CVE-2019-9812 in Firefox
Summary
by MITRE
Given a compromised sandboxed content process due to a separate vulnerability, it is possible to escape that sandbox by loading accounts.firefox.com in that process and forcing a log-in to a malicious Firefox Sync account. Preference settings that disable the sandbox are then synchronized to the local machine and the compromised browser would restart without the sandbox if a crash is triggered. This vulnerability affects Firefox ESR < 60.9, Firefox ESR < 68.1, and Firefox < 69.
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Analysis
by VulDB Data Team • 11/26/2025
This vulnerability represents a sophisticated sandbox escape mechanism that exploits the Firefox browser's synchronization capabilities to undermine security boundaries. The flaw operates through a multi-stage attack vector where an initial compromise of a sandboxed content process creates a foothold that can be leveraged to bypass the browser's core security protections. The attack specifically targets the Firefox Sync functionality, which is designed to synchronize user preferences and settings across multiple devices, but becomes a weapon when manipulated by malicious actors. The vulnerability demonstrates how legitimate browser features can be abused to create persistent security weaknesses that persist beyond the initial compromise.
The technical implementation of this exploit relies on the trust model inherent in Firefox's synchronization system, where user preferences are automatically synchronized between devices without proper validation of the source or integrity of the data. When a compromised process loads the accounts.firefox.com domain and forces a login to a malicious Firefox Sync account, the system's normal synchronization behavior becomes a vector for privilege escalation. The malicious account contains preference settings that disable the sandbox mechanism, which are then downloaded and applied to the local machine. This creates a persistent backdoor where the browser will automatically restart without sandbox protection if a crash occurs, effectively removing the security boundary that would normally isolate malicious code execution.
The operational impact of this vulnerability is significant as it transforms a temporary process compromise into a permanent system compromise. Once the malicious preferences are synchronized and the browser restarts without sandbox protection, the attacker gains unrestricted access to the system's resources and can execute arbitrary code with the privileges of the user account. The vulnerability affects multiple versions of Firefox, including both the Extended Support Release (ESR) versions and the regular release cycle, indicating a widespread exposure across different deployment scenarios. This makes it particularly dangerous for enterprise environments where Firefox ESR versions are commonly used for stability and security reasons.
Security mitigations for this vulnerability primarily focus on immediate patching of affected versions and implementation of additional controls around browser synchronization features. Organizations should ensure that all Firefox installations are updated to versions that contain the fix for this vulnerability, specifically Firefox 60.9, Firefox 68.1, and Firefox 69. The vulnerability aligns with CWE-284 Access Control Issues, specifically related to insufficient access control in synchronized preference settings. From an ATT&CK perspective, this vulnerability maps to T1059 Command and Scripting Interpreter and T1078 Valid Accounts, as it leverages legitimate account synchronization to achieve persistent access. Additional mitigations include implementing network-level controls to restrict access to Firefox Sync domains, monitoring for unusual synchronization behavior, and considering the use of more restrictive browser policies that limit synchronization capabilities. The vulnerability highlights the importance of validating and sanitizing data from synchronization sources and demonstrates how security boundaries can be circumvented through the exploitation of trusted synchronization mechanisms.