CVE-2026-6749: Mozilla Firefox Information Disclosure Bug

CVE-2026-6749 Overview

CVE-2026-6749 is an uninitialized memory use vulnerability in the Graphics: Canvas2D component of Mozilla Firefox and Thunderbird. This flaw allows for information disclosure when the browser fails to properly initialize memory before use in the Canvas 2D rendering context. Attackers can exploit this vulnerability remotely via malicious web content to potentially leak sensitive data from process memory.

Critical Impact

Uninitialized memory exposure in Canvas2D rendering can leak sensitive browser memory contents to attackers through specially crafted web pages, potentially exposing credentials, session tokens, or other sensitive data processed by the browser.

Affected Products

• Mozilla Firefox (versions prior to 150)
• Mozilla Firefox ESR (versions prior to 115.35 and 140.10)
• Mozilla Thunderbird (versions prior to 150 and 140.10)

Discovery Timeline

• April 21, 2026 - CVE-2026-6749 published to NVD
• April 22, 2026 - Last updated in NVD database

Technical Details for CVE-2026-6749

Vulnerability Analysis

This vulnerability is classified as CWE-908 (Use of Uninitialized Resource). The flaw exists in the Canvas 2D graphics rendering component of Mozilla's browser engine. When certain Canvas2D operations are performed, the rendering engine allocates memory buffers that are not properly initialized before being used. This uninitialized memory may contain residual data from previous allocations, including potentially sensitive information that was stored in that memory region by other browser processes or previously visited websites.

The network attack vector allows remote exploitation through malicious web content without requiring user interaction beyond visiting a compromised or malicious webpage. No special privileges are required for exploitation, making this vulnerability particularly dangerous for drive-by attacks targeting browser users.

Root Cause

The root cause of CVE-2026-6749 lies in improper memory initialization within the Canvas2D graphics component. When the rendering engine allocates memory for canvas operations, it fails to zero-initialize or otherwise sanitize the memory buffer before use. This oversight allows data from previous memory allocations to persist and potentially be read by JavaScript code operating on the canvas, enabling information disclosure attacks.

Attack Vector

The vulnerability is exploited through network-based attacks. An attacker can craft a malicious webpage containing JavaScript that performs specific Canvas2D operations designed to trigger the uninitialized memory read condition. The attack flow involves:

1. Victim visits a malicious or compromised website
2. JavaScript on the page creates a Canvas2D context and performs specific rendering operations
3. The browser allocates memory for canvas operations without proper initialization
4. The attacker's JavaScript reads pixel data from the canvas, extracting leaked memory contents
5. Sensitive data from uninitialized memory is exfiltrated to the attacker

This vulnerability affects the confidentiality of data processed by the browser, as uninitialized memory may contain fragments of passwords, session cookies, form data, or other sensitive information.

Detection Methods for CVE-2026-6749

Indicators of Compromise

• Unusual Canvas2D API usage patterns in web traffic or browser logs
• JavaScript attempting to read large amounts of pixel data from canvas elements
• Network traffic exfiltrating encoded image or binary data to suspicious external domains
• Browser crash dumps indicating memory access violations in graphics components

Detection Strategies

• Monitor for anomalous JavaScript execution patterns involving Canvas2D getImageData() calls
• Deploy web proxy rules to detect and block known exploit patterns targeting Canvas2D vulnerabilities
• Implement browser telemetry monitoring for unusual graphics subsystem behavior
• Use endpoint detection solutions to identify browsers running vulnerable versions

Monitoring Recommendations

• Enable browser crash reporting to identify potential exploitation attempts
• Monitor network traffic for base64-encoded data exfiltration following canvas operations
• Track browser version deployments across the organization to identify vulnerable installations
• Review web application firewall logs for requests containing suspicious canvas-related payloads

How to Mitigate CVE-2026-6749

Immediate Actions Required

• Update Mozilla Firefox to version 150 or later immediately
• Update Mozilla Firefox ESR to version 115.35 or 140.10 or later
• Update Mozilla Thunderbird to version 150 or 140.10 or later
• Consider temporarily disabling JavaScript on untrusted sites until patching is complete

Patch Information

Mozilla has released security patches addressing this vulnerability across multiple product lines. The fixes are documented in the following security advisories:

• Mozilla Security Advisory MFSA-2026-30
• Mozilla Security Advisory MFSA-2026-31
• Mozilla Security Advisory MFSA-2026-32
• Mozilla Security Advisory MFSA-2026-33
• Mozilla Security Advisory MFSA-2026-34

Additional technical details can be found in Mozilla Bug Report #2022610.

Workarounds

• Disable JavaScript in browser settings to prevent exploitation via malicious web content
• Use browser extensions to restrict Canvas2D API access on untrusted websites
• Enable strict site isolation features in browser security settings
• Consider using network-level filtering to block access to known malicious domains

# Firefox about:config workaround - Disable Canvas2D
# Navigate to about:config and set:
# gfx.canvas.azure.accelerated = false
# This may impact performance but reduces attack surface