CVE-2026-100776 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 safety issue within the Mozilla Gecko rendering engine, specifically affecting the JavaScript WebAssembly component implementation. This flaw is classified as an out-of-bounds write resulting from use-after-free conditions, which occurs when the application continues to utilize a pointer after the memory it references has been deallocated or freed. In the context of WebAssembly execution, this typically involves scenarios where garbage collection mechanisms or explicit memory management routines release resources that are still actively referenced by running code or pending asynchronous operations. The root cause lies in improper tracking of object lifecycles during complex interactions between JavaScript and native C++ components handling Wasm modules. When a reference to an already freed object is accessed, the program may write data to arbitrary locations in memory, leading to unpredictable behavior that can compromise system stability and security integrity.
From a technical perspective, this vulnerability aligns with Common Weakness Enumeration identifier CWE-416, which denotes use-after-free errors. These types of flaws are particularly dangerous because they allow attackers to manipulate the heap layout or overwrite critical data structures such as function pointers or object vtables. By carefully crafting malicious WebAssembly modules and JavaScript payloads, an attacker can exploit this race condition or logic error to achieve arbitrary code execution within the browser process sandbox. The impact extends beyond simple application crashes; successful exploitation could lead to remote code execution on the victim's machine, potentially allowing full compromise of the user environment if additional containment mechanisms are bypassed. This aligns with MITRE ATT&CK techniques related to memory corruption and privilege escalation, specifically those involving dynamic binary modification or heap spraying attacks designed to control program flow through corrupted pointers.
The operational impact of this vulnerability is severe due to its potential for remote exploitation via standard web browsing activities. Users visiting compromised websites that serve malicious WebAssembly content could trigger the flaw without any additional interaction beyond loading the page. This makes it a high-risk vector for drive-by download attacks or cross-site scripting enhancements where memory corruption facilitates further payload delivery. The vulnerability affects multiple versions of Firefox, including ESR 153.4 and earlier, as well as Firefox 157 and prior releases up to ESR 140.16. Organizations relying on these versions for enterprise environments face significant exposure unless immediate remediation steps are taken. Security teams must prioritize patching efforts across all endpoints running affected browser versions to mitigate the risk of exploitation by threat actors leveraging automated scanning tools or targeted campaigns against specific industries.
Mitigation strategies primarily involve updating the Firefox installation to a version where this issue has been resolved, specifically ensuring that systems run at least Firefox ESR 153.4, Firefox 157, or Firefox ESR 140.17 or later. For environments unable to update immediately due to compatibility constraints, implementing strict content security policies can help restrict the execution of untrusted WebAssembly modules and limit JavaScript capabilities that might trigger the vulnerability. Additionally, enabling advanced browser sandboxing features and ensuring operating-level memory protection mechanisms such as Data Execution Prevention are active can reduce the likelihood of successful exploitation even if the flaw is triggered. Regular monitoring for new advisories from Mozilla regarding security updates is essential to maintain a robust defense posture against evolving web-based threats targeting browser engine components.