File synchronization services, used to accommodate roaming employees inside organizations, can also be a weak point that attackers could exploit to remain undetected inside compromised networks.
Researchers from security firm Imperva found that attackers could easily hijack user accounts for services from Dropbox, Google Drive, Microsoft OneDrive and Box if they gain limited access to computers where such programs run -- without actually stealing user names and passwords.
Once the accounts are hijacked, attackers could use them to grab the data stored in them, and to remotely control the compromised computers without using any malware programs that could be detected by antivirus and other security products.
The Imperva researchers found that all of the file synchronization applications they looked at provide continued access to users' cloud storage accounts via access tokens that are generated after users log in for the first time. These tokens are stored on users' computers in special files, in the Windows registry or in the Windows Credential Manager, depending on the application.
The researchers developed a simple tool they dubbed Switcher, whose role is to perform what they call a "double switch" attack.
Switcher can be deployed on the system through a malicious email attachment or a drive-by download exploit that takes advantage of a vulnerability in a browser plug-in. If an exploit is used, the program doesn't even have to be written to disk. It can be loaded directly into the computer's memory and doesn't need high-level privileges to execute its routine.
The Switcher first makes a copy of the user's access token for the targeted file synchronization app and replaces it with one that corresponds to an account controlled by the attacker. It then restarts the application so that it synchronizes with the attacker's account.
The previously saved user token is copied to the synchronized folder so that the attacker receives a copy and then the Switcher app restores it back, forcing the app to be linked back to the user's real account -- hence the double switch name.
However, since the attacker now has a copy of the user's access token, he can use the Switcher on his own computer and synchronize it with the user's real account, getting a copy of all of the files stored in it.
The attack can be taken to the next step by having the Switcher create a scheduled task or a Windows Management Instrumentation (WMI) event that would be triggered when a specific file appears in the synchronized folder. That file could be created by the attacker and could contain commands to be executed by the scheduled task.
This mechanism would give the attacker persistent remote access to the computer even after Switcher deletes itself or is removed from memory. After executing a command and saving its output to the synchronized folder, the attacker could delete it, as well as the trigger file in order to cover his tracks.
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