Trusteer have recently discovered a sophisticated Man in the Browser (MitB) enterprise attack that targets VPN users at a major international airport hub.


Using the Citadel Trojan, criminals are targeting employees to steal their credentials for accessing internal airport applications. Trusteer has notified airport officials and the relevant government agencies of this attack. Due to the sensitive nature of these systems, the airport immediately disabled remote employee access through this VPN site – the site is currently down.

VPNs are often used to secure Internet connections between a business’ private network and a remote employee. VPNs provide users with secure remote access to applications and data that reside inside an enterprise’s firewall. However, 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.

This attack uses a combination of form grabbing and screen capture technologies to steal the victim’s username, password, and the one-time passcode generated by a strong authentication product (we have also contacted this vendor).

The first part of the attack uses form grabbing to steal the username and password entered into the login screen. The second part of the attack relies on screen capture capabilities to take a snapshot of the image presented to the victim by the strong authentication product.

The strong authentication product offers two authentication options: single channel and dual channel.

The dual channel mode sends users a one-time PIN via SMS or a mobile application.

The single channel mode is activated when users select the “Get Image” option at login. This prompts the strong authentication product to generate an on-screen CAPTCHA of 10 digits. The user then maps their original (static) password to the string of digits in the image to produce the “one time code”.

This security measure prevents the form grabber from capturing the actual static password. This is where the screen capturing feature in Citadel kicks in.

By capturing the image, the attacker uses the permutation of digits, along with the one-time code stolen by the form grabber, to reproduce the static password.

This is a clever use of form grabbing and screen grabbing techniques by attackers. It also demonstrates how enterprises that rely on strong authentication approaches are still at risk from targeted attacks if they lack cybercrime prevention security on endpoint devices.

This is especially true in the case of unmanaged or BYOD endpoints. Since these devices are exposed to threats that would otherwise be filtered at the enterprise perimeter, they are much more vulnerable to infection from advance malware like Citadel, Zeus, SpyEye, etc. When an infected unmanaged device connects to the enterprise via a VPN or some other remote access method, malware can circumvent strong authentication systems, as evidenced by the attack above.

The Citadel malware used in this attack is typically used to execute online banking and other forms of financial fraud. However, this is not the first time Trusteer have identified and reported on enterprises being targeted by screen-capturing/form grabbing financial malware. This attack is especially troubling given the potential impact on air travel security and border control.