Johannes Kliemann discovered a vulnerability in ownCloud Desktop Client, the client-side of the ownCloud file sharing services. The vulnerability allows man-in-the-middle attacks in situations where the server is using self-signed certificates and the connection is already established. If the user in the client side manually distrusts the new certificate, the file syncing will continue using the malicious server as valid.
For the stable distribution (jessie), this problem has been fixed in version 1.7.0~beta1+really1.6.4+dfsg-1+deb8u1.
For the testing distribution (stretch), this problem has been fixed in version 1.8.4+dfsg-1.
For the unstable distribution (sid), this problem has been fixed in version 1.8.4+dfsg-1.
We recommend that you upgrade your owncloud-client packages.