Mozilla’s Firefox 159 release, published on October 27, 2026, addresses 167 security vulnerabilities across the browser.
The security advisory identifies 108 High-severity, 52 Moderate-severity, and seven Low-severity vulnerabilities. Unlike the Chrome release discussed separately, Mozilla does not classify these findings as Critical.
The volume of fixes is significant, but the number alone does not determine the practical risk. Vulnerability severity, affected components, exploitability, browser exposure, and the availability of patches all influence remediation priorities.
Several of the most security-relevant findings involve memory safety, browser isolation, and privilege boundaries.
Vulnerabilities That Deserve Attention
1. CVE-2026-117861: Sandbox Escape in DOM Navigation
A browser sandbox restricts what content running within a browser process can access or execute.
A sandbox escape can undermine those restrictions, potentially allowing an attacker to cross a security boundary that should separate browser content from more privileged resources.
This vulnerability affects the DOM Navigation component.
The exact exploitation requirements and resulting impact should be assessed using Mozilla’s advisory. A sandbox escape does not, by itself, establish that an attacker can compromise the entire operating system.
2. CVE-2026-117860: Same-Origin Policy Bypass
The same-origin policy is a foundational browser security mechanism. It restricts how documents and scripts from one origin interact with resources belonging to another.
A bypass can undermine those restrictions and potentially expose information or functionality that should remain isolated between websites.
This is particularly relevant to users signed in to multiple web applications, where browser isolation helps protect authenticated sessions and sensitive data.
The precise impact depends on the affected navigation behaviour and the conditions required to trigger the flaw.
3. CVE-2026-117862: Privilege Escalation in Networking
Privilege boundaries prevent components from performing operations beyond their intended permissions.
Mozilla’s advisory identifies a privilege-escalation issue associated with incorrect boundary conditions in the Networking component.
Incorrect boundary handling can cause software to permit an operation that should have been rejected. The security consequences depend on the permissions involved and the component’s execution context.
This issue reinforces the importance of treating browser networking components as part of the security boundary rather than merely as connectivity infrastructure.
4. CVE-2026-117864: Sandbox Escape in Form Manager
Form Manager handles browser functionality associated with web forms.
A security defect in this area becomes particularly concerning when it undermines the isolation enforced by the browser sandbox.
Sandbox escapes can potentially allow access beyond the intended execution boundary. However, the available advisory information should guide any conclusions about exploitability, prerequisites, and impact.
5. CVE-2026-117865: Use-After-Free in JavaScript Garbage Collection
JavaScript engines manage objects and memory while executing scripts from web applications.
A use-after-free condition occurs when software continues to reference memory after it has been released. Such defects can cause crashes or memory corruption and may, under suitable conditions, be exploitable for code execution.
Memory-safety vulnerabilities in JavaScript processing deserve particular attention because browsers routinely execute scripts from websites.
The actual impact depends on the specific defect, memory layout, mitigations, and exploit conditions.
6. CVE-2026-117866: Privilege Escalation in Canvas2D
Canvas2D provides web applications with graphics-rendering functionality.
Mozilla identifies a use-after-free issue in Canvas2D associated with privilege escalation.
This combines a memory-lifetime defect with a security-impact classification that warrants careful attention. Nevertheless, the precise escalation path should not be inferred without further technical details from the vendor.
Why These Vulnerabilities Matter
The highlighted findings affect different browser security mechanisms:
- Memory safety: Helps prevent invalid memory access and potential memory corruption.
- Same-origin enforcement: Restricts interactions between different website origins.
- Sandbox isolation: Limits the actions available to browser content.
- Privilege boundaries: Prevent components from exceeding their intended permissions.
These mechanisms work together. A weakness in one area can sometimes increase the impact of a weakness in another, although the vulnerabilities listed in this advisory should not automatically be treated as a single exploit chain.
Mozilla’s release information does not establish that these particular vulnerabilities are being actively exploited in the wild. The priority is to apply the available fixes rather than speculate about attack scenarios.
Affected Versions and Remediation
Firefox 159 addresses the vulnerabilities covered by the October 27 advisory.
Organisations should also assess the relevant Firefox Extended Support Release (ESR) branches and other Mozilla products covered by their software inventory. The appropriate fixed version depends on the product and release channel.
Enterprise security teams should prioritise managed endpoints, validate patch deployment, and confirm that unsupported browser installations are addressed.
