CVE-2026-100811 in Firefox
Summary
by MITRE • 09/29/2026
Sandbox escape due to use-after-free in the DOM: Core & HTML 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 security flaw represents a critical memory management error within the Document Object Model and HTML parsing subsystems of Mozilla Firefox browsers. Specifically, this is an use-after-free vulnerability that occurs during the processing of DOM elements or HTML content. In software engineering terms, a use-after-free condition arises when a program continues to reference a pointer after the memory it points to has been deallocated or freed by the system. This typically happens due to improper lifecycle management of objects within the browser's rendering engine. When an attacker can trigger this specific sequence through crafted HTML or JavaScript code embedded in a webpage, they gain control over how that dangling pointer is utilized. The vulnerability resides deep within the core components responsible for interpreting and manipulating web content, which are fundamental to any modern browser operation.
From a technical perspective, the exploitation of this flaw allows an attacker to achieve arbitrary read and write access to memory regions associated with the browser process. By carefully crafting malicious input that causes the DOM or HTML parser to free an object while maintaining references to it elsewhere in the code execution flow, the attacker can manipulate these dangling pointers. This manipulation often leads to a state where the application executes instructions based on attacker-controlled data rather than legitimate system memory structures. Such control over memory layout and instruction pointer values is the primary mechanism for achieving remote code execution without user interaction beyond visiting the malicious site. The complexity of exploiting this specific vulnerability lies in the precise timing required to trigger the free operation while ensuring the subsequent use occurs before the operating system can reclaim or overwrite that memory segment, a technique often referred to as heap grooming or race condition exploitation depending on the exact implementation details within the Firefox engine.
The operational impact of successfully leveraging this sandbox escape is severe and comprehensive. Browsers operate with multiple security layers designed to isolate web content from the underlying host operating system. A successful exploit of this vulnerability allows malicious code running in a restricted, low-privilege browser context to break out into a higher-privileged environment or gain direct access to sensitive memory spaces that were previously protected by sandboxing mechanisms. This effectively neutralizes one of the most important defense-in-depth strategies employed by web browsers. An attacker could potentially install malware, steal credentials stored within the browser session, exfiltrate private data from other tabs or applications accessible via shared resources, and establish persistent access to the victim's machine. The risk is particularly acute for users who frequently visit untrusted websites or click on links in phishing emails that host such malicious payloads.
Industry standards classify this type of flaw under CWE-416, which denotes Use After Free vulnerabilities. This classification highlights the fundamental error in memory management where a pointer is used after its associated storage has been reused. Furthermore, from an adversary simulation perspective aligned with the MITRE ATT&CK framework, this vulnerability facilitates techniques related to Initial Access and Execution. Specifically, it supports Remote Code Execution (T1203) by allowing attackers to run arbitrary code on the target system through a web-based vector. It also contributes to Defense Evasion (T1620) if used in conjunction with other tools to hide malicious activity within legitimate browser processes. The existence of such vulnerabilities underscores the ongoing challenge browsers face in maintaining robust memory safety guarantees while processing complex and often untrusted HTML structures generated by dynamic websites.
Mitigation for this vulnerability relies primarily on timely software updates provided by Mozilla. As indicated, the flaw has been addressed in Firefox ESR version 153.4, standard Firefox release 157, and Firefox Extended Support Release (ESR) version 140.17. Organizations and individual users must ensure their browsers are updated to one of these patched versions immediately upon availability. For enterprise environments utilizing the Extended Support Release channel, administrators should verify that automatic update policies are configured correctly to push this specific patch to all endpoints without manual intervention. Additionally, while software updates provide the definitive fix, supplementary defensive measures such as enabling strict Content Security Policies (CSP) on internal web applications and using browser extensions that restrict script execution from untrusted domains can reduce the attack surface. However, these controls are secondary to the necessity of applying the vendor-provided patch which resolves the underlying memory management defect in the DOM and HTML parsing engines.