EDIT by Trantor:
Based on our current observations, this issue only affects Synology NAS servers running some older versions of DSM (DSM 4.3-3810 or earlier), by exploiting a security vulnerability that was fixed and patched in December, 2013. At present, we have not observed this vulnerability in DSM 5.0.
For users who have not encountered any of the symptoms stated above, we highly recommend downloading and installing DSM 5.0, or any version below:
-For DSM 4.3, please install DSM 4.3-3827 or later
-For DSM 4.1 or DSM 4.2, please install DSM 4.2-3243 or later
-For DSM 4.0, please install DSM 4.0-2259 or later
Source : Synology forum