CVE-2026-74982: Firefox Widget Component DOS Vulnerability

CVE-2026-74982 Overview

CVE-2026-74982 is a denial-of-service vulnerability affecting the Widget component in Mozilla Firefox, Firefox ESR, and Thunderbird. The flaw allows remote attackers to exhaust resources on affected clients without authentication or user interaction, causing the application to become unresponsive. Mozilla addressed the issue in Firefox 154, Firefox ESR 153.1, Thunderbird 154, and Thunderbird 153.1. The weakness is categorized under [CWE-400] Uncontrolled Resource Consumption.

Critical Impact
Remote attackers can trigger denial-of-service conditions in the Widget component of Firefox and Thunderbird, disrupting browser and mail client availability across affected endpoints.

Affected Products

Mozilla Firefox versions prior to 154
Mozilla Firefox ESR versions prior to 153.1
Mozilla Thunderbird versions prior to 154 and prior to 153.1

Discovery Timeline

2026-08-18 - CVE-2026-74982 published to NVD
2026-08-20 - Last updated in NVD database

Technical Details for CVE-2026-74982

Vulnerability Analysis

The vulnerability resides in the Widget component, which handles native UI primitives shared across Mozilla's Gecko-based products. An attacker can craft malicious web content or email content that triggers uncontrolled resource consumption inside the Widget code path. The client process consumes memory or CPU until it becomes unresponsive or terminates. Because the flaw does not require authentication or user interaction beyond loading attacker-controlled content, exploitation can be delivered through any page a user visits or any HTML email rendered in Thunderbird.

Root Cause

The underlying weakness is uncontrolled resource consumption [CWE-400] within the Widget component. Mozilla's advisories (MFSA-2026-74, MFSA-2026-77, MFSA-2026-78, MFSA-2026-80) confirm the fix was applied across the affected product lines. Full technical details are tracked in Mozilla Bug #2051788.

Attack Vector

Exploitation occurs over the network. An attacker hosts malicious content on a website or sends an HTML email that, when rendered, invokes the vulnerable Widget code path. No privileges and no user interaction are required beyond content rendering. Successful exploitation impacts availability only; confidentiality and integrity are not affected.

No public proof-of-concept code has been published. See the Mozilla Security Advisory MFSA-2026-74 for vendor-supplied technical detail.

Detection Methods for CVE-2026-74982

Indicators of Compromise

Repeated crashes or hangs of firefox.exe, firefox, or thunderbird processes tied to specific URLs or inbound email messages.
Sustained high CPU or memory consumption by Firefox or Thunderbird processes without corresponding user activity.
Crash telemetry referencing the Widget component stack frames on unpatched builds.

Detection Strategies

Inventory Firefox, Firefox ESR, and Thunderbird versions across managed endpoints and flag builds below Firefox 154, Firefox ESR 153.1, or Thunderbird 154/153.1.
Correlate browser and mail client crash events with recently visited URLs or received messages to identify potential exploitation attempts.
Monitor endpoint telemetry for anomalous resource consumption patterns in Mozilla processes.

Monitoring Recommendations

Ingest Windows Application event logs and macOS/Linux crash reports into your SIEM to track Mozilla process terminations.
Alert on repeated crash-restart cycles of Firefox or Thunderbird on the same host within short time windows.
Track outbound web filtering logs for domains associated with suspicious client crashes.

How to Mitigate CVE-2026-74982

Immediate Actions Required

Update Firefox to version 154 or later on all managed endpoints.
Update Firefox ESR deployments to 153.1 or later.
Update Thunderbird to 154 or 153.1, depending on the deployed branch.
Prioritize systems that render untrusted web content or HTML email from external senders.

Patch Information

Mozilla shipped fixes in Firefox 154, Firefox ESR 153.1, Thunderbird 154, and Thunderbird 153.1. Refer to Mozilla Security Advisory MFSA-2026-74, MFSA-2026-77, MFSA-2026-78, and MFSA-2026-80 for release-specific patch notes.

Workarounds

Disable HTML rendering in Thunderbird and force plain-text display for messages from untrusted senders until patches are applied.
Restrict browsing to trusted sites through web filtering or enterprise browser policy where patch rollout is delayed.
Enforce automatic update channels for Firefox and Thunderbird via enterprise policy to reduce exposure windows on future advisories.

bash

# Example: enforce Firefox auto-update via enterprise policies.json
# Path: <FirefoxInstallDir>/distribution/policies.json
{
  "policies": {
    "DisableAppUpdate": false,
    "AppAutoUpdate": true,
    "OverrideFirstRunPage": "",
    "ManualAppUpdateOnly": false
  }
}