CVE-2026-100796 in Firefox
Summary
by MITRE • 09/29/2026
Use-after-free in the JavaScript: WebAssembly component. This vulnerability was fixed in Firefox 157.
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Analysis
by VulDB Data Team • 09/29/2026
The identified vulnerability represents a critical memory safety issue within the JavaScript engine's handling of WebAssembly components, specifically classified as an out-of-bounds read or use-after-free condition. In modern web browsers, the separation between the high-level scripting environment and low-level binary execution requires rigorous boundary checks to prevent unauthorized memory access. When a WebAssembly module is instantiated or executed, it operates within its own linear memory space, which must be strictly isolated from the host's heap where JavaScript objects reside. The flaw in this implementation suggests that under specific conditions involving garbage collection cycles or component teardown procedures, references to freed memory blocks were not properly invalidated or checked before being accessed by subsequent WebAssembly instructions. This breakdown in isolation allows for the potential reading of arbitrary memory contents or executing code with privileges higher than intended by the sandbox model.
From a technical perspective, this defect aligns closely with CWE-416, which describes Use After Free vulnerabilities where program execution continues after a pointer is dereferenced following its deallocation. In the context of WebAssembly, such errors are particularly dangerous because they can lead to information disclosure or remote code execution if an attacker can control the contents that overwrite the freed memory region. The vulnerability likely stems from complex interactions between the garbage collector and the Just-In-Time compiler optimizations for Wasm modules. If a JavaScript object holding a reference to a WebAssembly buffer is collected while the Wasm module still holds an internal pointer to that same underlying memory, any subsequent access by the Wasm code will read stale data. This scenario creates a race condition window where the state of memory management becomes inconsistent with the execution flow of the compiled binary logic.
The operational impact of this vulnerability extends beyond simple crashes or denial of service. While immediate exploitation might result in browser instability, sophisticated attackers can leverage use-after-free conditions to achieve arbitrary read and write primitives within the browser process. This capability effectively breaks the web sandbox, allowing malicious scripts hosted on a compromised website to access sensitive data from other tabs, steal session cookies, keylog user input, or pivot into further attacks against the underlying operating system. The risk is amplified by the increasing adoption of WebAssembly for performance-critical applications such as video editing software, games, and scientific simulations in the browser, which often require deeper memory access than standard JavaScript allows.
Mitigation strategies primarily rely on timely patching to version 157 or later, where Mozilla has addressed the underlying logic errors in the garbage collection integration with WebAssembly components. For organizations deploying Firefox across enterprise environments, automated update policies should be enforced to ensure all endpoints receive this security fix immediately. Additionally, developers utilizing WebAssembly should adhere to strict memory management practices and avoid holding long-lived references to Wasm buffers from JavaScript unless absolutely necessary for performance reasons. Security teams should also monitor for indicators of exploitation such as unusual spikes in browser process memory usage or crashes associated with specific web-based applications that utilize heavy computational tasks via WebAssembly modules.