CVE-2021-47961: Synology SSL VPN Client Password Storage Flaw

CVE-2021-47961 Overview

A plaintext storage of a password vulnerability exists in Synology SSL VPN Client before version 1.4.5-0684. This security flaw allows remote attackers to access or influence the user's PIN code due to insecure storage mechanisms. When exploited in combination with user interaction, this vulnerability may lead to unauthorized VPN configuration changes and potential interception of subsequent VPN traffic.

Critical Impact

Remote attackers can access sensitive PIN code credentials stored in plaintext, potentially enabling unauthorized VPN access and traffic interception.

Affected Products

• Synology SSL VPN Client versions prior to 1.4.5-0684

Discovery Timeline

• 2026-04-10 - CVE CVE-2021-47961 published to NVD
• 2026-04-13 - Last updated in NVD database

Technical Details for CVE-2021-47961

Vulnerability Analysis

This vulnerability is classified under CWE-256 (Unprotected Storage of Credentials), which describes weaknesses where an application stores sensitive authentication credentials in a manner that allows unauthorized access. In the case of Synology SSL VPN Client, user PIN codes are stored in plaintext format rather than being encrypted or securely hashed.

The vulnerability requires network access and user interaction to exploit successfully. An attacker who gains access to the stored credentials can compromise the user's VPN authentication, potentially leading to unauthorized configuration changes and the ability to intercept VPN traffic.

Root Cause

The root cause of this vulnerability lies in the application's failure to implement proper credential storage mechanisms. Instead of utilizing encryption, secure hashing, or platform-specific credential management systems (such as Windows Credential Manager or macOS Keychain), the Synology SSL VPN Client stores user PIN codes in plaintext. This design flaw violates fundamental security principles around sensitive data protection and credential management.

Attack Vector

The attack vector for this vulnerability is network-based, requiring user interaction to successfully exploit. An attacker could potentially:

1. Gain access to the system where the VPN client is installed through various means (malware, physical access, or network compromise)
2. Locate and read the plaintext PIN code from the insecure storage location
3. Use the obtained credentials to authenticate to the VPN service
4. Modify VPN configurations or intercept VPN traffic

The vulnerability impacts both confidentiality and integrity, as attackers can both read sensitive credentials and potentially influence VPN configurations.

Detection Methods for CVE-2021-47961

Indicators of Compromise

• Unexpected access to VPN configuration files or credential storage locations
• Unauthorized VPN connection attempts from unrecognized devices or locations
• Modifications to VPN client configuration files without user action
• Unusual file access patterns targeting VPN client installation directories

Detection Strategies

• Monitor file access events to VPN client configuration and credential storage locations
• Implement endpoint detection rules for unauthorized reading of VPN client data files
• Review VPN server logs for connection attempts from unexpected sources or using compromised credentials
• Deploy file integrity monitoring on VPN client installation directories

Monitoring Recommendations

• Enable logging and alerting for VPN authentication events, particularly failed attempts followed by successful logins
• Implement network traffic analysis to detect anomalous VPN connection patterns
• Configure endpoint protection solutions to monitor access to sensitive application data directories
• Regularly audit VPN client installations for versions vulnerable to CVE-2021-47961

How to Mitigate CVE-2021-47961

Immediate Actions Required

• Upgrade Synology SSL VPN Client to version 1.4.5-0684 or later immediately
• Reset PIN codes for all users who have used affected versions of the VPN client
• Audit VPN access logs for any signs of unauthorized access during the vulnerability window
• Review and rotate any credentials that may have been stored insecurely

Patch Information

Synology has addressed this vulnerability in SSL VPN Client version 1.4.5-0684. Organizations should immediately upgrade all installations to this patched version or later. For detailed patch information, refer to the Synology Security Advisory SA-26-05.

Workarounds

• If immediate patching is not possible, consider temporarily disabling the VPN client until the update can be applied
• Implement additional authentication factors for VPN access to reduce reliance on potentially compromised PIN codes
• Restrict local file system access to VPN client directories using operating system-level permissions
• Monitor for unauthorized access to VPN client storage locations and alert on suspicious activity