CVE-2026-2758: Mozilla Firefox Use-After-Free Vulnerability

CVE-2026-2758 Overview

CVE-2026-2758 is a use-after-free vulnerability in the JavaScript garbage collection (GC) component of Mozilla Firefox and Mozilla Thunderbird. The flaw is classified under CWE-416 and affects memory management routines during garbage collection cycles. Mozilla addressed the issue in Firefox 148, Firefox ESR 115.33, Firefox ESR 140.8, Thunderbird 148, and Thunderbird 140.8. An attacker who serves crafted JavaScript content to a vulnerable browser can trigger memory corruption that may lead to arbitrary code execution within the renderer process.

Critical Impact
Remote attackers can exploit this flaw through malicious web content with no authentication or user interaction beyond visiting a page, potentially achieving code execution in the browser context.

Affected Products

Mozilla Firefox (versions prior to 148)
Mozilla Firefox ESR (versions prior to 115.33 and 140.8)
Mozilla Thunderbird (versions prior to 148 and 140.8)

Discovery Timeline

2026-02-24 - CVE-2026-2758 published to NVD
2026-04-13 - Last updated in NVD database

Technical Details for CVE-2026-2758

Vulnerability Analysis

The vulnerability resides in the JavaScript garbage collector (GC) inside Mozilla's SpiderMonkey engine. A use-after-free condition occurs when the GC reclaims memory associated with a JavaScript object while another reference to that memory remains active. Subsequent access to the freed allocation reads or writes attacker-influenced data, corrupting engine state.

The Mozilla bug tracking entry Bug 2009608 is currently restricted while patches propagate to downstream consumers. Mozilla rates issues in this class as exploitable for remote code execution within the content process sandbox.

Root Cause

The root cause is improper object lifetime tracking during garbage collection. The GC fails to account for an outstanding reference held by a different execution context or runtime structure. When the collector frees the object, the dangling reference is later dereferenced, producing the classic use-after-free primitive described in CWE-416.

Attack Vector

Exploitation requires an attacker to deliver malicious JavaScript to the victim's browser. This is typically accomplished through a compromised website, a malicious advertisement, or attacker-controlled email content rendered by Thunderbird. The attacker crafts JavaScript that forces specific GC behavior, allocates and frees a target object, then reuses the freed memory to gain a read/write primitive. Successful exploitation runs code at the privilege level of the browser content process.

No verified public proof-of-concept code is available. Refer to Mozilla Security Advisory MFSA-2026-13 and related advisories MFSA-2026-14 through MFSA-2026-17 for vendor technical context.

Detection Methods for CVE-2026-2758

Indicators of Compromise

Unexpected child processes spawned by firefox.exe, thunderbird.exe, or platform equivalents shortly after browsing or message rendering activity.
Renderer or content process crashes with access-violation signatures referencing SpiderMonkey GC frames such as js::gc::*.
Outbound network connections from browser child processes to previously unseen infrastructure.
Browser version strings on endpoints reporting versions below Firefox 148, Firefox ESR 115.33/140.8, or Thunderbird 148/140.8.

Detection Strategies

Inventory installed Firefox and Thunderbird builds across the fleet and flag any host running a version below the patched releases.
Monitor process telemetry for browser processes performing code execution, file writes to startup locations, or credential store access.
Inspect crash reporting data for repeated SpiderMonkey GC faults that may indicate exploitation attempts in progress.

Monitoring Recommendations

Centralize browser crash and Watson-style telemetry to identify clustering of GC-related faults across users.
Alert on browser parent or child processes launching shell interpreters, scripting hosts, or LOLBins.
Track DNS and HTTP egress from browser processes to newly registered or low-reputation domains.

How to Mitigate CVE-2026-2758

Immediate Actions Required

Upgrade Firefox to version 148 or later on all endpoints.
Upgrade Firefox ESR deployments to 115.33 or 140.8, depending on the ESR channel in use.
Upgrade Thunderbird to 148 or 140.8 across all mail clients.
Force a browser restart after deployment to ensure the patched binaries are loaded.

Patch Information

Mozilla released fixes in Firefox 148, Firefox ESR 115.33, Firefox ESR 140.8, Thunderbird 148, and Thunderbird 140.8. Patch metadata is documented in Mozilla Security Advisory MFSA-2026-13, MFSA-2026-14, MFSA-2026-15, MFSA-2026-16, and MFSA-2026-17.

Workarounds

Disable JavaScript execution in Firefox or Thunderbird where feasible by setting javascript.enabled to false in about:config, accepting the loss of functionality on most modern sites.
Configure Thunderbird to render messages as plain text and block remote content to reduce the attack surface from email-borne JavaScript.
Restrict browser usage to trusted sites through enterprise URL filtering until patches are fully deployed.

bash

# Example: enforce minimum Firefox version through enterprise policy on Windows
# HKLM\SOFTWARE\Policies\Mozilla\Firefox
# DisableAppUpdate = 0
# Enable silent auto-update so endpoints receive 148+ automatically
reg add "HKLM\SOFTWARE\Policies\Mozilla\Firefox" /v DisableAppUpdate /t REG_DWORD /d 0 /f