CVE-2026-100787 in Firefox
Summary
by MITRE • 09/29/2026
Sandbox escape in the XUL component. This vulnerability was fixed in Firefox ESR 153.4 and Firefox 157.
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Analysis
by VulDB Data Team • 09/29/2026
The identified security issue represents a critical sandbox escape within the Mozilla XUL (XML User Interface Language) component, which is responsible for rendering graphical user interfaces in applications such as Firefox and Thunderbird. This vulnerability allows an attacker to break out of the restricted execution environment that typically isolates untrusted content from privileged system resources. By exploiting flaws in how the browser handles certain UI elements or script interactions within this framework, a malicious actor can execute arbitrary code with higher privileges than intended by the security model. Such escapes are particularly dangerous because they undermine the fundamental isolation principles designed to protect users from web-based attacks, potentially leading to full device compromise if not properly contained.
From a technical perspective, sandbox escape vulnerabilities often stem from improper validation of inputs or incorrect handling of object references within the rendering engine. In the context of XUL, this may involve manipulating specific DOM structures or event handlers that trigger privileged operations without adequate checks on their origin or intent. The flaw likely permits an attacker to bypass security boundaries by tricking the browser into executing code in a trusted context using data sourced from untrusted origins. This type of vulnerability is frequently categorized under CWE-250, which denotes execution with unnecessary privileges, and aligns with MITRE ATT&CK techniques related to privilege escalation or defense evasion through process injection or API misuse within the application layer.
The operational impact of this vulnerability is severe, as it enables remote code execution scenarios where a user merely visiting a crafted webpage could have their system compromised without any additional interaction beyond loading the malicious content. Attackers can leverage this access to install malware, exfiltrate sensitive data stored locally by the browser such as cookies and passwords, or use the infected machine as part of a botnet. Because XUL is deeply integrated into the core functionality of Firefox-based applications, the exploitability surface is broad, affecting multiple versions until patched. The lack of immediate mitigation for users who have not yet updated their software leaves them exposed to sophisticated phishing campaigns that specifically target browser vulnerabilities for initial access and lateral movement within enterprise environments.
To mitigate this risk, organizations must ensure that all instances of Firefox ESR 153.4 and later, as well as Firefox 157 and subsequent releases, are deployed across end-user devices. Patch management processes should prioritize these updates due to the high severity associated with sandbox escapes. Additionally, implementing application control policies can restrict the execution of unsigned scripts or limit browser capabilities in untrusted zones. Security teams should also monitor for indicators of compromise that suggest exploitation attempts, such as unusual network connections originating from browser processes or unexpected spawning of system utilities by Firefox binaries. Regular vulnerability scanning and adherence to strict update schedules are essential defenses against this class of threats until all systems are fully remediated.