CVE-2026-84121 in Firefox
Summary
by MITRE • 09/01/2026
Sandbox escape due to use-after-free in the DOM: Security component. This vulnerability was fixed in Firefox 155, Firefox ESR 115.40, Firefox ESR 140.15, and Firefox ESR 153.2.
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Analysis
by VulDB Data Team • 09/01/2026
The identified security flaw represents a critical memory management error within the Document Object Model implementation of Mozilla Firefox browsers, specifically categorized as an use-after-free vulnerability in the DOM Security component. This type of defect occurs when the application continues to reference a memory block after it has been deallocated or freed by the system. In the context of web browser engines, such errors are particularly dangerous because they allow maliciously crafted web content to manipulate how the browser handles internal data structures during rendering and script execution. The vulnerability stems from improper lifecycle management of DOM objects where references persist beyond their valid existence window, creating a race condition or logic error that leaves dangling pointers accessible to attacker-controlled code paths.
From an operational perspective, this flaw enables potential sandbox escape scenarios, which is among the most severe consequences for browser-based vulnerabilities. A successful exploitation would allow remote attackers to break out of the restricted execution environment imposed by the browser's security model and execute arbitrary code with the privileges of the user running the application. This effectively neutralizes key defense-in-depth mechanisms such as site isolation and content sandboxing, which are designed to limit the blast radius of a compromise. An attacker could leverage this flaw to install malware, steal sensitive credentials stored in the browser, or pivot into internal network resources depending on the victim's access levels. The impact is compounded by the fact that DOM manipulation often occurs during normal browsing activities, meaning exploitation does not necessarily require complex social engineering beyond hosting malicious content or being redirected via a compromised website.
The technical classification of this issue aligns with Common Weakness Enumeration standard CWE-416, which defines use-after-free conditions as a result of improper invalidation of references to dynamically allocated memory. In the ATT&CK framework for enterprise security, this vulnerability facilitates techniques associated with Initial Access and Execution phases, specifically leveraging browser vulnerabilities to bypass application sandboxing controls. The persistence of these dangling pointers allows attackers to perform arbitrary read or write operations in memory space that should otherwise be protected from user-level processes. This capability is often a precursor to more advanced attacks such as heap spraying or ROP chain construction, which are necessary for achieving reliable code execution on modern operating systems with mitigations like ASLR and DEP enabled.
To mitigate the risks associated with this vulnerability, organizations must ensure that all instances of Firefox, including standard releases and Extended Support Release versions, are updated to their respective patched versions: Firefox 155, Firefox ESR 115.40, Firefox ESR 140.15, or Firefox ESR 153.2. These updates contain the necessary code corrections that enforce proper memory deallocation protocols and prevent references from remaining valid after object destruction. In environments where immediate patching is not feasible due to compatibility testing requirements, network-level controls such as web filtering proxies can be configured to block access to known malicious domains or restrict JavaScript execution capabilities for untrusted sites. Additionally, enabling automatic updates ensures that users receive these critical security fixes promptly without requiring manual intervention, thereby reducing the window of exposure during which attackers could exploit this flaw in the wild.