Master Key for Hive Ransomware Retrieved Using a Flaw in its Encryption Algorithm

Researchers have detailed what they call the “first successful
attempt” at decrypting data infected with Hive ransomware
without relying on the private key used to lock access to the
content.

“We were able to recover the master key for generating the file
encryption key without the attacker’s private key, by using a
cryptographic vulnerability identified through analysis,” a group
of academics from South Korea’s Kookmin University said^[1] in a new paper analyzing
its encryption process.

Hive, like other cybercriminals groups, operates a
ransomware-as-a-service that uses different mechanisms to
compromise business networks, exfiltrate data, and encrypt data on
the networks, and attempt to collect a ransom in exchange for
access to the decryption software.

It was first observed^[2]
in June 2021, when it struck a company called Altus Group. Hive
leverages a variety of initial compromise methods, including
vulnerable RDP servers, compromised VPN credentials, as well as
phishing emails with malicious attachments.

The group also practices the increasingly lucrative scheme of
double extortion^[3], wherein the actors go
beyond just encryption by also exfiltrating sensitive victim data
and threatening to leak the information on their Tor site,
“HiveLeaks.”

As of October 16, 2021, the Hive RaaS program has victimized at
least 355 companies, with the group securing the eighth spot^[4]
among the top 10 ransomware strains by revenue in 2021, according
to blockchain analytics company Chainalysis.

The malicious activities associated with the group have also
prompted the U.S. Federal Bureau of Investigation (FBI) to release
a Flash report^[5]
detailing the attacks’ modus operandi, noting how the ransomware
terminates processes related to backups, anti-virus, and file
copying to facilitate encryption.

The cryptographic vulnerability identified by the researchers
concerns the mechanism by which the master keys are generated and
stored, with the ransomware strain only encrypting select portions
of the file as opposed to the entire contents using two keystreams^[6]
derived from the master key.

“For each file encryption process, two keystreams from the
master key are needed,” the researchers explained. “Two keystreams
are created by selecting two random offsets from the master key and
extracting 0x100000 bytes (1MiB) and 0x400 bytes (1KiB) from the
selected offset, respectively.”

The encryption keystream, which is created from an XOR operation^[7]
of the two keystreams, is then XORed with the data in alternate
blocks to generate the encrypted file. But this technique also
makes it possible to guess the keystreams and restore the master
key, in turn enabling the decode of encrypted files sans the
attacker’s private key.

The researchers said that they were able to weaponize the flaw
to devise a method to reliably recover more than 95% of the keys
employed during encryption.

“The master key recovered 92% succeeded in decrypting
approximately 72% of the files, the master key restored 96%
succeeded in decrypting approximately 82% of the files, and the
master key restored 98% succeeded in decrypting approximately 98%
of the files,” the researchers said.