A new campaign is spreading the LimeRAT Remote Access Trojan using an old Excel file encryption technique. LimeRAT is a simple Trojan designed for computers Windows. This malware can install backdoors on infected computers and encrypt files in exactly the same way as other types ransomware, add computers to botnets and install miners cryptocurrencies. In addition, the LimeRAT Trojan can spread through connected drives USB, uninstall if a virtual machine (VM) is detected, "lock" screens and steal data that is then sent to a command and control server (C2) via AES (Advanced Encryption Standard) encryption. In a new campaign discovered by Mimecast, the Trojan appears as payload in Excel documents and spread through Phishing messages. Researchers said in a related blog post on the company that Excel documents are read-only - not locked - as Excel encrypts them without requiring users to enter a password.
To decrypt the file, when opened, Excel will try to use a built-in password, "VelvetSweatshop", which was previously used by developers of Microsoft. If this succeeds, Excel decrypts the file and allows macros to be started and malicious payload injected, while keeping the document read-only.
Usually, if decryption through VelvetSweatshop fails, then users must set a password. However, the feature that allows only reading a file bypasses this step, thereby reducing the steps required to access a Windows computer. According to the researchers, the advantage of having a hacker feature that only allows reading in Excel files is that it requires no user input, and the Microsoft Office system will display no warning but only a warning that says file is read-only.
The new campaign created for the dissemination of LimeRAT uses this technique, which first appeared in 2013 and was presented at a conference of Virus Bulletin. In addition, there is a vulnerability identified as CVE-2012-0158, which is exploited by hackers. It is worth noting that this issue has been presented a long time ago. However, Sophos notes that hackers are still exploiting this vulnerability in a case that is considered "remarkable". Mimecast reports that hackers are also using a host of other techniques in an attempt to trick users' systems into encrypting spreadsheet content to hide exploitation and payload. Finally, Microsoft has been informed that this vulnerability is being used again.