Synology Patches Critical Vulnerability in Replication Service and Unified Controller (NCSC-2025-0090)

Synology has recently addressed a critical vulnerability in its Synology Replication Service and Synology Unified Controller, which could allow remote attackers to execute arbitrary code on affected systems. The vulnerability, identified as NCSC-2025-0090, stems from an off-by-one error in the software, potentially leading to unauthorized access and full system compromise. This article provides a detailed analysis of the vulnerability, its implications, and remediation steps for system administrators and security professionals.

TL;DR: Key Points

• Vulnerability: Off-by-one error in Synology Replication Service and Unified Controller.
• Impact: Remote code execution (RCE) leading to unauthorized system access.
• Severity: Rated as Medium/High (M/H) by the NCSC.
• Affected Systems: Synology Replication Service and Unified Controller.
• Remediation: Apply the latest updates from Synology.

Vulnerability Details

The vulnerability, identified as NCSC-2025-0090, is an off-by-one error in the Synology Replication Service and Unified Controller. This type of error occurs when a program writes data outside the bounds of a buffer, often leading to memory corruption. In this case, the flaw allows remote attackers to execute arbitrary code on the affected systems, potentially gaining full control over the device¹².

Technical Breakdown

• Off-by-One Error: A common programming mistake where a loop or buffer operation exceeds its intended boundary by one. This can lead to memory corruption and, in some cases, remote code execution.
• Attack Vector: The vulnerability can be exploited remotely, meaning attackers do not need physical access to the system.
• Impact: Successful exploitation could result in unauthorized access, data exfiltration, or system compromise.

Relevance to Security Professionals

For Red Teamers, this vulnerability presents an opportunity to test the resilience of Synology systems in controlled environments. Exploiting such flaws can help identify weaknesses in network defenses. For Blue Teamers and SOC Analysts, understanding the vulnerability is crucial for patching and monitoring affected systems. System administrators should prioritize applying the latest updates to mitigate risks.

Proof of Concept (PoC)

While no public PoC is available at the time of writing, the following Python snippet demonstrates how an off-by-one error might be exploited in a controlled environment:

# Example of a buffer overflow exploit (for educational purposes only)
import socket

target_ip = "192.168.1.1"
target_port = 8080

payload = b"A" * 1024  # Buffer overflow payload

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect((target_ip, target_port))
sock.send(payload)
response = sock.recv(1024)
print(response.decode())
sock.close()

Note: This code is for educational purposes only and should not be used maliciously.

Remediation Steps

Synology has released updates to address this vulnerability. System administrators should take the following steps to secure their systems:

1. Update Immediately: Apply the latest patches from Synology. Check the Synology Security Advisory for details³.
2. Monitor Logs: Review system logs for unusual activity, particularly in the Replication Service and Unified Controller.
3. Restrict Access: Limit access to these services to trusted IP addresses only.
4. Implement Network Segmentation: Isolate critical systems to minimize the impact of potential breaches.

Conclusion

The NCSC-2025-0090 vulnerability in Synology Replication Service and Unified Controller highlights the importance of timely patching and robust security practices. For security professionals, this serves as a reminder to stay vigilant and proactive in identifying and mitigating vulnerabilities. By applying the recommended updates and implementing best practices, organizations can significantly reduce their risk exposure.

References

1. NCSC Advisories. “[NCSC-2025-0090 [1.00] [M/H] Kwetsbaarheid verholpen in Synology Replication Service en Synology Unified Controller”. NCSC. Retrieved 2025-03-19.
2. Cocoon Risk Management. “[NCSC-2025-0090 [1.00] [M/H] Kwetsbaarheid verholpen in Synology Replication Service en Synology Unified Controller”. Cocoon. Retrieved 2025-03-19.
3. Synology. “[Synology Security Advisory Synology_SA_24_22”. Synology. Retrieved 2025-03-19.