Zero-Day Advisory

Fortinet Discovers Synology Cloud Station Backup DLL Preloading Vulnerability


Fortinet's FortiGuard Labs has discovered a DLL preloading vulnerability in Synology Cloud Station Backup.

Synology's Cloud Station Backup is a backup service that allows you to back up your files from multiple client computers to a centralized Synology product.

Synology Cloud Station Backup before 4.2.5-4396 is susceptible to multiple DLL preloading vulnerabilities. The issue occurs when the application looks to load a DLL for execution and loads an attacker provided malicious DLL instead of the legitimate DLL. The application generally follows a specific search path to locate the DLL. The vulnerability can be exploited by a simple file write (or potentially an over-write) which results in a foreign DLL (located in the current working directory) running under the context of the application.


Users should apply the solution provided by Synology.

Additional Information

Fortinet reported the vulnerability to Synology on July 7, 2017.

Synology released patch for it on August 30, 2017.


This vulnerability was discovered by Kushal Arvind Shah of Fortinet's FortiGuard Labs.

IPS Subscription

Fortinet customers who subscribe to Fortinet's intrusion prevention (IPS) service should be protected against this vulnerability with the appropriate configuration parameters in place. Fortinet's IPS service is one component of FortiGuard Subscription Services, which also offer comprehensive solutions such as antivirus, Web content filtering and antispam capabilities. These services enable protection against threats on both application and network layers. FortiGuard Services are continuously updated by FortiGuard Labs, which enables Fortinet to deliver a combination of multi-layered security intelligence and true zero-day protection from new and emerging threats. These updates are delivered to all FortiGate, FortiMail and FortiClient products. Fortinet strictly follows responsible disclosure guidelines to ensure optimum protection during a threat's lifecycle.