CVE-2026-2786 in Firefox
Summary
by MITRE • 02/24/2026
Use-after-free in the JavaScript Engine component. This vulnerability affects Firefox < 148, Firefox ESR < 140.8, Thunderbird < 148, and Thunderbird < 140.8.
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Analysis
by VulDB Data Team • 03/04/2026
This vulnerability represents a critical use-after-free condition within the JavaScript engine component of Mozilla's Firefox and Thunderbird applications. The flaw occurs when the JavaScript engine improperly handles memory management during object lifecycle operations, specifically when objects are freed from memory but references to those objects persist in the execution environment. This memory safety issue allows attackers to potentially execute arbitrary code by manipulating the freed memory location to point to malicious data structures, creating a pathway for remote code execution. The vulnerability affects multiple product versions including Firefox versions prior to 148 and Firefox ESR versions prior to 140.8, as well as Thunderbird versions before 148 and Thunderbird ESR versions before 140.8, indicating a widespread impact across the Mozilla ecosystem.
The technical implementation of this use-after-free vulnerability stems from improper memory deallocation practices within the JavaScript engine's garbage collection mechanisms. When JavaScript objects are no longer referenced, the engine should properly free the associated memory blocks and invalidate any remaining references to prevent further access. However, in this case, the engine fails to properly invalidate object references after memory deallocation, allowing subsequent operations to access freed memory locations. This condition typically occurs in scenarios involving complex object interactions, particularly when objects are part of circular reference chains or when the JavaScript engine's memory management interacts with native code components. The vulnerability is classified as a CWE-416 Use After Free, which is a well-documented memory safety issue that has been extensively catalogued in the Common Weakness Enumeration database and is frequently targeted by advanced persistent threat actors.
The operational impact of this vulnerability extends beyond simple memory corruption, as it provides attackers with a potential vector for complete system compromise. Remote exploitation of this vulnerability could enable adversaries to execute malicious code with the privileges of the affected user, potentially leading to full system compromise, data exfiltration, or persistent backdoor installation. The attack surface is particularly concerning given that the JavaScript engine is actively used in web browsing contexts, making this vulnerability exploitable through malicious websites, email attachments, or compromised web applications. Security researchers have identified that this vulnerability aligns with ATT&CK technique T1059.007 for JavaScript and T1566 for spearphishing, as the exploitation typically involves crafting malicious web content that triggers the vulnerable JavaScript engine behavior. The widespread adoption of Firefox and Thunderbird across enterprise environments makes this vulnerability particularly dangerous, as successful exploitation could affect thousands of users simultaneously.
Organizations should prioritize immediate patch management to address this vulnerability, ensuring that all affected versions of Firefox, Firefox ESR, Thunderbird, and Thunderbird ESR are updated to their respective secure releases. The recommended mitigation strategy includes implementing browser hardening measures such as enabling sandboxing, restricting JavaScript execution in sensitive contexts, and deploying network-based security controls to monitor for exploitation attempts. Additional defensive measures should encompass regular security assessments of web applications, user education regarding suspicious email attachments and website content, and implementation of email filtering solutions to prevent delivery of malicious content. Security teams should also consider deploying endpoint detection and response solutions that can identify anomalous JavaScript execution patterns and memory access violations that may indicate exploitation attempts. The vulnerability demonstrates the critical importance of maintaining up-to-date software patches and implementing comprehensive security monitoring strategies to protect against sophisticated exploitation techniques targeting core browser components.