CVE-2026-4725: Mozilla Firefox Use-After-Free Vulnerability

CVE-2026-4725 Overview

CVE-2026-4725 is a use-after-free vulnerability in the Graphics: Canvas2D component of Mozilla Firefox and Thunderbird. The flaw enables a sandbox escape, allowing attackers to break out of the content process boundary. Mozilla addressed the issue in Firefox 149 and Thunderbird 149. The vulnerability carries a maximum CVSS score and requires no authentication or user interaction beyond visiting a malicious web page. The weakness is tracked under CWE-416 and is described in Mozilla Security Advisory MFSA-2026-20.

Critical Impact
Remote attackers can escape the Firefox content sandbox by exploiting freed memory in Canvas2D, gaining the ability to execute code with broader system privileges.

Affected Products

Mozilla Firefox versions prior to 149
Mozilla Thunderbird versions prior to 149
Mozilla Firefox ESR (per MFSA-2026-23)

Discovery Timeline

2026-03-24 - CVE-2026-4725 published to NVD
2026-04-13 - Last updated in NVD database

Technical Details for CVE-2026-4725

Vulnerability Analysis

The vulnerability resides in the Canvas2D rendering path within the Firefox graphics subsystem. A use-after-free condition occurs when code retains a reference to a Canvas2D object after its backing memory has been released. Subsequent operations on the dangling pointer let an attacker manipulate freed heap contents.

Because Canvas2D is reachable from any web page through standard <canvas> JavaScript APIs, an attacker can trigger the condition remotely. Successful exploitation breaks out of the content process sandbox, giving the attacker code execution outside the renderer's restricted boundary. The CVSS scope change reflects this cross-boundary impact on confidentiality, integrity, and availability.

Root Cause

The defect is an object lifetime management error in the Canvas2D component, classified as [CWE-416] Use After Free. The renderer frees graphics resources while other code paths still hold references to them. Mozilla's fix in Firefox 149 corrects the ownership and reference handling so that freed objects can no longer be reused. Details are tracked in Mozilla Bug 2017108.

Attack Vector

The attack vector is network-based and requires the victim to load attacker-controlled web content in a vulnerable Firefox or Thunderbird build. JavaScript executed on the page invokes Canvas2D operations in a sequence that frees an underlying object and then re-accesses it. After establishing a controlled heap layout, the attacker overwrites the freed allocation to corrupt program state and pivots through the sandbox escape primitive. No prior authentication or additional user interaction is needed beyond visiting the page.

No public proof-of-concept code is available at the time of writing. Refer to MFSA-2026-20 and MFSA-2026-23 for vendor technical detail.

Detection Methods for CVE-2026-4725

Indicators of Compromise

Firefox or Thunderbird child process crashes referencing Canvas2D, gfx, or SkiaGL modules in crash telemetry.
Unexpected spawning of child processes from firefox.exe or thunderbird.exe that perform file system or network operations outside normal browser behavior.
Browser sessions establishing outbound connections to uncategorized or newly registered domains shortly after rendering canvas-heavy pages.

Detection Strategies

Inventory all Firefox and Thunderbird installations and flag any build earlier than version 149 as vulnerable.
Hunt endpoint telemetry for browser content processes writing to disk locations outside their normal sandbox scratch directories.
Correlate web proxy logs with endpoint process trees to identify pages that immediately precede browser crashes or anomalous child process activity.

Monitoring Recommendations

Enable browser crash reporting and forward Windows Error Reporting or macOS crash logs to a central SIEM for review.
Alert on Firefox or Thunderbird parent processes spawning cmd.exe, powershell.exe, bash, or other shells, which is uncommon during normal browsing.
Track version compliance through software inventory tooling and generate alerts when unpatched builds reappear.

How to Mitigate CVE-2026-4725

Immediate Actions Required

Upgrade all Firefox installations to version 149 or later and all Thunderbird installations to version 149 or later.
Apply the corresponding Firefox ESR update referenced in MFSA-2026-23 for environments pinned to the extended support channel.
Restart browser and mail client processes after updating to ensure the patched binaries are loaded.

Patch Information

Mozilla released fixes in Firefox 149 and Thunderbird 149. The vendor advisory is published as MFSA-2026-20, with the ESR companion advisory at MFSA-2026-23. The underlying code change is tracked in Mozilla Bug 2017108.

Workarounds

If immediate patching is not possible, restrict browsing to trusted internal sites using web proxy allow-lists.
Disable JavaScript on untrusted sites through enterprise policy or extensions such as NoScript to block the Canvas2D code path.
Deploy enterprise policy to enforce auto-update so that future Firefox and Thunderbird releases install without user action.

bash

# Enforce Firefox auto-update via enterprise policies.json
# Place in the distribution directory next to the Firefox binary
{
  "policies": {
    "DisableAppUpdate": false,
    "AppAutoUpdate": true,
    "DisableJavaScriptOnUntrustedSites": true
  }
}