CVE-2019-9792 in Firefox
Summary
by MITRE
The IonMonkey just-in-time (JIT) compiler can leak an internal JS_OPTIMIZED_OUT magic value to the running script during a bailout. This magic value can then be used by JavaScript to achieve memory corruption, which results in a potentially exploitable crash. This vulnerability affects Thunderbird < 60.6, Firefox ESR < 60.6, and Firefox < 66.
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Analysis
by VulDB Data Team • 11/26/2025
The vulnerability described in CVE-2019-9792 represents a critical memory safety issue within the IonMonkey JIT compiler component of Mozilla's JavaScript engine. This flaw exists in the interaction between the optimized compilation process and the bailout mechanism that occurs when the JIT compiler encounters conditions it cannot handle efficiently. The vulnerability specifically targets the handling of internal magic values that should remain isolated within the compiler's internal operations but are inadvertently exposed to user-facing JavaScript code during exceptional execution paths.
The technical flaw manifests when the IonMonkey compiler performs a bailout operation, which is a process where the JIT compiler reverts to interpreted execution due to encountering complex or unexpected code patterns. During this bailout process, an internal JS_OPTIMIZED_OUT magic value intended for compiler-internal use is not properly sanitized or removed from the execution context before returning control to the JavaScript runtime. This magic value contains specific memory layout information that, when accessed through normal JavaScript operations, can be manipulated to create controlled memory access patterns that lead to memory corruption.
The operational impact of this vulnerability extends beyond simple crash conditions to potentially enable remote code execution exploits. When JavaScript code gains access to this magic value, it can leverage the information to craft precise memory manipulation attacks that may allow an attacker to overwrite critical memory locations, manipulate pointers, or corrupt heap structures. This creates a pathway for privilege escalation and system compromise, particularly since the affected software includes major web browsers and email clients that execute untrusted content from remote sources. The vulnerability affects versions of Firefox and Thunderbird that were widely deployed, making it a significant threat to end-user security.
This vulnerability aligns with CWE-125: Out-of-Bounds Read and CWE-787: Out-of-bounds Write, as it involves improper handling of memory access patterns during JIT compilation. From an ATT&CK framework perspective, this vulnerability maps to T1059.007: Command and Scripting Interpreter: JavaScript and T1203: Exploitation for Client Execution, representing how an attacker can exploit memory corruption to execute arbitrary code on target systems. The exploitation requires a sophisticated understanding of the JavaScript engine's internal workings and memory layout, typically achieved through browser-based attacks where malicious web content triggers the vulnerable code path. The affected versions indicate that this vulnerability was present for an extended period, making it a prime target for attackers who could leverage it in zero-day exploitation campaigns before the patch was released and deployed.
Organizations should prioritize immediate patching of all affected versions, implementing browser hardening measures, and monitoring for exploitation attempts. The vulnerability demonstrates the critical importance of maintaining strict isolation between internal compiler operations and user-accessible JavaScript execution contexts, as even seemingly benign internal values can become dangerous when improperly exposed to user code.