CVE-2020-3556 Overview
CVE-2020-3556 is a vulnerability in the interprocess communication (IPC) channel of Cisco AnyConnect Secure Mobility Client Software. An authenticated, local attacker can exploit the flaw to force a targeted AnyConnect user to execute a malicious script. The issue stems from a lack of authentication on the IPC listener, allowing crafted IPC messages to trigger script execution under the targeted user's privileges. Exploitation requires valid local credentials and an active AnyConnect session by the targeted user. Cisco has not released software updates addressing this vulnerability, making workarounds and detection controls the primary defensive options for affected organizations.
Critical Impact
An authenticated local attacker can execute arbitrary scripts in the security context of another AnyConnect user, leading to local privilege misuse and lateral compromise on shared systems.
Affected Products
- Cisco AnyConnect Secure Mobility Client 4.9(3052)
- Cisco AnyConnect Secure Mobility Client 98.145(86)
- Cisco AnyConnect Secure Mobility Client deployments with active user sessions
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
The vulnerability resides in the IPC channel used by the Cisco AnyConnect Secure Mobility Client to coordinate between its user-mode and service-mode components. The IPC listener accepts messages without verifying the identity or authorization of the sender. An authenticated local attacker can send crafted IPC messages to the listener and instruct the targeted AnyConnect user's session to execute a script. The script runs with the privileges of the targeted AnyConnect user, not the attacker. This creates an opportunity for lateral movement on multi-user systems and abuse of trust relationships on workstations where multiple identities operate. The flaw is classified under [CWE-20] Improper Input Validation, reflecting the listener's failure to validate the origin and content of IPC requests.
Root Cause
The root cause is the absence of authentication on the AnyConnect IPC listener. The listener trusts inbound IPC messages without verifying the sending process or user. As a result, any local process running under valid credentials can interact with the listener and direct script execution against an active AnyConnect session.
Attack Vector
Exploitation requires local access with valid user credentials and an active AnyConnect session belonging to a different user on the same system. The attacker delivers crafted IPC messages to the AnyConnect client IPC listener. The targeted session then executes the supplied script under that user's privileges. The vulnerability is not exploitable remotely and requires user interaction in the form of an existing AnyConnect session.
No verified public proof-of-concept code is available. Refer to the Cisco Security Advisory for vendor technical details.
Detection Methods for CVE-2020-3556
Indicators of Compromise
- Unexpected child processes spawned by the AnyConnect client process tree, particularly script interpreters such as cmd.exe, powershell.exe, wscript.exe, or cscript.exe.
- Local IPC connections to the AnyConnect listener originating from processes outside the AnyConnect installation directory.
- Script files written to user-writable paths immediately before AnyConnect-spawned execution events.
Detection Strategies
- Monitor process lineage where AnyConnect components are parents of script interpreters or unsigned binaries.
- Correlate local logon events with IPC activity targeting AnyConnect to surface cross-user interactions on shared systems.
- Apply behavioral analytics to flag AnyConnect sessions executing commands inconsistent with normal VPN client operation.
Monitoring Recommendations
- Enable command-line auditing and PowerShell script block logging on systems running AnyConnect.
- Forward endpoint telemetry to a centralized analytics platform for correlation across users and hosts.
- Track installed AnyConnect versions in asset inventory and alert on the affected builds 4.9(3052) and 98.145(86).
How to Mitigate CVE-2020-3556
Immediate Actions Required
- Restrict interactive and remote local logon rights on systems running AnyConnect to trusted users only.
- Audit shared workstations and terminal servers where multiple users may have concurrent AnyConnect sessions.
- Apply application control policies that prevent unauthorized scripts from being launched in user contexts.
Patch Information
Cisco has not released software updates that address CVE-2020-3556. Consult the Cisco Security Advisory for the latest vendor guidance and any future updates.
Workarounds
- Limit local user accounts on endpoints running AnyConnect to reduce the population of potential authenticated attackers.
- Enforce least privilege so that AnyConnect users do not hold elevated rights that an exploited session could abuse.
- Use endpoint protection and application allowlisting to block execution of unauthorized scripts under AnyConnect user contexts.
- Disable AnyConnect on multi-user systems where session isolation cannot be guaranteed.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
