CVE-2026-109776 in Firefox
Summary
by MITRE • 10/13/2026
Race condition in the XPCOM component. This vulnerability was fixed in Firefox 158 and Firefox ESR 153.5.
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Analysis
by VulDB Data Team • 10/13/2026
The identified security issue resides within the XPCOM (Cross-Platform Component Object Model) subsystem of Mozilla Firefox, representing a critical race condition that arises from improper synchronization mechanisms during concurrent access to shared resources. XPCOM serves as the foundational architecture for many core browser components, managing object lifecycle and inter-process communication. When multiple threads or asynchronous operations attempt to read or modify state within this component without adequate locking or atomicity guarantees, a window of vulnerability opens where one thread may observe an inconsistent or partially updated state while another is in the middle of a modification operation. This specific flaw was addressed in Firefox version 158 and Firefox Extended Support Release (ESR) version 153.5, indicating that the underlying synchronization logic has been corrected to ensure data integrity under concurrent execution scenarios.
From a technical perspective, race conditions in component models like XPCOM often stem from missing mutex locks, incorrect use of non-atomic variables, or flawed assumptions about thread scheduling order. In this instance, the vulnerability likely allows an attacker who can trigger specific sequences of events within the browser environment to exploit the timing gap between check and update operations. Such exploitation could potentially lead to memory corruption, arbitrary code execution if the corrupted state influences pointer dereferences or function calls, or denial of service through crashes caused by accessing invalid memory addresses derived from inconsistent object states. The severity is heightened because XPCOM components often handle sensitive browser data and interact with lower-level system APIs, making successful exploitation a significant threat to both user privacy and system stability.
The operational impact of this vulnerability extends beyond simple application instability. If successfully exploited, it could allow an attacker to bypass security controls that rely on the integrity of internal state variables within XPCOM-managed objects. This might facilitate privilege escalation within the browser sandbox or enable data exfiltration by manipulating how information is processed before being sent over network connections or written to disk. Furthermore, because race conditions are inherently difficult to reproduce and debug, they pose a persistent risk even after initial patches if similar patterns exist elsewhere in the codebase. The fix implemented in versions 158 and ESR 153.5 likely involves introducing proper synchronization primitives such as mutexes or atomic operations to serialize access to the affected resources, thereby eliminating the race window and ensuring that all threads observe a consistent view of the data at any given point in time.
To mitigate risks associated with this class of vulnerabilities, organizations should ensure immediate deployment of Firefox 158 or ESR 153.5 across all endpoints. Beyond patching, developers working on similar component-based architectures should adhere to strict concurrency best practices, including minimizing shared mutable state and using thread-safe data structures where possible. Security testing frameworks should incorporate dynamic analysis tools capable of detecting race conditions through fuzzing with multi-threaded workloads. This vulnerability aligns with CWE-362 (Concurrent Execution Using Shared Resource with Improper Synchronization) in the Common Weakness Enumeration standard, highlighting the need for rigorous code review focused on thread safety and atomicity guarantees in high-concurrency environments like modern web browsers.