External risk intelligence

Firefox Windows Sandbox Escape Vulnerability

CVE advisorySeverity: CRITICAL (CVSS 10.0)

CVE-2025-2857

This vulnerability is located within the browser's internal Inter-Process Communication (IPC) code and requires a compromised child process to be triggered. As a client-side application feature, it is not a network-facing service or an externally reachable management interface.

Mozilla Firefox

before 136.0.4before 115.21.1128.1.0 to before 128.8.1

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 vulnerability has been identified in Firefox on Windows that could allow a compromised process to gain elevated privileges, potentially enabling a sandbox escape. While the original vulnerability was exploited in the wild, this specific pattern was found in Firefox's inter-process communication code. The primary concern is confirming if this specific flaw affects our deployed instances and understanding the potential exposure.

A bug could let one part of Firefox attack another.
This specific flaw could be a severe risk.
Confirm if Firefox on Windows is in use and exposed.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this by first compromising a child process within Firefox on Windows. This compromised process could then trick the parent process into returning a powerful handle, allowing the attacker to escape the browser's sandbox. This could lead to significant system compromise.

Entry condition: Compromised child process.
Trigger point: Unintentionally powerful handle returned.
Resulting risk: Sandbox escape and system compromise.

Live Threat

Current exploitation, exposure, and threat context

A vulnerability in Firefox on Windows could allow a compromised child process to gain elevated privileges, potentially leading to a complete escape from the browser's security sandbox. This could affect the integrity and confidentiality of user data and the overall system when the affected version of Firefox is running.

Compromised parent process functionality.
Unintentionally powerful handle returned.
Sandbox escape and system compromise.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability affects the Firefox browser on Windows, specifically related to its Inter-Process Communication (IPC) code. Owners of Firefox deployments on Windows should first identify all instances of the affected browser versions, determine their reachability and business criticality, and then coordinate with the vendor for remediation.

Identify Firefox on Windows deployments.
Verify browser reachability and criticality.
Plan vendor-coordinated remediation.

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 Firefox and how does it manage tasks?

Firefox is a widely used web browser that relies on a multi-process architecture to improve stability and security. It separates various tasks—like rendering webpages or handling plugins—into isolated child processes managed by a central parent process. This structure is intended to keep potentially unsafe web content contained within a restricted sandbox, preventing it from interacting directly with your operating system or sensitive local files.

What does CWE-668 mean for CVE-2025-2857?

CWE-668 represents the weakness of Exposure of Resource to Wrong Sphere. In the context of CVE-2025-2857, this means the browser's internal communication system mistakenly allowed a low-privilege child process to access a highly privileged handle belonging to the parent process. Because the boundary between these two areas was not properly enforced, the child process gained permissions it should never have had, effectively breaking the security sandbox.

How is this Firefox sandbox escape triggered?

An attacker must first successfully compromise a child process within the browser. Once they control that process, they can send specific commands that trick the parent process into handing over unauthorized, powerful access tokens. It is important to note that simply visiting a standard website does not trigger this; the attacker needs to establish a foothold by first subverting the browser's internal process isolation.

Do I need to worry if I use Firefox on Windows?

According to Halo Surface Signal, this vulnerability is very unlikely to be triggered over a network because it exists deep within internal Inter-Process Communication code. It is not an externally reachable service, meaning remote attackers cannot easily target it directly. Your primary concern should be local security, specifically ensuring your Windows workstations remain patched against processes that could lead to browser compromise.

When should I update my Firefox installation?

You should update immediately if you are running Firefox or Firefox ESR on a Windows system. Check your version against the fixed releases—136.0.4, 128.8.1, or 115.21.1—to see if your deployment is covered. The first step is to audit your environment to locate all Windows-based Firefox instances, then prioritize updating those machines to the latest version to eliminate the underlying communication flaw.

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.