malicious website or opening a specially crafted email. Simply having the software installed is not enough; the vulnerability is triggered only when the application processes the malicious content. It does not trigger if you are not actively viewing a compromised site or interacting with a malicious message.

Is my system at risk of internet-facing exposure from this?

According to Halo Surface Signal, this vulnerability is not an internet-facing service or listener. Because it lives within client-side applications like browsers and email clients, it does not create a direct network listener that an attacker can scan for on the open internet. Risk is primarily tied to user behavior and the specific content users choose to view.

Do I need to update my browser or email client immediately?

Yes, your first step is to identify all installations of Firefox and Thunderbird within your environment. Verify which systems are running versions older than the patches provided by Mozilla. Once you have an inventory, coordinate with your team to apply the updates—specifically Firefox ESR 153.4, Thunderbird 157, or equivalent versions—during your next standard maintenance window.

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.