Man-in-the-middle (MitM) attacks have become commonplace; most organizations now deploy strong authentication strategies and encryption tactics to combat this problem. Malicious actors, however, are getting more sophisticated. According to CSO Online, cybercriminals are launching man-in-the-cloud (MitC) attacks that leverage popular file sync services to gain complete and persistent access to stored files, all without the need for user credentials.

File Sync Gains Token Appreciation

For many companies, cloud-based file services are the ideal way to accommodate increasingly mobile employees — when workers can sync files from any device, anywhere, increased productivity is the natural result. As noted by SecurityWeek, however, there’s a problem: Popular options like Dropbox, Google Drive, Microsoft OneDrive and Box are all vulnerable to cybercriminal hijackings.

To streamline file sync, these services rely on access tokens generated after users log in for the first time. These tokens are then stored in the Windows registry or Windows Credential Manager. The result is that employees aren’t required to continually re-authenticate, but as discovered by researchers from Imperva, tokens provide an ideal access point for attackers.

The Imperva team created a tool called Switcher that infects a system via email or drive-by download. A copy is made of the user’s access token, which is then replaced by one from an attacker-controlled account. After an app restart, the file service is synchronized with the attacker’s account and then the process happens in reverse — a copy of the original token is copied to the sync folder and the app is restarted again, giving cybercriminals total access to user files.

Even more worrisome? The ten lines of code needed for Switcher don’t look like malware, meaning it’s unlikely the program would get caught by current threat detection software. There’s no need for malicious actors to go after user credentials, either, since the token does all the work.

Persistent Problems

While Imperva CTO Amichai Shulman said file sync solutions “aren’t dangerous or insecure,” he noted that “it’s kind of a trade-off between usability and security.” According to ZDNet, the kind of persistent tokenization used by cloud services comes with a number of security concerns. First is the problem of passwords. Since most tokens are linked to a device rather than a session, even changing passwords won’t prevent attacker access. Users need to fully log out and change devices — or delete their accounts entirely — to get ahead of criminals.

In addition to the risks of stolen or ransomed files, there’s the problem of persistent access. It’s possible for programs like Switcher to create scheduled tasks or Windows Management Instrumentation (WMI) events triggered by specific files in the sync folder, giving them both ongoing access and the ability to leverage systems as cloud command-and-control (C&C) servers.

Ultimately, file sync services offer a huge advantage for businesses looking to leverage the power of both mobile workers and devices. Tokenization is a necessary evil here. It’s a way to simplify user access without leaving files vulnerable to standard attack vectors, but MitC attacks are a natural evolution of the medium. Bottom line? There’s no quick fix, but if companies do better than token security investments — for example, deploying file- and database-monitoring tools — it’s possible to minimize the risks of hijacked cloud accounts.

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