OOHU Ransomware (.oohu File Extension)

What is Oohu?

The Oohu ransomware is a malicious program that encrypts your files and demands payment for their decryption. It not only locks your data but also alters system configurations, weakening your computer's defenses. Preventing its intrusion is far easier than dealing with its damaging effects.

Most infections of Oohu occur through the use of pirated software. Hackers bypass software license verifications, allowing unauthorized use without payment. In doing so, they often embed malware into the program's code. The Oohu ransomware is a common example of malware distributed this way.

This ransomware employs the Salsa20 encryption algorithm to encrypt your files. While Salsa20 isn't the most advanced encryption method, the decryption key is stored on a remote server controlled by the attackers. This makes decrypting your files without paying the ransom virtually impossible. Brute-force attempts are ineffective due to the immense number of possible keys. File decryption tools offer minimal hope, and recovering your files without the key is highly unlikely.

The authors of the Oohu have Russian roots. The frauds use the Russian language and Russian words written in English and related domains registered through Russian domain-registration companies.

Oohu Encryption process

The Oohu 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 - ".oohu". 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 Oohu 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.

Oohu 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)