CVE-2026-109733 in Firefox
Summary
by MITRE • 10/13/2026
JIT miscompilation in the JavaScript Engine: JIT component. This vulnerability was fixed in Firefox 158, Firefox ESR 115.43, and Firefox ESR 153.5.
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Analysis
by VulDB Data Team • 10/13/2026
The identified vulnerability represents a critical failure within the Just-In-Time compilation subsystem of the browser's JavaScript engine, specifically manifesting as a miscompilation error. In modern web browsers, the JIT compiler is responsible for translating high-level JavaScript code into optimized machine code to ensure rapid execution and responsive user experiences. This process involves complex static analysis, type inference, and optimization passes designed to eliminate redundant operations or inline functions. However, in this specific instance, the optimizer incorrectly transformed valid source code into semantically incorrect machine instructions. Such miscompilations are particularly dangerous because they bypass standard runtime checks that would normally catch logical errors, allowing flawed logic to execute with full privileges within the sandboxed environment of the browser process.
From a technical perspective, JIT miscompilation often stems from edge cases in type inference or assumptions made during optimization passes that do not hold true for all possible input states. When the compiler incorrectly assumes a variable's type or state, it may remove necessary bounds checks, null pointer validations, or security-critical guards. This creates a discrepancy between what the JavaScript specification dictates should happen and what actually occurs at the machine code level. The flaw is inherently non-deterministic in its exploitation potential depending on the specific JIT optimization path taken by the engine for different inputs, making it difficult to reproduce consistently without precise control over the execution context or compiler flags.
The operational impact of this vulnerability is severe, primarily because JavaScript engines operate with significant privileges relative to web content restrictions. A successful exploit could allow an attacker to achieve arbitrary code execution within the browser process sandbox. While modern browsers employ multiple layers of defense such as WebAssembly sandboxing and site isolation, a JIT miscompilation can potentially bypass these controls by corrupting memory structures or manipulating control flow in ways that lead to information disclosure or further privilege escalation steps. This effectively neutralizes the security boundaries intended to isolate malicious web content from the underlying operating system resources.
This type of vulnerability is commonly categorized under CWE-754: Improper Check for Unusual or Exceptional Conditions, as it involves a failure to correctly handle specific execution paths that lead to incorrect program behavior. Furthermore, in the context of attack techniques, this aligns with ATT&CK technique T1059: Command and Scripting Interpreter, specifically through JavaScript exploitation vectors where malicious scripts leverage engine flaws to execute arbitrary commands or exfiltrate data from the victim's session. The miscompilation serves as a foundational primitive for more complex exploits that might aim to escape the browser sandbox entirely.
Mitigation strategies rely heavily on timely patch deployment. Users must update their browsers to Firefox version 158, Firefox Extended Support Release (ESR) version 115.43, or ESR version 153.5, as these releases contain the corrected JIT compiler logic that resolves the specific optimization bug responsible for the miscompilation. For organizations managing large fleets of browsers, automated patch management systems should be configured to prioritize this update due to its severity. Additionally, enabling strict Content Security Policy (CSP) headers and disabling unnecessary JavaScript features can reduce the attack surface by limiting the types of scripts that are allowed to execute within the browser environment, thereby mitigating the risk even if similar vulnerabilities were present in older versions.