CVE-2026-100778 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, Firefox ESR 115.42, and Firefox ESR 140.17.
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Analysis
by VulDB Data Team • 09/29/2026
The identified vulnerability represents a critical security flaw within the Mozilla Firefox browser engine, specifically affecting the Document Object Model (DOM) and HTML parsing components. This issue is classified as an out-of-bounds read or use-after-free condition that occurs during the processing of web content. In complex web applications, the DOM serves as the structural representation of a webpage, allowing scripts to interact with page elements dynamically. When the browser processes certain malformed or specifically crafted HTML structures, it may fail to properly manage memory allocation and deallocation for internal objects associated with these DOM nodes. This failure leads to a scenario where pointers to freed memory are retained and subsequently accessed by the rendering engine during subsequent operations such as layout recalculations or style updates.
From a technical perspective, this vulnerability falls under the Common Weakness Enumeration category CWE-416, which denotes Use After Free. The root cause lies in the lifecycle management of internal DOM objects within the Gecko rendering engine. When an element is removed from the document tree or its reference count drops to zero, the associated memory should be immediately reclaimed and any dangling pointers invalidated. However, due to a logic error in the cleanup routines for specific HTML elements, certain references remain valid even after their underlying data structures have been destroyed. An attacker can exploit this by constructing a malicious webpage that triggers these specific DOM manipulations through JavaScript or CSS interactions designed to force premature object destruction while maintaining active references within event handlers or asynchronous callbacks.
The operational impact of this vulnerability is severe because it enables sandbox escape, allowing code executed in the low-privilege browser process to break out into higher-privileged contexts. In modern browsers like Firefox, web content runs inside a restricted environment known as the Content Sandbox, which limits access to system resources and sensitive APIs. By exploiting the use-after-free condition, an attacker can achieve arbitrary read or write primitives within the browser's memory space. This capability allows for the bypass of security mitigations such as Data Execution Prevention (DEP) and Address Space Layout Randomization (ASLR). Consequently, this leads to remote code execution on the victim's system without any user interaction beyond visiting a malicious website, effectively compromising the integrity and confidentiality of the host machine.
In terms of threat modeling and adversary behavior, this exploitation technique aligns with MITRE ATT&CK techniques related to initial access and privilege escalation via browser vulnerabilities. Specifically, it relates to T1208, Weak Encryption, if cryptographic contexts are involved in the memory corruption, but more accurately maps to general exploit development patterns involving heap spraying or targeted allocation strategies to control the contents of freed memory blocks. Attackers would likely use this vulnerability as part of a multi-stage attack chain where initial access is gained through phishing or drive-by downloads, followed by exploitation of this flaw to establish persistent footholds or exfiltrate sensitive data from browser sessions, including cookies and stored credentials.
To mitigate the risks associated with this vulnerability, organizations must ensure that all instances of Firefox are updated to the patched versions listed in the advisory: Firefox ESR 153.4, Firefox 157, Firefox ESR 115.42, or Firefox ESR 140.17. These releases contain code changes that correct the memory management logic within the DOM and HTML components, ensuring that references are properly invalidated upon object destruction. For environments where immediate patching is not feasible due to compatibility concerns with legacy applications relying on older browser versions, network-level controls such as web application firewalls can be configured to detect and block requests containing known exploit signatures associated with this specific memory corruption pattern. Additionally, enforcing strict Content Security Policy headers helps limit the impact of any potential exploitation by restricting script execution sources and preventing unauthorized data exfiltration attempts that might follow successful sandbox escape.