Synology has released a security advisory addressing two important vulnerabilities in its SSL VPN Client that could allow remote attackers to access sensitive files and expose locally stored PINs. Both flaws require user interaction—specifically, visiting a crafted web page while the vulnerable client is running—but their consequences range from quietly reading configuration files and certificates to enabling interception of VPN traffic by abusing exposed credentials. Administrators and users should prioritize the patch immediately to prevent potential network compromise.
What the vulnerabilities are
CVE-2021-47960 (CVSS 6.5) and CVE-2021-47961 (CVSS 8.1) are the two issues fixed by Synology’s update. The first is a directory and file exposure problem that can let an attacker read files from the SSL VPN Client installation directory. The second is a more serious weakness involving plaintext storage of credentials or PIN-related data on the local machine, which could allow an attacker to obtain or manipulate the user’s PIN and thereby authorize malicious VPN configurations. Both flaws were reported to Synology by security researcher Laurent Sibilla.
How the attacks work
Neither vulnerability can be exploited completely without user action. An attacker must trick a user into visiting a specially crafted web page while the vulnerable Synology client is running. For the file-access issue, the attacker leverages a local HTTP service bound to the loopback interface to retrieve sensitive artifacts—configuration files, certificates, logs—that are normally intended to remain local. For the PIN-related issue, the insecure storage exposes credentials that can be used by an attacker to push or authorize rogue VPN settings, potentially allowing interception or redirection of subsequent VPN traffic.
Who is at risk
Any user or organization running older versions of the Synology SSL VPN Client is potentially affected, particularly those who use the client on laptops or desktops that regularly browse the web while connected to corporate resources. Remote attackers benefit from a vector that combines social-engineering (malicious web content) with client-side weaknesses, so environments with aggressive web access controls and user education have a relative advantage but remain at risk until patched.
Patch availability and immediate steps
Synology has published a security update that remedies these issues. According to the advisory, there are no reliable temporary mitigations or workarounds; applying the vendor-supplied patch is the only effective remedy. Administrators should:
- Upgrade the Synology SSL VPN Client to version 1.4.5-0684 or later on all affected endpoints as soon as possible.
- Enforce and verify client version compliance with endpoint management tools or patch management systems.
- If you suspect compromise, consider reissuing any affected credentials and inspect VPN configurations for unauthorized entries.
Operational recommendations and monitoring
Beyond applying the patch, teams should take pragmatic steps to reduce exposure and detect suspicious activity:
- Educate users about the risk of clicking unknown links while a VPN client is running, and provide examples of common social-engineering tactics.
- Monitor VPN and network access logs for unusual configuration changes, unexpected PIN or credential resets, anomalous connection patterns, or traffic redirection that could indicate misuse of compromised clients.
- Apply network segmentation and least-privilege access controls so a single compromised workstation cannot easily pivot to critical systems.
- Review local security policies that govern storage of credentials and sensitive data on endpoints; where feasible, favor secure credential storage mechanisms and limit local plaintext secrets.
If you’ve already been targeted
If indicators suggest a successful exploit—unexpected VPN configuration changes, presence of unknown certificates, or evidence of exfiltrated configuration files—treat the incident as potentially serious. Isolate affected machines from sensitive networks, perform forensic analysis to determine scope, rotate credentials, and coordinate remediation with your identity and network security teams.
Conclusion
The Synology SSL VPN Client vulnerabilities underscore how client-side weaknesses combined with simple web-based social-engineering can create powerful footholds for attackers. Because there are no practical mitigations other than the vendor patch, rapid upgrade to version 1.4.5-0684 or newer is essential. Follow the operational guidance above to harden environments and detect any signs of exploitation while you roll out fixes.
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