TCBU Ransomware (.tcbu File Extension)

What is Tcbu?

The Tcbu virus can accurately be categorized as ransomware. It encrypts your files and then demands a ransom for their release. Tcbu is part of the STOP/Djvu ransomware family, currently among the most prolific. This group predominantly preys on individuals downloading software from third-party websites.

During an attack by the Tcbu malware, your files are encrypted using the Salsa20 algorithm. This encryption technique implies approximately 2^256 possible decryption keys, equating to a 78-digit number. Regrettably, obtaining the key through brute force or other common hacking methods is practically unfeasible.

The distribution method of the Tcbu malware is critical to understand. It's established that dubious websites offering "cracked" software and unreliable tools are the main distribution channels for the entire STOP/Djvu family, and Tcbu is no exception. This ransomware is typically embedded in software presented as "free", despite being paid products on official sites. Users who bypass license verification in these programs inadvertently incorporate malware into the "final product", as virus developers compensate them for this. Similarly, questionable tools like cheat engines, Windows activators, or keygens often require disabling your antivirus software, leaving you vulnerable and likely unaware of the infection until it's too late.

Tcbu Encryption process

The Tcbu uses the Salsa20 encryption algorithm. That is not the strongest method, but it still provides an overwhelming amount of possible decryption keys. To brute force the 78-digit number of keys, you need 3.5 unvigintillion years (1*10^65), even if you use the most powerful regular PC. Quantum computers can show a bit better results, but it is still too slow to get your files back while you are alive.

The exact algorithm of encryption is next: malware scans each folder for the files it is able to encrypt. Then, when it finds the target, it makes a copy of your file, removes the original one, encrypts the copy and leaves it instead of the removed original. Such a procedure is done to prevent the situation when you have already opened the file, so ransomware is not able to read it because of the Windows restrictions. To each encrypted copy, the virus adds the specific extension - ".tcbu". Then, ransomware creates a _readme.txt file in the folder where the encrypted file is located, and goes to the next folder.

Such an encryption method can be exploited for file recovery. Since the original file is deleted, you may try to recover it using the file recovery tools. The less time is passed - the bigger the chance to get your files back, so hurry up!

Another specific moment that can help you to use the files even after the encryption is the fact that Tcbu ransomware encrypts only the first 150KB of each file. Hence, you can try to run a big file, such as video or music, without the encryption. Similar feature also works with other ransomware families - Dharma, Conti and Makop encrypt the same 150KB.

Tcbu note:

Ransom note is the same for the whole STOP/DJVU family. In fact, it is one of the main signs of to which family the certain ransomware belongs. Here is the typical note for STOP/Djvu family:

External Links:

Indicators of Compromise(IOC)