According to Wiki, after ten years in orbit NASA investigators were reported to have found that the ROSAT failure was linked to a cyber-intrusion at Goddard Space Flight Center in 2008. This was also reported through Bruce Schneier's blog, a highly-regarded commentary on IT security issues.
"Hostile activities compromised [NASA] computer systems that directly and indirectly deal with the design, testing, and transferring of satellite package command-and-control codes." — Talleur, Thomas J. (18 January 1999).