CVE-2026-74976 in Firefox
Summary
by MITRE • 08/18/2026
JIT miscompilation in the JavaScript Engine: JIT component. This vulnerability was fixed in Firefox 154, Firefox ESR 140.14, and Firefox ESR 153.1.
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Analysis
by VulDB Data Team • 08/18/2026
The identified vulnerability represents a critical failure within the Just-In-Time compilation subsystem of the Mozilla JavaScript engine, specifically manifesting as a miscompilation error. This type of flaw occurs when the compiler generates machine code that deviates from the semantic intent of the original source code or bytecode instructions. In high-performance engines like SpiderMonkey, which powers Firefox and Thunderbird, the JIT compiler is responsible for translating dynamic JavaScript into optimized native CPU instructions to ensure rapid execution. When this translation process fails due to incorrect assumptions about variable types, control flow, or memory layout, it results in undefined behavior that can be exploited by malicious actors. The issue was addressed in recent updates including Firefox version 154 and the Extended Support Release versions 140.14 and 153.1, indicating a significant effort to restore correctness to the execution pipeline.
From a technical perspective, JIT miscompilations are particularly dangerous because they often bypass standard security checks that rely on predictable program behavior. Unlike memory corruption vulnerabilities such as buffer overflows which have clear boundaries, logic errors in compiled code can lead to arbitrary code execution or information disclosure without triggering traditional safeguards like stack canaries or address space layout randomization protections immediately. The flaw likely stems from an incorrect optimization pass where the compiler assumes a certain state of variables that does not hold true at runtime under specific conditions. This could involve type confusion, where the engine treats data as one type when it is actually another, leading to invalid memory accesses or execution of unintended instructions. Such errors are notoriously difficult to detect through static analysis alone and often require sophisticated fuzzing techniques targeting the JIT compiler's edge cases to reproduce reliably.
The operational impact of this vulnerability is severe, potentially allowing remote attackers to execute arbitrary code on a victim's system simply by visiting a crafted webpage. Since JavaScript runs in the browser sandbox, successful exploitation typically requires breaking out of that sandbox to gain access to the underlying operating system resources. However, JIT bugs often serve as reliable primitives for such escapes because they can manipulate memory structures in ways that confuse security boundaries. An attacker could leverage this miscompilation to achieve arbitrary read and write capabilities within the process memory space, which is a critical step toward full system compromise. This affects not only individual users but also organizations relying on Firefox-based browsers for secure web interactions, as it undermines trust in the browser's ability to isolate malicious content from sensitive data.
To mitigate this risk, immediate patching of all affected systems is essential. Administrators should ensure that Mozilla Firefox and Thunderbird are updated to version 154 or later, including ESR branches 140.14 and 153.1 respectively. These updates contain the necessary corrections to the JIT compiler logic that prevent the generation of incorrect machine code under the identified conditions. Beyond patching, organizations should consider implementing strict content security policies to limit the execution of inline scripts and restrict access to potentially dangerous APIs. Additionally, enabling automatic update mechanisms ensures that future fixes for similar vulnerabilities are applied promptly without requiring manual intervention from end-users or IT staff.
This vulnerability aligns with Common Weakness Enumeration categories such as CWE-471 which relates to modification of assumed immutable data, and more broadly falls under the domain of logic errors in software implementation. In terms of the MITRE ATT&CK framework, this flaw facilitates techniques associated with initial access via browser exploitation and potentially privilege escalation if sandbox escape is achieved. The presence of such bugs highlights the complexity involved in maintaining high-performance interpreters and compilers where performance optimizations must not compromise correctness or security guarantees. Continuous monitoring for similar reports from Mozilla's security team and adherence to recommended configuration baselines remain vital components of a robust defense strategy against browser-based attacks.