Even as Facebook constantly monitors its security system, it is still vulnerable to being penetrated by malicious hackers. Recently, a white hat hacker tried to gain backdoor entry and steal employees' login details, but to his surprise, he learnt that a bug found in the Accellion File Transfer Appliance had already been planted in the social media giant's system.

Taking advantage of the Menlo Park-headquartered firm's bug bounty program, white hat hacker Orange Tsai managed to breach a Facebook employee's account, only to discover that someone had already planted the backdoor bug.

Fortunately, the threat was a non-issue and Facebook's security researcher Reginaldo Silva confirmed that the malware Tsai discovered was actually installed by another security researcher.

Silva said: "On this case, the software we were using is third party. As we don't have full control of it, we ran it isolated from the systems that host the data people share on Facebook. We do this precisely to have better security, as chromakode mentioned. After incident response, we determined that the activity Orange detected was in fact from another researcher who participates in our bounty program. Neither of them were able to compromise other parts of our infra-structure so, the way we see it, it's a double win: two competent researchers assessed the system, one of them reported what he found to us and got a good bounty, none of them were able to escalate access."

Tsai, who works at a Taiwan-based firm called Devcore, discovered that the file transfer app had a few vulnerabilities that would essentially provide intruders access to key corporate information. Tsai also found that the hackers used a code that allowed them to access around 300 employees' credentials between 1 and 7 February 2016.

He also noted that the hackers had – on two previous occasions – attempted to penetrate Facebook's system. However, he was unable to ascertain if the attack was carried out by the same hacker. Facebook rewarded Tsai with $10,000 for his work, the Tech Times reported.