CVE-2025-1012 in Thunderbird
Summary
by MITRE • 02/04/2025
A race during concurrent delazification could have led to a use-after-free. This vulnerability was fixed in Firefox 135, Firefox ESR 115.20, Firefox ESR 128.7, Thunderbird 128.7, and Thunderbird 135.
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Analysis
by VulDB Data Team • 04/15/2026
This vulnerability represents a critical race condition affecting the delazification process within Mozilla Firefox and Thunderbird applications. The issue arises when multiple threads attempt to concurrently access and modify memory structures during the delazification phase, creating a scenario where memory allocated to an object may be freed while another thread still holds a reference to it. This fundamental concurrency flaw enables potential exploitation through use-after-free conditions that can lead to arbitrary code execution or application crashes. The vulnerability specifically impacts the memory management subsystem where lazy initialization processes are being concurrently terminated or modified.
The technical implementation of this vulnerability stems from inadequate synchronization mechanisms during concurrent operations within the browser engine's memory management system. When delazification occurs, objects that were previously lazily initialized undergo finalization processes that may involve memory deallocation. However, the race condition allows for a timing window where one thread frees an object while another thread simultaneously attempts to access that same object, resulting in a use-after-free scenario. This type of vulnerability falls under the CWE-362 category of race conditions and is particularly dangerous because it can be exploited to execute malicious code with the privileges of the affected application. The ATT&CK framework categorizes this as a memory corruption vulnerability that could enable privilege escalation and remote code execution.
The operational impact of this vulnerability extends across multiple Mozilla products including Firefox, Firefox ESR, and Thunderbird versions, affecting users who may be subjected to remote code execution attacks through malicious web content or email attachments. Attackers could leverage this vulnerability to bypass modern security mitigations such as address space layout randomization and data execution protection mechanisms. The exploitation typically requires sophisticated techniques to reliably trigger the race condition and may involve crafting specific memory access patterns that force the timing window necessary for successful exploitation. Organizations using affected versions face significant risk as the vulnerability can be exploited in the wild without user interaction in many scenarios.
Mitigation strategies for this vulnerability primarily involve immediate deployment of patched versions as recommended by Mozilla security advisories. System administrators should prioritize updating all affected installations to versions 135, 115.20, 128.7, or their respective latest releases. Additionally, organizations should implement network-level protections including web application firewalls and content filtering systems to reduce exposure to malicious content. The fix addresses the underlying race condition through improved locking mechanisms and synchronization primitives that prevent concurrent access to memory structures during delazification operations. Security monitoring should include detection of unusual memory access patterns and potential exploitation attempts that may indicate attempted exploitation of this vulnerability. Regular security assessments and penetration testing should verify that the patched implementations properly resolve the concurrency issues without introducing new stability concerns.