CVE-2021-1567 in AnyConnect Secure Mobility Client
Summary
by MITRE • 06/17/2021
A vulnerability in the DLL loading mechanism of Cisco AnyConnect Secure Mobility Client for Windows could allow an authenticated, local attacker to perform a DLL hijacking attack on an affected device if the VPN Posture (HostScan) Module is installed on the AnyConnect client. This vulnerability is due to a race condition in the signature verification process for DLL files that are loaded on an affected device. An attacker could exploit this vulnerability by sending a series of crafted interprocess communication (IPC) messages to the AnyConnect process. A successful exploit could allow the attacker to execute arbitrary code on the affected device with SYSTEM privileges. To exploit this vulnerability, the attacker must have valid credentials on the Windows system.
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Analysis
by VulDB Data Team • 06/21/2021
This vulnerability resides in the Windows implementation of Cisco AnyConnect Secure Mobility Client where a race condition exists within the DLL loading mechanism that enables privilege escalation through DLL hijacking techniques. The flaw specifically affects systems where the VPN Posture (HostScan) Module is installed, creating an exploitable path for authenticated local attackers who possess valid Windows system credentials. The vulnerability stems from insufficient signature verification processes during dynamic link library loading, allowing malicious code to masquerade as legitimate system components. The race condition occurs during the verification sequence when the system temporarily loads DLL files without proper validation, creating a window of opportunity for attackers to inject malicious payloads.
The technical exploitation requires an attacker to leverage interprocess communication mechanisms to send crafted IPC messages to the AnyConnect process, effectively manipulating the DLL loading sequence. This attack vector specifically targets the temporal gap in the signature verification workflow where the system loads DLL files before completing their authenticity checks. The vulnerability is classified as a privilege escalation issue that can be escalated to SYSTEM level privileges, representing a critical security risk. According to CWE standards, this vulnerability aligns with CWE-427 Uncontrolled Search Path Element and CWE-1229 Improper Handling of DLL Loading in Windows, both of which describe weaknesses in dynamic library loading mechanisms. The attack pattern follows the MITRE ATT&CK framework's technique T1059.001 for Command and Scripting Interpreter and T1068 for Exploitation for Privilege Escalation, demonstrating how local authentication can be leveraged to achieve system-level control.
The operational impact of this vulnerability extends beyond simple code execution, as it provides attackers with complete system compromise capabilities through the SYSTEM privileges that are normally restricted to authorized administrators. Once successfully exploited, the attacker gains the ability to modify system files, install persistent backdoors, access sensitive data, and potentially move laterally within the network. The vulnerability affects organizations that rely on Cisco AnyConnect for remote access, particularly those with security modules enabled for posture assessment. The requirement for valid credentials limits the attack surface but does not eliminate the risk, as compromised accounts or credential theft scenarios can quickly lead to exploitation. Organizations with multiple users accessing the VPN client are particularly vulnerable, as the attack only requires local access rather than network-based exploitation.
Mitigation strategies should focus on immediate patch deployment from Cisco, which addresses the race condition in the DLL loading process and strengthens signature verification. System administrators should also implement strict access controls and monitor for unusual IPC communication patterns that might indicate exploitation attempts. The principle of least privilege should be enforced to limit the impact of any successful attack, ensuring that even if exploitation occurs, the attacker cannot escalate privileges beyond the initial compromised account. Network segmentation and monitoring solutions should be deployed to detect anomalous behavior in the AnyConnect process, particularly around DLL loading activities. Additional defensive measures include disabling unnecessary Posture modules when not required, implementing application whitelisting policies, and conducting regular security assessments of VPN client configurations to identify and remediate similar vulnerabilities in the broader attack surface.