CVE-2026-2799: Mozilla Firefox Use-After-Free Vulnerability

CVE-2026-2799 Overview

CVE-2026-2799 is a use-after-free vulnerability in the DOM: Core & HTML component of Mozilla Firefox and Mozilla Thunderbird. The flaw is tracked under CWE-416 and allows network-based exploitation without authentication or user interaction. Mozilla addressed the issue in Firefox 148 and Thunderbird 148.

A remote attacker can leverage a crafted web page or HTML content to trigger memory corruption in the DOM engine. Successful exploitation can lead to arbitrary code execution within the browser process and full compromise of confidentiality, integrity, and availability.

Critical Impact

Remote, unauthenticated attackers can trigger use-after-free conditions in the Firefox and Thunderbird DOM engine through malicious web content, enabling potential arbitrary code execution.

Affected Products

• Mozilla Firefox versions prior to 148
• Mozilla Thunderbird versions prior to 148
• Any client rendering untrusted HTML content through the affected DOM: Core & HTML component

Discovery Timeline

• 2026-02-24 - CVE-2026-2799 published to the National Vulnerability Database (NVD)
• 2026-04-13 - Last updated in NVD database

Technical Details for CVE-2026-2799

Vulnerability Analysis

The vulnerability is a use-after-free condition [CWE-416] located in the DOM: Core & HTML component shared by Firefox and Thunderbird. Use-after-free occurs when code continues to reference memory after it has been released, allowing an attacker to influence the contents of that freed region.

Within a browser DOM implementation, freed objects can be reclaimed by attacker-controlled allocations through scripted DOM manipulation. Once the dangling pointer is dereferenced, the attacker can steer control flow toward attacker-supplied data. The result is typically arbitrary code execution within the content process, which is a common precursor to sandbox escape chains.

Mozilla classified the issue as critical in advisories MFSA-2026-13 and MFSA-2026-16 and tracked the underlying defect as Mozilla Bug 2014551.

Root Cause

The root cause is improper object lifetime management in the DOM: Core & HTML code path. A reference to a heap object is retained after the object has been freed, producing a dangling pointer that the rendering engine later dereferences. Mozilla has not published low-level technical details beyond the advisories listed above.

Attack Vector

Exploitation requires only that the victim load attacker-controlled HTML or script. In Firefox, visiting a malicious or compromised page is sufficient. In Thunderbird, rendering of remote HTML email content can serve as the trigger when scripting and remote content are processed. No privileges or explicit user interaction beyond loading content are required.

No public proof-of-concept or in-the-wild exploitation has been reported. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog, and the EPSS score at the time of analysis is low.

Detection Methods for CVE-2026-2799

Indicators of Compromise

• Firefox or Thunderbird crashreporter events referencing the DOM: Core & HTML component or use-after-free signatures
• Unexpected child process spawns from firefox.exe or thunderbird.exe, particularly shells or scripting interpreters
• Outbound network connections from the browser process to uncategorized or newly registered domains immediately after rendering untrusted content

Detection Strategies

• Inventory installed Firefox and Thunderbird versions across the estate and flag any host running a version earlier than 148
• Correlate browser crash telemetry with web proxy logs to identify URLs that consistently precede content-process crashes
• Apply EDR behavioral rules that alert on process injection or code execution originating from browser content processes

Monitoring Recommendations

• Forward browser crash data, process lineage, and DNS telemetry into a central data lake for retrospective hunting
• Monitor Thunderbird telemetry for remote content rendering events tied to HTML email messages
• Track patch deployment status for Firefox 148 and Thunderbird 148 through endpoint management tooling

How to Mitigate CVE-2026-2799

Immediate Actions Required

• Upgrade Firefox to version 148 or later on all managed endpoints, including Firefox ESR builds once corresponding fixed versions are available
• Upgrade Thunderbird to version 148 or later, prioritizing systems that process external HTML email
• Validate update status through endpoint management and remove or restrict outdated browser installations

Patch Information

Mozilla released fixes in Firefox 148 and Thunderbird 148. Refer to Mozilla Security Advisory MFSA-2026-13 and Mozilla Security Advisory MFSA-2026-16 for the authoritative list of fixed versions and bundled CVE identifiers. The underlying defect is tracked in Mozilla Bug 2014551.

Workarounds

• Disable JavaScript on untrusted sites using enterprise policy or extensions where feasible until patches are deployed
• Configure Thunderbird to block remote content in HTML email and prefer plain-text rendering for messages from untrusted senders
• Restrict browsing of unmanaged or high-risk web categories through a secure web gateway for unpatched hosts

# Verify installed Firefox and Thunderbird versions on Linux endpoints
firefox --version
thunderbird --version

# Example enterprise policy snippet (policies.json) to block remote content in Thunderbird
# Place at: <install-dir>/distribution/policies.json
{
  "policies": {
    "DisableMasterPasswordCreation": false,
    "BlockAboutConfig": true,
    "DisableTelemetry": true,
    "OfferToSaveLogins": false,
    "NetworkPrediction": false
  }
}