CVE-2020-3556: Cisco AnyConnect Privilege Escalation Flaw

CVE-2020-3556 Overview

CVE-2020-3556 is a vulnerability in the interprocess communication (IPC) channel of Cisco AnyConnect Secure Mobility Client Software that could allow an authenticated, local attacker to cause a targeted AnyConnect user to execute a malicious script. The vulnerability stems from a lack of authentication to the IPC listener, enabling attackers to send crafted IPC messages to the AnyConnect client and trigger arbitrary script execution with the privileges of the targeted user.

Critical Impact

Successful exploitation allows local attackers to execute arbitrary scripts with victim user privileges, potentially leading to credential theft, lateral movement, or complete system compromise.

Affected Products

• Cisco AnyConnect Secure Mobility Client 4.9(3052)
• Cisco AnyConnect Secure Mobility Client 98.145(86)
• Cisco AnyConnect Secure Mobility Client (multiple versions)

Discovery Timeline

• 2020-11-06 - CVE-2020-3556 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-3556

Vulnerability Analysis

This vulnerability represents a significant security flaw in the IPC mechanism used by Cisco AnyConnect Secure Mobility Client. The IPC channel, designed to facilitate communication between different components of the AnyConnect software, lacks proper authentication controls. This design weakness allows any authenticated local user on the system to send malicious IPC messages to another user's AnyConnect client session.

The attack requires several conditions to be met: the attacker must have valid local user credentials on the target system, and there must be an active AnyConnect session by the targeted user at the time of the attack. Despite these prerequisites, the vulnerability is concerning because it enables privilege abuse within multi-user environments where AnyConnect is commonly deployed.

Root Cause

The root cause of CVE-2020-3556 is improper input validation (CWE-20) in the IPC listener component. The AnyConnect client fails to authenticate the source of incoming IPC messages, accepting and processing commands from any local process without verifying the sender's identity or authorization level. This missing authentication mechanism creates an attack surface where malicious local users can inject arbitrary commands into other users' AnyConnect sessions.

Attack Vector

The attack vector is local, requiring the attacker to have authenticated access to the same system where the targeted AnyConnect user has an active session. The exploitation workflow involves:

1. The attacker identifies an active AnyConnect session belonging to another user on the system
2. The attacker crafts malicious IPC messages designed to trigger script execution
3. These messages are sent to the AnyConnect client's IPC listener
4. The victim's AnyConnect client processes the malicious messages without authentication
5. A malicious script executes with the privileges of the targeted AnyConnect user

The vulnerability can be exploited to execute scripts that perform actions such as harvesting VPN credentials, establishing persistence, or pivoting to access protected network resources that the victim's AnyConnect session provides access to.

Detection Methods for CVE-2020-3556

Indicators of Compromise

• Unusual IPC traffic patterns involving the AnyConnect client process (vpnagent.exe or vpnui.exe)
• Unexpected script execution spawned as child processes of AnyConnect components
• Anomalous local user activity targeting AnyConnect IPC channels
• Scripts or executables launched with privileges different from the originating user context

Detection Strategies

• Monitor for suspicious child process creation from AnyConnect binaries using endpoint detection tools
• Implement behavioral analysis to detect unexpected script execution triggered by IPC calls
• Deploy SentinelOne's behavioral AI to identify anomalous process chains involving AnyConnect
• Alert on cross-user IPC communication attempts targeting AnyConnect components

Monitoring Recommendations

• Enable detailed logging for AnyConnect client operations and IPC events
• Configure SentinelOne Singularity to monitor process relationships and detect privilege abuse scenarios
• Implement user behavior analytics to identify patterns consistent with local privilege exploitation
• Audit system events for unusual process spawning from VPN client software

How to Mitigate CVE-2020-3556

Immediate Actions Required

• Review the Cisco Security Advisory for the latest guidance
• Restrict local user access on systems running AnyConnect to minimize attack surface
• Implement least privilege principles to limit the number of users with local access
• Deploy endpoint protection solutions capable of detecting script-based attacks
• Consider network segmentation to isolate systems with VPN client software

Patch Information

As of the last update, Cisco has not released software updates that address this vulnerability. Organizations should monitor the Cisco Security Advisory for updates regarding patches or workarounds. In the absence of a patch, implementing compensating controls is critical to reducing risk.

Workarounds

• Disable the Auto Update feature in AnyConnect if not required, as referenced in the Cisco advisory
• Restrict which users can log on to systems running AnyConnect to trusted personnel only
• Implement application whitelisting to prevent unauthorized script execution
• Use SentinelOne's application control features to restrict script execution paths
• Consider alternative VPN solutions if the vulnerability poses unacceptable risk to your environment

Organizations should continuously monitor Cisco's security advisory for any updates regarding patches or additional workarounds. Implementing defense-in-depth strategies with endpoint detection and response (EDR) solutions like SentinelOne can help detect and prevent exploitation attempts even in the absence of a vendor patch.