As an arbitrage trader, Kerviel made money off price differences between different financial products. Société Générale said Kerviel balanced real and fake trades in order to avoid setting off internal alarms.
Kerviel has been described in some press reports as a computer genius. However, most attackers used unsophisticated methods for exploiting systemic vulnerabilities in applications, processes and procedures, according to the 2005 "Insider Threat Study" by Carnegie Mellon Software Engineering Institute.
That report notes that sophisticated tools are also used in some attacks, which would demand that internal financial systems need to be designed on a more defensive footing.
Programmers should code under the assumption that a hacker or employee will use every means in order to break in, said Ben Rothke, senior security consultant at BT's International Network Services.
"The underlying issue is that many systems are designed to stop honest people from making mistakes, but do not take into account those with malicious intent," Rothke said.
It makes insider jobs one of the toughest to defend against. The psychological profile of an insider tends to be a disgruntled employee who feels wronged by the company, according to the Insider Threat Report.
That in turn can lead to a suspicious behavior such as staying late at work, which paradoxically might only signal a committed employee.
"It's always the insider," Rothke said. "It's often harder to steal $10,000 from a bank than $10 million."