Security researchers today published the details of how a ransomware attack could abuse the Windows Encrypting File System (EFS). Several major security vendors have released patches to protect machines from this attack after anti-malware tools failed to defend against the technique.
The discovery comes from SafeBreach Labs, where researchers were brainstorming new, more sophisticated ways to implement ransomware. "It's important we understand what can be done so we can develop better controls around it," says co-founder and CTO Itzik Kotler. One of their goals was to find attack vectors that today's defenses lack capabilities to defend against.
Starting in Windows 2000, Microsoft began to offer EFS to business customers using the Windows Pro, Professional, Business, Ultimate, Enterprise, and Education editions. EFS enables encryption of specific folders and files keyed to the Windows user. Encryption and decryption are done in the NTFS driver, under the file system filter drivers. Part of the encryption key is stored in a file the user can access; part is computed from the account password. EFS should not be confused with BitLocker, which is a full-disk encryption feature.
Researchers created their concept ransomware in a lab environment to test whether antivirus software could defend against it. Because this malware uses EFS functionality, as opposed to the typical ransomware tactic of overwriting the file, it uses a different set of system calls.
"We thought there was good potential there for completely evading security controls," says Amit Klein, vice president of security research. "Indeed, that turned out to be the case."
The malware they developed first generates a key to be used by EFS, as well as a certificate for that key, which is added to the personal certificate store. It then sets the current EFS key to the certificate the malware created; now, this key can be invoked on specific files and folders to encrypt them. The ransomware saves the key files to memory and deletes them from two folders:
- %APPDATA% \Microsoft\Crypto\RSA\sid\ (where sid is the user SID)
- %ProgramData% \Microsoft\Crypto\RSA\MachineKeys\
From there, the ransomware erases the EFS data from memory, rendering the encrypted files inaccessible to the victim. Ideally, the researchers explain, it also wipes slack parts of the disk to ensure data from the EFS key files and temporary files used by EncryptFile can't be retrieved. The malware can now encrypt data it stole from the two previously mentioned files using a public key hardwired into the ransomware and send encrypted data to the attacker. Files are encrypted at a deep level of the kernel and won't be noticed by file-system filter drivers. The attack doesn't require admin rights or human interaction, Klein writes in a blog post.
Every ransomware should have a way to restore the files, Klein explains, and this one is no different. An attacker would need to decrypt the key files using their private key to restore them to their original state. When this happens, Windows will be able to read the user files.
Researchers tested the EFS ransomware on Windows 10 64-bit versions 1803, 1809, and 1903. It should also work on Windows 32-bit operating systems and on earlier version of Windows — likely Windows 8.x, Windows 7, and Windows Vista.
Inside the Patching Process
The team tested its malware with three anti-ransomware tools from well-known vendors: ESET (Internet Security 126.96.36.199), Kaspersky (Anti Ransomware Tool for Business 188.8.131.521a), and Microsoft (Windows 10 Controlled Folder Access on Windows 10 64-bit version 1809, build 17763). All three failed to defend against this type of ransomware attack.
SafeBreach then notified 17 major anti-malware and anti-ransomware vendors for Windows endpoints, provided its proof of concept, and found many products were affected.
"The whole business of disclosing this threat to major solution vendors is about reducing the threat of this being used in the wild at some later point," Klein says. SafeBreach disclosed the attack to companies in June and July 2019, he notes. More than six months passed between the time of disclosure to the last vendor and SafeBreach's public disclosure today.
"Some vendors took a very short while to figure out what the problem is and how they want to address it," he notes. "Other vendors took quite a bit of time to start working on addressing the issue." Most affected vendors deployed updates to defend against this attack technique.
One possible workaround for this attack is to disable ESF entirely, which is possible with admin rights. Klein advises taking this route if your organization does not actively use the feature.
As ransomware evolves, security vendors must also adapt to defend against new and changing threats. Signature-based tools "are not up to this job," Klein writes in his post, and while heuristics-based solutions hold promise, additional research is required to train them to protect against future threats.
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