Hell hath no fury like an IT support administrator scorned. At least that's the message being heard loud and clear by firms that are finding their networks at risk of attack from former employees.

Thirty-year-old Lesmany Nunez has been sentenced to a year and a day in federal prison after being found guilty of breaking into servers at Quantum Technology Partners (QTP) and shutting down its servers. Nunez had been a computer support technician at the Miami-based firm, which provides IT services to companies in Florida, until May 2007. Three months later he was able to breach the company's network security by logging in with an administrator password.

The former employee then changed the passwords of all of the system administrators, shut down the servers, and deleted files that would have made recovery of lost data from backup tapes simpler.

The court heard that Nunez's actions meant QTP and its customers were unable to conduct normal business for almost a week, with costs exceeding $30,000. Ultimately, Nunez was identified as the suspect by tracing activity on the firm's network to his home computer, where additional electronic evidence was uncovered.

The story of Nunez and QTP is just the latest example of an ex-worker accessing his former employer's computer network after he left its employment. Other recent examples include the tale of David Ernest Everett Jr., who, after leaving his employment at a firm that produces integrated solutions for fast-food outlets, sought revenge by breaking into its servers and planting malicious files, which managed to crash operations at 25 different restaurants.

As belts tighten and the credit crunch continues to hit around the world, more and more companies will be making the decision to make staff and contractors redundant. A disaffected employee could create havoc inside your organization, so make sure appropriate security is in place.

The importance of killing off user names and passwords when individuals leave your business can't be emphasized enough - it's all too easy (and sometimes tempting) for people to break back into your system -- either for reasons of revenge, commercial advantage, or to look clever in front of their mates.

Graham Cluley is senior technology consultant at Sophos, and has been working in the computer security field since the early 1990s. When he's not updating his other blog on the Sophos website, you can find him on Twitter at @gcluley. Special to Dark Reading.