CVE-2021-47961: Synology SSL VPN Client Plaintext Stora

Vendor-confirmed - CVE-2021-47961 is a high-severity PIN disclosure in Synology SSL VPN Client prior to 1.4.5-0684 that leaks stored authentication secrets to any local user. Upgrade immediately to block credential theft.

Overview

CVE-2021-47961 is a high-severity vulnerability (CVSS 8.1) in the Synology SSL VPN Client. Versions prior to 1.4.5-0684 insecurely store a user’s PIN code in plaintext on the system. This flaw allows a remote attacker with access to the system to potentially read this sensitive data.

Vulnerability Details

The core issue is a failure to encrypt or properly protect authentication secrets. The VPN client stores the PIN code-used for authentication-in a plaintext format within its configuration. With an Attack Vector of NETWORK and Attack Complexity of LOW, an attacker could potentially retrieve this PIN without needing any prior privileges on the system. However, successful exploitation requires user interaction, meaning an attacker would need to trick a user into performing an action, such as opening a malicious file or link, to facilitate access to the stored data.

Impact

If exploited, this vulnerability allows an attacker to obtain a valid user PIN. With this information, they could configure unauthorized VPN connections, potentially gaining access to the organization’s internal network. Furthermore, if combined with other techniques, this could lead to the interception of VPN traffic, compromising data confidentiality and integrity for the affected user. This highlights the risk of credential storage flaws, a common theme in many security incidents detailed in our breach reports.

Remediation and Mitigation

The primary remediation is to apply the vendor update immediately. Synology has addressed this vulnerability in SSL VPN Client version 1.4.5-0684 and later. All users should upgrade to this version or a subsequent secure release.

Actionable Steps:

1. Patch: Update Synology SSL VPN Client to version 1.4.5-0684 or higher. Obtain the update directly from Synology’s official download center.
2. Inventory: Identify all endpoints using the affected Synology SSL VPN Client software.
3. Monitor: While the EPSS score indicates a very low probability (0.0%) of exploitation in the next 30 days, and it is not listed on CISA’s Known Exploited Vulnerabilities catalog, applying the patch eliminates the risk.

For administrators managing multiple vulnerabilities, staying informed through security news is recommended for broader threat context.

Security Insight

This vulnerability is a classic example of the persistent risk posed by insecure credential storage, even in security-focused applications like VPN clients. It mirrors past incidents where vendors prioritized connection functionality over the fundamental security of stored secrets. The high CVSS score, driven by the lack of required privileges and network attack vector, underscores that the mere presence of a plaintext credential on a system significantly lowers the barrier for a network-based attacker, transforming a simple data discovery into a potential network intrusion.

Further Reading

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