CVE-2026-100818 in Firefox
Summary
by MITRE • 09/29/2026
Sandbox escape due to use-after-free in the Widget: Gtk 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 • 09/30/2026
The identified security flaw represents a critical memory management error within the Gecko rendering engine's handling of GTK-based widgets, specifically categorized under the Widget: Gtk component. This vulnerability stems from an improper lifecycle management sequence that results in a use-after-free condition. In software engineering terms, this occurs when a program continues to use a pointer after the memory it points to has been freed or reallocated. The root cause lies in the complex interaction between Firefox's internal widget abstraction layer and the underlying GTK toolkit on Linux systems. When specific UI elements are destroyed or updated dynamically during user interactions such as scrolling, resizing windows, or interacting with form controls, the reference counting mechanism for these objects fails to properly invalidate dangling pointers before the associated memory is released back to the system allocator.
From a technical perspective, this use-after-free vulnerability allows an attacker who can trick a victim into visiting a specially crafted web page to execute arbitrary code within the context of the browser process. The exploitability relies on precise heap grooming techniques where the attacker allocates new data in the freed memory region before it is reused by legitimate application logic. By controlling the content written into this reclaimed memory block, an adversary can manipulate function pointers or object vtables that are subsequently accessed by the vulnerable code path. This manipulation enables the injection of malicious instructions that bypass standard browser sandboxing mechanisms designed to isolate web content from the host operating system.
The operational impact of this vulnerability is severe due to its potential for remote code execution without user interaction beyond clicking a link or loading a page. If successfully exploited, an attacker could gain full control over the victim's machine, leading to data theft, installation of malware, or use of the compromised device as part of a botnet. The severity is further amplified by the widespread adoption of Firefox on Linux distributions used in enterprise and government environments where strict security boundaries are expected. Although modern browsers employ various mitigation techniques such as heap randomization and sandboxing, sophisticated exploits can often bypass these defenses when precise memory corruption primitives like use-after-free are available to an attacker.
This vulnerability aligns with Common Weakness Enumeration (CWE) ID 416, which defines Use After Free errors, a category of bugs that frequently lead to arbitrary code execution vulnerabilities in complex applications. In the context of the MITRE ATT&CK framework, this flaw facilitates techniques associated with Initial Access and Execution, specifically leveraging browser-based attack vectors such as Drive-by Code Execution or Exploitation for Client Execution. The vulnerability highlights the ongoing challenges in maintaining memory safety within large C++ codebases that interface directly with native system libraries like GTK.
To mitigate the risks posed by this vulnerability, organizations must ensure that all instances of Firefox are updated to version 153.4 (ESR), version 157, or version 140.17 (ESR) as soon as possible. These releases contain patches that correct the reference counting logic and add additional checks to prevent access to freed memory regions within the GTK widget component. Additionally, administrators should enforce strict update policies across all endpoints running Firefox on Linux systems. While browser sandboxing provides a layer of defense against exploitation attempts, relying solely on these mitigations is insufficient given the high exploitability potential of use-after-free bugs. Regular patch management and monitoring for emerging threat intelligence related to zero-day exploits targeting Gecko-based browsers are essential components of a comprehensive security posture.