CVE-2026-74936 in Firefox
Summary
by MITRE • 08/18/2026
Use-after-free in the JavaScript: WebAssembly 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 • 08/19/2026
The identified vulnerability represents a critical memory safety issue within the JavaScript engine's handling of WebAssembly components, specifically manifesting as an use-after-free condition. This type of flaw occurs when program code attempts to access or manipulate memory that has already been deallocated by the system. In the context of Firefox and its underlying SpiderMonkey engine, this typically involves the complex lifecycle management of objects associated with WebAssembly modules, where pointers may remain valid in application logic even after the underlying memory regions have been returned to the heap allocator for reuse. When an attacker can trigger a scenario where these stale references are dereferenced, they gain unauthorized access to previously freed memory structures. This is particularly dangerous because modern browsers rely heavily on sandboxing and isolation mechanisms that assume such fundamental memory safety guarantees hold true across all execution contexts including those involving compiled WebAssembly code.
The technical root cause lies in the intricate interaction between the JavaScript garbage collector and the native C++ components responsible for managing WebAssembly linear memory and object references. During specific operations, such as module instantiation or function invocation within a WebAssembly component, reference counts may be decremented prematurely or not updated correctly when objects are moved or resized. This leads to a state where an internal pointer continues to point to a location in the heap that has been marked for deallocation but not yet overwritten by new data. An attacker who can control the input leading to this specific sequence of events can exploit this window of opportunity. By carefully crafting malicious WebAssembly code and JavaScript interactions, it is possible to force the browser into executing arbitrary native code or reading sensitive memory contents from other tabs or processes if sandbox boundaries are breached through subsequent exploitation steps like heap spraying or type confusion techniques that leverage the corrupted pointer state.
The operational impact of this vulnerability is severe due to its potential for remote code execution and privilege escalation within the browser's security model. If successfully exploited, a malicious website hosting compromised WebAssembly content could execute arbitrary commands on the victim's machine with the privileges of the user running the browser. This effectively bypasses the sandbox protections designed to contain web-based threats. The attacker could install malware, steal sensitive information such as cookies and session tokens stored in memory, or use the compromised browser instance as a pivot point for further network attacks. Given that WebAssembly is increasingly used for performance-critical applications on the web, including graphics rendering, video processing, and complex simulations, this vulnerability affects a wide range of modern web experiences. The risk is amplified by the fact that many users may not be aware they are executing compiled binary code within their browser environment, assuming it to be as safe as standard JavaScript execution which benefits from different security properties like same-origin policies but lacks the direct memory access risks inherent in lower-level languages unless properly sandboxed.
Mitigation strategies primarily focus on ensuring timely patching of the Firefox browser and its Extended Support Release versions. Users must update to Firefox version 154 or later, as well as Firefox ESR 140.14 and ESR 153.1 where this specific flaw has been addressed by Mozilla engineers through rigorous code review and memory safety improvements in the SpiderMonkey engine. For organizations managing large fleets of devices, automated deployment tools should be configured to push these updates immediately upon availability. Additionally, security teams can employ content security policies that restrict the loading of untrusted WebAssembly modules or disable WebAssembly execution entirely if it is not strictly required for business operations, thereby reducing the attack surface. Enabling browser features like ASan (Address Sanitizer) in development builds helps detect such issues early in the software lifecycle before they reach production releases.
From a classification perspective, this vulnerability aligns with CWE-416 which describes use-after-free errors, a category of bugs that are notoriously difficult to debug and exploit reliably but offer high impact when successful. In terms of adversary tactics as defined by MITRE ATT&CK, this flaw facilitates initial access via drive-by compromise or malicious web content delivery, potentially leading to execution through technique T1203 if the vulnerability allows for arbitrary code execution within the browser process. The exploitation chain likely involves techniques related to memory corruption and sandbox escape, which are critical components in advanced persistent threat campaigns targeting high-value environments. Continuous monitoring of browser update channels and strict adherence to patch management schedules remain the most effective defense against such zero-day or recently disclosed vulnerabilities that target core engine functionalities rather than just application-level logic flaws.