In Qt5's plugin loader code as found in qtbase-opensource-src, it was possible to (side-)load plugins from the local folder in addition to a system-widely defined library path.

For Debian 8 Jessie, this problem has been fixed in version 5.3.2+dfsg-4+deb8u4.

We recommend that you upgrade your qtbase-opensource-src packages.

Further information about Debian LTS security advisories, how to apply these updates to your system and frequently asked questions can be found at: https://wiki.debian.org/LTS