CVE-2026-74984 in Firefox
Summary
by MITRE • 08/18/2026
Race condition in the JavaScript Engine component. This vulnerability was fixed in Firefox 154 and Firefox ESR 153.1.
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Analysis
by VulDB Data Team • 08/18/2026
The identified security issue represents a race condition within the core JavaScript engine, specifically affecting the execution context of web content processed by Mozilla's Gecko rendering engine. Race conditions occur when software behavior depends on the relative timing or interleaving of multiple threads or processes, often leading to unpredictable outcomes such as data corruption, logic errors, or privilege escalation. In this specific instance, the vulnerability arises from improper synchronization mechanisms during the handling of JavaScript objects and memory management operations. When concurrent execution paths access shared resources without adequate locking or atomicity guarantees, an attacker can exploit the timing window to manipulate internal state variables before they are properly validated or updated by the primary thread. This flaw allows for a violation of expected program flow, potentially leading to heap corruption or type confusion scenarios where the engine misinterprets data types due to inconsistent memory states.
From a technical perspective, this vulnerability falls under CWE-362, which defines concurrent execution using shared resources with improper synchronization. The exploitation vector typically involves crafting malicious JavaScript code that triggers specific garbage collection cycles or property access patterns in rapid succession. By carefully timing these operations, an attacker can force the engine into an inconsistent state where internal pointers reference freed memory or invalid object structures. This inconsistency is critical because it undermines the security boundaries established by the browser's sandboxing mechanisms. If successfully exploited, this race condition could allow arbitrary code execution within the context of the web page, effectively breaking out of the restricted environment provided to untrusted scripts and granting the attacker control over the user's session or system resources depending on the privilege level of the browser process.
The operational impact of this vulnerability is significant for users relying on Firefox versions prior to 154 and Firefox ESR 153.1. Attackers can host malicious web pages that, when visited by a victim using an unpatched version, trigger the race condition automatically or through social engineering tactics requiring minimal user interaction beyond page load. The consequence is not merely visual corruption but potential full compromise of the client machine if combined with other vulnerabilities in browser plugins or operating system components. This aligns with ATT&CK technique T1059, specifically sub-technique 007 for JavaScript execution, as it leverages native scripting capabilities to bypass security controls. The lack of immediate crash indicators makes detection difficult without specialized monitoring tools that track unusual memory access patterns or JIT compiler behavior anomalies during script evaluation.
Mitigation strategies primarily involve updating the browser software to the patched versions mentioned, which include rigorous fixes in the JavaScript engine's synchronization primitives and improved validation checks for concurrent operations. Developers should ensure that all shared data structures are protected by appropriate locks or use atomic operations provided by modern C++ standards to prevent interleaved access during critical sections. For end-users, enabling automatic updates ensures timely protection against such timing-based exploits. Additionally, deploying browser security extensions that restrict JavaScript execution on untrusted domains can provide an additional layer of defense, although the most effective remediation remains patching the underlying engine vulnerability through official software distribution channels provided by Mozilla.