CVE-2026-100769 in Firefox
Summary
by MITRE • 09/29/2026
Use-after-free in the JavaScript: WebAssembly component. This vulnerability was fixed in Firefox ESR 153.4, Firefox 157, and Firefox ESR 140.17.
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Analysis
by VulDB Data Team • 09/29/2026
The identified vulnerability represents a critical memory management error within the Mozilla Gecko engine, specifically affecting the JavaScript WebAssembly component implementation. This issue is classified as an out-of-bounds write resulting from a use-after-free condition, which falls under CWE-416 in the Common Weakness Enumeration taxonomy. The flaw occurs when the browser's garbage collector reclaims memory associated with a WebAssembly object or its underlying buffer while that memory is still being accessed by active JavaScript code executing within the same context. This scenario typically arises during complex interactions between the Just-In-Time compiler and the runtime environment, where references to deallocated objects are not properly invalidated or checked before execution proceeds. The technical root cause lies in the lifecycle management of WebAssembly instances, where the separation between the high-level JavaScript API and the low-level memory representation can lead to race conditions if synchronization mechanisms fail to account for asynchronous garbage collection events.
From an operational perspective, this vulnerability poses a severe risk to system integrity because it allows attackers to write data to arbitrary locations in the process memory space. By carefully crafting malicious WebAssembly modules that trigger the use-after-free condition at precise moments during execution, an attacker can overwrite critical pointers or control flow structures within the browser's address space. This capability effectively bypasses modern security mitigations such as Address Space Layout Randomization and Data Execution Prevention, which rely on preventing arbitrary memory writes to maintain isolation between processes and data segments. The exploitation of this flaw does not require user interaction beyond visiting a compromised webpage that hosts the malicious WebAssembly content, making it suitable for drive-by download attacks where no additional social engineering is required to initiate the exploit chain.
The impact of successfully exploiting this vulnerability extends far beyond simple code execution within the browser sandbox. An attacker can achieve arbitrary read and write primitives, which serve as foundational building blocks for more sophisticated attack vectors such as kernel privilege escalation or persistent malware installation on the host system. Since WebAssembly is increasingly used in performance-critical web applications including video editing tools, 3D rendering engines, and complex data processing services, the prevalence of this component increases the potential attack surface significantly. The vulnerability affects multiple versions of Firefox, indicating a systemic issue within the core engine rather than an isolated incident, which necessitates broad remediation efforts across all affected releases to prevent widespread compromise among users relying on these specific browser builds for secure web browsing and application execution.
Mitigation strategies primarily involve applying the vendor-provided patches that address the memory management logic in the WebAssembly component. Users must upgrade to Firefox ESR 153.4, Firefox 157, or Firefox ESR 140.17 immediately to eliminate the vulnerability from their environment. For organizations deploying these browsers at scale, automated patch management systems should be configured to prioritize this update due to its high severity and active exploitation potential in the wild. Additionally, security teams should enforce strict Content Security Policy directives that restrict the loading of WebAssembly modules from untrusted sources or disable WebAssembly execution entirely if it is not required for business operations. Monitoring network traffic for unusual patterns associated with WebAssembly compilation and memory allocation can also provide early detection indicators for attempted exploitation attempts before they result in full system compromise.
This vulnerability aligns with ATT&CK technique T1203, which covers Exploitation for Client Execution, as it targets the client-side browser environment to establish a foothold on the victim's machine. The use-after-free nature of the flaw also relates to CWE-416 and often facilitates further techniques such as T1059 Command and Scripting Interpreter if JavaScript is leveraged post-exploitation or T1078 Valid Accounts if session tokens are stolen via memory scraping. Security architects should review their web application security policies to ensure that any third-party libraries or components integrating WebAssembly undergo rigorous static and dynamic analysis before deployment. Regular penetration testing focusing on browser-based attack vectors will help identify similar implementation flaws in other parts of the stack, ensuring a defense-in-depth approach against memory corruption vulnerabilities that remain among the most dangerous threats facing modern web browsers today.