The code in src/sftpserver.c did not verify the validity of certain pointers and expected them to be valid. A NULL pointer dereference could have been occured that typically causes a crash and thus a denial-of-service.
For Debian 8 jessie, these problems have been fixed in version 0.6.3-4+deb8u5.
We recommend that you upgrade your libssh packages.
Further information about Extended LTS security advisories can be found at: https://deb.freexian.com/extended-lts/