Ever since security giant RSA was hacked last March, anti-virus researchers have been trying to get a copy of the malware used for the attack to study its method of infection. But RSA wasn’t cooperating, nor were the third-party forensic experts the company hired to investigate the breach.

This week Finnish security company F-Secure discovered that the file had been under their noses all along. Someone—the company assumes it was an employee of RSA or its parent firm, EMC—had uploaded the malware to an online virus scanning site back on March 19, a little over two weeks after RSA is believed to have been breached on March 3. The online scanner, VirusTotal, shares malware samples it receives with security vendors and malware researchers.

RSA had already revealed that it had been breached after attackers sent two different targeted phishing e-mails to four workers at its parent company EMC. The e-mails contained a malicious attachment that was identified in the subject line as “2011 Recruitment plan.xls.”

None of the recipients were people who would normally be considered high-profile or high-value targets, such as an executive or an IT administrator with special network privileges. But that didn’t matter. When one of the four recipients clicked on the attachment, the attachment used a zero-day exploit targeting a vulnerability in Adobe Flash to drop another malicious file—a backdoor—onto the recipient’s desktop computer. This gave the attackers a foothold to burrow farther into the network and gain the access they needed.

“The e-mail was crafted well enough to trick one of the employees to retrieve it from their Junk mail folder, and open the attached excel file,” RSA wrote on its blog in April.

The intruders succeeded in stealing information related to the company’s SecurID two-factor authentication products. SecurID adds an extra layer of protection to a login process by requiring users to enter a secret code number displayed on a keyfob, or in software, in addition to their password. The number is cryptographically generated and changes every 30 seconds.

The company initially said that none of its customers were at risk, since the attackers would need more than the data they got from RSA to break into customer systems. But three months later, after defense contractor Lockheed Martin discovered hackers trying to breach their network using duplicates of the SecurID keys that RSA had issued the company—and other defense contractors such as L-3 were targeted in similar attacks—RSA announced it would replace most of its security tokens.

So just how well crafted was the e-mail that got RSA hacked? Not very, judging by what F-Secure found.

The attackers spoofed the e-mail to make it appear to come from a “web master” at Beyond.com, a job-seeking and recruiting site. Inside the e-mail, there was just one line of text: “I forward this file to you for review. Please open and view it.” This was apparently enough to get the intruders the keys to RSAs kingdom.

F-Secure produced a brief video showing what happened if the recipient clicked on the attachment. An Excel spreadsheet opened, which was completely blank except for an “X” that appeared in the first box of the spreadsheet. The “X” was the only visible sign that there was an embedded Flash exploit in the spreadsheet. When the spreadsheet opened, Excel triggered the Flash exploit to activate, which then dropped the backdoor—in this case a backdoor known as Poison Ivy—onto the system.

Poison Ivy would then reach out to a command-and-control server that the attackers controlled at good.mincesur.com, a domain that F-Secure says has been used in other espionage attacks, giving the attackers remote access to the infected computer at EMC. From there, they were able to reach the systems and data they were ultimately after.

F-Secure notes that neither the phishing e-mail nor the backdoor it dropped onto systems were advanced, although the zero-day Flash exploit it used to drop the backdoor was advanced. And ultimately, the fact that the attackers hacked a giant like RSA just to gain the information they needed to hack Lockheed Martin and other defense contractors exhibited a high level of advancement, not to mention chutzpah.