Online backup is seen as a good strategy for preventing data loss, in case of a disaster at a local datacenter or on a local machine. But apparently the software used by over 100 services is vulnerable to a man in the middle attack, even though it uses SSL to secure the connection:

Tests by heise Security show that four of the six services tested were vulnerable to attack.

While all of the tested systems encrypt communication with the backup server using SSL, external attackers can sniff the access code as plain text by acting as a man-in-the-middle (MITM) if the locally installed backup software does not perform sufficiently rigorous checks on the authenticity of the server’s certificates. In the vulnerable systems, we were able to hijack the connection from the client software to the backup servers.

Four of six may not be a large test sample, but it does raise concerns about trust between customers and their service providers. If you’re providing or purchasing this kind of service, you might want to look into it closely to make sure your data is secure.