ZOUU Ransomware (.zouu File Extension)

What is Zouu?

Zouu ransomware is a member of the STOP/Djvu ransomware family, utilizing the Salsa20 cipher to encrypt your files and append a .zouu extension to them. Such files become inaccessible through conventional means. Beyond encrypting files, this malware also manipulates system settings, undermining security mechanisms. These modifications could potentially cause system instability and complicate routine computer usage.

The encryption strength of Zouu ransomware makes brute force attempts futile, given the impracticality of generating and testing the vast array of possible keys. While certain decryption tools, possessing pre-compromised keys, might offer a slim chance of file recovery, the broader threat lies in the ransomware's system configuration alterations. These changes can severely limit your ability to install new programs or access specific websites, posing a significant problem, particularly for those without critical data backups.

The most effective defense against ransomware is prevention. Many infections by the Zouu virus stem from the use of pirated software and illegal tools. Such programs are often distributed via peer-to-peer networks and third-party "software collector" sites, offered for free after being cracked to bypass license verification. Hackers, compensated for embedding malicious code into these applications, have a clear incentive to continue these practices. Thus, understanding and avoiding the sources of such threats is crucial.

Zouu Encryption process

The Zouu 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 - ".zouu". 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 Zouu 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.

Zouu 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)