Two critical Synology SSL VPN Client vulnerabilities have been disclosed, raising serious concerns for organizations relying on VPN software for secure remote access.
VPN clients are often the first line of defense for enterprise connectivity, making them a high-value target for attackers seeking access to sensitive systems and internal networks.
These vulnerabilities can allow remote attackers to:
- Access sensitive local files
- Steal stored credentials
- Intercept VPN-related network traffic
- Manipulate VPN configurations
Because VPN clients operate at a privileged network boundary, exploitation can quietly expose entire enterprise environments without immediate detection.
What Are the Synology SSL VPN Client Vulnerabilities?
The vulnerabilities affect older versions of the Synology SSL VPN Client and are classified as “Important” severity issues.
They include two distinct flaws:
- CVE-2021-47960 → Local file disclosure vulnerability
- CVE-2021-47961 → Insecure credential storage vulnerability
Both issues increase the risk of data leakage and session compromise.
CVE-2021-47960: Local File Disclosure Vulnerability
What is the issue?
CVE-2021-47960 allows unauthorized access to files and directories within the VPN client’s installation environment.
How it works
A remote attacker can:
- Exploit a locally bound HTTP service (loopback interface)
- Access sensitive files within the VPN client directory
- Extract configuration and system data
What attackers can access
- VPN configuration files
- Digital certificates
- System logs
- Local application data
Impact
This vulnerability can silently leak sensitive information that helps attackers:
- Understand VPN architecture
- Steal authentication material
- Prepare further attacks on enterprise networks
CVE-2021-47961: Plaintext Credential Storage Flaw
What is the issue?
CVE-2021-47961 is a high-severity credential storage vulnerability caused by storing sensitive authentication data in plaintext.
How it works
An attacker may:
- Retrieve stored PIN codes from the local machine
- Exploit insecure credential handling mechanisms
- Manipulate or reuse VPN authentication data
Why this is dangerous
Once credentials are exposed, attackers can:
- Create rogue VPN sessions
- Intercept enterprise traffic
- Impersonate legitimate users
- Gain persistent network access
How the Attack Works (Real-World Scenario)
Unlike purely remote exploits, these vulnerabilities require user interaction.
Attack chain:
1. User is tricked into visiting a malicious website
While VPN client is active
2. Local service is triggered
A loopback HTTP service is exploited
3. Sensitive data is extracted
Including:
- Files
- Credentials
- VPN configuration
4. VPN session is hijacked
Attacker gains access to internal traffic
Why VPN Client Vulnerabilities Are High Risk
VPN clients are critical because they:
- Bridge external and internal networks
- Handle sensitive authentication data
- Operate with elevated trust levels
Risk impact table:
| Vulnerability | Type | Impact | Severity |
|---|---|---|---|
| CVE-2021-47960 | File disclosure | Sensitive data leakage | Medium |
| CVE-2021-47961 | Credential storage flaw | VPN session compromise | High |
Affected Versions
- Synology SSL VPN Client versions before 1.4.5-0684
Patch and Mitigation Guidance
1. Immediate update (critical)
Upgrade to:
- Synology SSL VPN Client 1.4.5-0684 or later
2. User awareness training
Educate users to:
- Avoid clicking unknown links while VPN is active
- Avoid untrusted websites during remote sessions
- Report suspicious behavior immediately
3. Monitor VPN activity
Security teams should track:
- Unauthorized VPN configuration changes
- Abnormal login or session patterns
- Unexpected traffic routing changes
4. Endpoint security enforcement
- Use endpoint detection and response (EDR)
- Restrict local service exploitation attempts
- Monitor loopback interface activity
Expert Security Insights
This vulnerability highlights a key issue in modern VPN security:
VPN security is only as strong as the endpoint client.
Key lessons:
- Local file access vulnerabilities can be as dangerous as network exploits
- Credential storage must never rely on plaintext mechanisms
- User interaction is often the weakest link in attack chains
Mapped to:
- OWASP Sensitive Data Exposure
- MITRE ATT&CK: T1555 (Credential Access)
- NIST endpoint security guidelines
FAQs
What are Synology SSL VPN Client vulnerabilities?
They are two flaws that allow file disclosure and credential theft in VPN clients.
Can attackers access VPN traffic?
Yes, CVE-2021-47961 may allow VPN session hijacking.
Is user interaction required?
Yes, victims must visit a malicious page while the VPN client is active.
Which versions are affected?
All versions before 1.4.5-0684.
What is the risk to organizations?
Data leakage, credential theft, and potential network compromise.
How can organizations protect themselves?
By patching, monitoring VPN logs, and enforcing endpoint security controls.
Conclusion
The Synology SSL VPN Client vulnerabilities demonstrate how endpoint-level flaws can undermine the entire security of remote access systems.
CVE-2021-47960 enables sensitive file leakage, while CVE-2021-47961 exposes credentials that can lead to full VPN compromise.
Immediate patching and user awareness are essential to prevent exploitation.
