Description

Incorrect boundary conditions in the JavaScript Engine: JIT component. This vulnerability was fixed in Firefox ESR 153.4 and Firefox 157.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "8.8",
  "pubDate": "2026-09-29T13:17:46.550Z",
  "pubdate": "2026-09-29T13:17:46.550Z",
  "executiveSummary": "This vulnerability involves incorrect boundary condition checks within the JavaScript Engine's Just-In-Time (JIT) compilation component in Mozilla Firefox.\nThe flaw allows an attacker to bypass memory safety guarantees, potentially leading to arbitrary code execution or out-of-bounds memory access.\nAffected products include Firefox ESR versions prior to 153.4 and standard Firefox versions prior to 157.\nThe vulnerability represents a critical risk as it enables attackers to execute malicious code within the browser's context by providing specially crafted JavaScript payloads.\nSuccessful exploitation requires the victim to visit a malicious website or interact with compromised web content that triggers the flawed JIT optimization logic.\nNo specific authentication is required for exploitation; the vulnerability is reachable through standard browser interactions.",
  "technicalDetails": "The root cause of this vulnerability lies in the JIT compiler's handling of boundary conditions during the optimization of JavaScript code. In the JIT component, the compiler attempts to optimize hot paths by predicting variable types and range constraints. An incorrect boundary condition check occurs when the JIT engine fails to properly validate the indices or lengths of array-like structures during speculative optimization.\nThe flaw specifically resides in the optimization phase where the engine generates machine code. When the JIT compiler generates code that assumes specific bounds on integer operations or array access, it may omit necessary runtime checks if it incorrectly determines that the bounds are implicitly safe. An attacker can manipulate this behavior by utilizing JavaScript patterns that force the JIT compiler to make incorrect assumptions about the range of an index or the size of a buffer.\nThe attack flow begins with the delivery of a malicious JavaScript payload designed to trigger the JIT engine's speculative optimization processes. By crafting a sequence of operations that repeatedly stresses the type and range inference mechanisms, the attacker forces the compiler to generate machine code that lacks the required bounds checks. Once the 'optimized' machine code is executed, the lack of validation allows for out-of-bounds reads or writes in the heap or stack memory allocated to the JavaScript engine.\nBy performing an out-of-bounds write, an attacker can overwrite critical data structures, such as pointers or function tables, to divert the execution flow. This control flow hijacking allows the attacker to execute arbitrary machine code with the privileges of the Firefox process. Given the nature of JIT vulnerabilities, this typically results in a transition from JavaScript-level control to native code execution.\nThis vulnerability affects Firefox ESR prior to 153.4 and Firefox versions prior to 157. Exploitation is remote and does not require local authentication. The post-exploitation impact includes complete system compromise relative to the browser's privilege level, potential sandbox escape, and theft of user data stored within the browser environment."
}