The latest exploit of the Citadel Trojan is yet more evidence that enterprise perimeter security is only as strong as the weakest endpoint device of its employees.Which is another way of saying, not very strong.Amit Klein, CTO of security vendor Trusteer announced in a blog post on Tuesday that the company had discovered a man-in-the-browser attack using the Citadel Trojan that had compromised the virtual private network (VPN) of a major international airport hub.Oren Kedem, director of product marketing for Trusteer, said the company would not name the airport or even the country where it is located. "It is the major international hub," he said, "so to name the country would be to name the airport."Kedem said the attack was serious enough to prompt the airport to shut down the VPN, essentially leaving 5,000 employees without outside access to the network, and also to involve federal agencies.Both Klein and Kedem said the recent attack is part of a troubling trend -- the Citadel malware used in this attack has typically been used in the past for online banking and other forms of financial fraud, but is now being aimed at enterprises."This attack is especially troubling, given the potential impact on air travel security and border control," Klein wrote.Kedem said it is difficult to speculate on the motive, "but other things than money pop into mind. Maybe some group did it on commission from somebody else -- they might do it without knowing the motive." He said it could involve anything from industrial espionage to drug trafficking to, in the case of an airport, national security."At the least, somebody inside could get a list of employees and their emails," he said.What makes such attacks so effective is that they do not even attempt to breach a VPN perimeter directly. They do it by infecting endpoint devices -- smartphones, tablets or laptops -- of employees, and then stealing the employee credentials for accessing internal applications.[See also:\u00a0Wireless VPNs -- Protecting the wireless wanderer]Kedem said the first step is to infect an employee device. "It can happen in a variety of ways," he said. It could be a form of social engineering, where employees are forwarded to website and then infected with a drive-by download."Most occur this way," he said. "But people could also be driven to a malicious website, or a site that is legit, but been infected. It could also be from social networking, where you're told to check out this cool website that looks benign enough. Another is an email campaign where you're asked to download something. It tells you they found a virus on your system."Kedem said in most cases, antivirus software will not block or even notify a user, since each version of an attack is new. He said Trusteer checked one exploit against 40 antivirus products, "and only four found it. That's more or less the way it is. If it's (the infection) new, it typically wouldn't be detected by antivirus.Whatever the method, once the device is infected, an enterprise's firewall is vulnerable, since the purpose of a VPN is to provide users with secure remote access to applications and data that reside inside an enterprise's firewall."Once an attacker steals a victim's VPN credentials they can login as the authorized user and have unfettered access to the information and resources associated with the account," Amit Klein wrote.In the case of the airport, that was accomplished first through form grabbing, which steals the victim's user name and password, and then screen capture, which takes a snapshot of the image presented to the victim by the strong authentication product.Oren Kedem said this should be a warning to all enterprises with VPNs. "They are definitely all susceptible to this," he said. "The major gap that exists in securing enterprises is outside the perimeter. Inside there is a lot of scrutiny. But once you open it up to BYOD (Bring Your Own Device) and unmanaged devices, you need to consider and apply specific technology and security controls."The same process [to improve security] that happened with online banking needs to happen in enterprise," he said.