CVE-2026-6759: Mozilla Firefox Use-After-Free Vulnerability

CVE-2026-6759 Overview

CVE-2026-6759 is a use-after-free vulnerability in the Widget: Cocoa component of Mozilla Firefox and Thunderbird. This memory corruption flaw occurs when the application references memory after it has been freed, potentially leading to application crashes or denial of service conditions. The vulnerability affects multiple versions of Firefox and Thunderbird, including their Extended Support Release (ESR) variants, and was addressed in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10.

Critical Impact

This use-after-free vulnerability can be exploited remotely without authentication, potentially causing application crashes and denial of service on affected macOS systems running vulnerable versions of Firefox or Thunderbird.

Affected Products

• Mozilla Firefox versions prior to 150
• Mozilla Firefox ESR versions prior to 140.10
• Mozilla Thunderbird versions prior to 150
• Mozilla Thunderbird ESR versions prior to 140.10

Discovery Timeline

• April 21, 2026 - CVE-2026-6759 published to NVD
• April 22, 2026 - Last updated in NVD database

Technical Details for CVE-2026-6759

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption issue that occurs when a program continues to use a pointer after the memory it references has been freed. In the context of CVE-2026-6759, the flaw exists within the Widget: Cocoa component, which is responsible for handling macOS-specific UI widget operations in Mozilla applications.

Use-after-free vulnerabilities are particularly dangerous because they can lead to unpredictable behavior when the freed memory is reallocated for other purposes. When the application attempts to access the previously freed memory, it may read or write to memory that now contains different data, leading to crashes, data corruption, or in some cases, code execution.

The vulnerability can be triggered remotely via network access, requires no user interaction, and does not require authentication, making it accessible to remote attackers. The primary impact is on availability, as successful exploitation leads to denial of service through application crashes.

Root Cause

The root cause of CVE-2026-6759 lies in improper memory management within the Widget: Cocoa component. The component fails to properly track object lifetimes, resulting in a scenario where a reference to a widget object persists after the underlying memory has been deallocated. When subsequent operations attempt to access this dangling reference, the application reads from invalid memory locations, triggering the use-after-free condition.

Attack Vector

The attack vector for this vulnerability is network-based. An attacker can craft malicious web content that triggers specific sequences of widget operations in the Cocoa component. When a victim visits a malicious webpage or opens a specially crafted email in Thunderbird, the vulnerability can be triggered, causing the application to crash.

The exploitation scenario involves:

1. Attacker hosts malicious content designed to manipulate widget object lifecycle
2. Victim accesses the malicious content through Firefox or Thunderbird
3. The Widget: Cocoa component processes the content, leading to premature memory deallocation
4. Subsequent access to the freed memory causes application crash

Technical details regarding the specific exploitation method can be found in the Mozilla Bug Report #2016164.

Detection Methods for CVE-2026-6759

Indicators of Compromise

• Unexpected Firefox or Thunderbird crashes on macOS systems, particularly when loading specific web content
• Application crash reports referencing the Widget: Cocoa component or related memory access violations
• Repeated browser crashes when users visit certain websites or open specific emails
• Memory access violation errors in application logs

Detection Strategies

• Monitor for unusual patterns of browser or email client crashes on macOS endpoints
• Implement application version tracking to identify systems running vulnerable Firefox or Thunderbird versions
• Deploy endpoint detection solutions capable of identifying memory corruption exploitation attempts
• Review crash dump data for signatures consistent with use-after-free exploitation

Monitoring Recommendations

• Enable crash reporting and centralized logging for all Firefox and Thunderbird installations
• Monitor network traffic for patterns associated with exploit delivery attempts
• Implement SentinelOne's behavioral AI to detect anomalous application behavior indicative of memory corruption attacks
• Track Mozilla security advisories for updates related to this vulnerability

How to Mitigate CVE-2026-6759

Immediate Actions Required

• Update Firefox to version 150 or later immediately
• Update Firefox ESR to version 140.10 or later
• Update Thunderbird to version 150 or later
• Update Thunderbird ESR to version 140.10 or later
• Prioritize patching macOS systems where the Cocoa widget component is utilized

Patch Information

Mozilla has released security patches addressing this vulnerability across multiple product lines. Organizations should consult the official Mozilla Security Advisories for comprehensive patch details:

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

The vulnerability is fixed in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10.

Workarounds

• If immediate patching is not possible, consider temporarily restricting access to untrusted web content on affected systems
• Implement network-level controls to block access to known malicious domains
• Configure content security policies to limit exposure to potentially malicious scripts
• Consider using alternative browsers on critical macOS systems until patches can be applied

# Check Firefox version on macOS
/Applications/Firefox.app/Contents/MacOS/firefox --version

# Check Thunderbird version on macOS
/Applications/Thunderbird.app/Contents/MacOS/thunderbird --version

# Verify installed version meets minimum patched version
# Firefox should be >= 150 or ESR >= 140.10
# Thunderbird should be >= 150 or ESR >= 140.10