CVE-2026-12325: Mozilla Firefox DoS Vulnerability

CVE-2026-12325 Overview

CVE-2026-12325 is a denial-of-service vulnerability in the Graphics: ImageLib component of Mozilla Firefox and Thunderbird. The flaw is classified under CWE-400 (Uncontrolled Resource Consumption). A remote attacker can trigger the issue by getting a user to load crafted image content, causing the browser or mail client to consume excessive resources and become unresponsive. Mozilla addressed the issue in Firefox 152, Firefox ESR 140.12, Firefox ESR 115.37, Thunderbird 152, and Thunderbird 140.12.

Critical Impact
Remote attackers can crash or freeze Firefox and Thunderbird clients by delivering crafted image content through web pages or HTML email, disrupting availability for affected users.

Affected Products

Mozilla Firefox (versions prior to 152)
Mozilla Firefox ESR (versions prior to 140.12 and 115.37)
Mozilla Thunderbird (versions prior to 152 and 140.12)

Discovery Timeline

2026-06-16 - CVE-2026-12325 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2026-12325

Vulnerability Analysis

The vulnerability resides in the Graphics: ImageLib component, the image decoding subsystem shared across Mozilla products. Processing a crafted image consumes excessive resources, producing a denial-of-service condition affecting availability of the application. Exploitation requires user interaction, such as visiting a malicious web page in Firefox or rendering HTML content containing a remote image reference in Thunderbird.

The issue does not expose confidentiality or integrity, but its network attack vector and low complexity make mass delivery practical through advertisements, embedded resources, or phishing email. EPSS data places exploitation probability at a low level at publication time.

Root Cause

The defect is categorized as uncontrolled resource consumption [CWE-400] within ImageLib. A crafted image triggers a decode path that allocates or processes resources without adequate bounds, causing the renderer process to exhaust memory or CPU. Mozilla's bug tracker entry Mozilla Bug Report #2039443 contains the technical fix details.

Attack Vector

An attacker hosts or distributes a crafted image and induces a victim to load it. In Firefox, this can occur through navigation to an attacker-controlled page or through a third-party resource embedded in a benign page. In Thunderbird, the image can be referenced in HTML email, where rendering of remote content triggers the same decoder path.

No authentication is required, and no privileges are needed on the target system. The result is loss of availability for the application process handling the image.

Detection Methods for CVE-2026-12325

Indicators of Compromise

Repeated Firefox or Thunderbird process crashes or unresponsive states correlated with image-heavy web traffic or inbound HTML email.
High memory or CPU usage by firefox.exe, thunderbird.exe, or their content/renderer child processes without corresponding user activity.
Crash reports referencing the ImageLib or graphics decoding stack frames.

Detection Strategies

Monitor endpoint telemetry for abnormal browser and mail client process termination patterns across multiple users in a short window.
Inspect proxy and email gateway logs for repeated retrieval of identical image URLs preceding client instability.
Correlate Mozilla crash reporter submissions with the affected versions to identify exposed hosts.

Monitoring Recommendations

Track installed Mozilla product versions across the fleet and alert on hosts still running pre-patch builds.
Enable browser and mail client crash telemetry collection to your SIEM or data lake for trend analysis.
Review web filtering categories for image-hosting domains delivering unusually large or malformed payloads.

How to Mitigate CVE-2026-12325

Immediate Actions Required

Upgrade Firefox to version 152, Firefox ESR to 140.12 or 115.37, and Thunderbird to 152 or 140.12 as documented in the Mozilla Security Advisory MFSA-2026-57 through MFSA-2026-61.
Inventory endpoints and identify systems running vulnerable Mozilla builds before forcing the update cycle.
Communicate to users that opening untrusted HTML email or visiting untrusted pages can crash the client until patching completes.

Patch Information

Mozilla released fixes in Firefox 152, Firefox ESR 140.12, Firefox ESR 115.37, Thunderbird 152, and Thunderbird 140.12. Refer to the vendor advisories: MFSA-2026-57, MFSA-2026-58, MFSA-2026-59, MFSA-2026-60, and MFSA-2026-61.

Workarounds

In Thunderbird, disable automatic loading of remote content in messages to prevent passive triggering through email.
Use enterprise policy to block known malicious image-hosting domains at the web proxy or DNS layer until patches are deployed.
Restrict Firefox to managed extensions and enable strict content blocking modes to reduce exposure to third-party image resources.

bash

# Configuration example: Thunderbird enterprise policy to block remote content
# policies.json located in the distribution directory
{
  "policies": {
    "DisableTelemetry": false,
    "BlockAboutConfig": true,
    "Preferences": {
      "mailnews.message_display.disable_remote_image": {
        "Value": true,
        "Status": "locked"
      }
    }
  }
}