The remote Debian host is missing a security-related update.
Alexander Peslyak discovered that insufficient input sanitising of RFB packets in LibVNCServer could result in the disclosure of memory contents.
Upgrade the libvncserver packages. For the oldstable distribution (jessie), this problem has been fixed in version 0.9.9+dfsg2-6.1+deb8u3. For the stable distribution (stretch), this problem has been fixed in version 0.9.11+dfsg-1+deb9u1.