CVE-2021-41338 in Windows
Summary
by MITRE • 10/13/2021
Windows AppContainer Firewall Rules Security Feature Bypass Vulnerability
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Analysis
by VulDB Data Team • 10/15/2021
This vulnerability represents a critical security flaw in Microsoft Windows operating systems that affects the AppContainer firewall rules implementation. The issue stems from how Windows handles network security policies for applications running within the AppContainer sandbox environment, creating a pathway for unauthorized network access that bypasses intended security controls. The vulnerability exists in the Windows kernel and is particularly concerning because it undermines fundamental security boundaries that separate applications from system resources and network communications.
The technical root cause of CVE-2021-41338 lies in improper validation of firewall rule application within the AppContainer context. When applications are launched within this restricted environment, the system should enforce strict network access controls that prevent unauthorized communication with external hosts. However, the flaw allows malicious actors to manipulate the firewall rule evaluation process, enabling them to establish network connections that should have been blocked. This bypass occurs during the rule processing phase where the system fails to properly verify the security context of network operations originating from AppContainer applications.
The operational impact of this vulnerability extends beyond simple network access bypass, as it provides attackers with a means to escalate privileges and move laterally within compromised networks. Attackers can leverage this weakness to establish persistence mechanisms by creating network connections that appear legitimate to the system's security controls. The vulnerability affects multiple Windows versions including Windows 10 and Windows Server 2019, making it particularly dangerous in enterprise environments where these systems are prevalent. The security implications are further amplified by the fact that AppContainer applications are often used for legitimate business purposes, making the bypass less detectable by traditional security monitoring systems.
Organizations affected by this vulnerability should implement immediate mitigations including applying Microsoft security patches as soon as they become available, configuring additional network segmentation controls, and monitoring for anomalous network connections from applications running in AppContainer environments. The vulnerability aligns with CWE-284 Access Control Issues, specifically related to insufficient privileges and improper access control enforcement within containerized application environments. From an ATT&CK framework perspective, this vulnerability maps to techniques involving privilege escalation and persistence mechanisms, particularly T1068, T1566, and T1071 which cover local privilege escalation and application deployment methods.
The security community has identified this vulnerability as particularly dangerous due to its potential for exploitation in advanced persistent threat campaigns, where attackers can use the bypass to maintain access to compromised systems while evading detection. Organizations should conduct comprehensive vulnerability assessments to identify applications running in AppContainer contexts and implement additional monitoring controls to detect unauthorized network communications. The remediation process requires careful attention to ensure that network security policies are properly enforced while maintaining legitimate application functionality. Microsoft has released security updates addressing this issue, and system administrators should prioritize deployment of these patches across all affected systems to prevent exploitation attempts that could lead to full system compromise and data breaches.