Not long ago, news appeared online of a younger sibling for the sensational vulnerability EternalBlue. The story was about a new vulnerability for *nix-based systems – EternalRed (aka SambaCry). This vulnerability (CVE-2017-7494) relates to all versions of Samba, starting from 3.5.0, which was released in 2010, and was patched only in the latest versions of the package (4.6.4/4.5.10/4.4.14).
On May 30th our honeypots captured the first attack to make use of this particular vulnerability, but the payload in this exploit had nothing in common with the Trojan-Crypt that was EternalBlue and WannaCry. Surprisingly, it was a cryptocurrency mining utility!
In order to check that an unauthorized user has permissions to write to the network drive, the attackers first try to write a text file, consisting of 8 random symbols. If the attempt is successful they delete the file.
Writing and deleting the text file
After this check, it is time for the exploit's payload (it is assembled as a Samba plugin). After successful exploitation of the vulnerability, this runs with super-user privileges, although first the attackers have to guess the full path to the dropped file with their payload, starting from the root directory of the drive. We can see such attempts in the traffic captured on our honeypot. They are just brute-forcing the most obvious paths (specified in different manuals, etc.), where files can be stored on the drive.
Bruteforcing the path to the payload
After the path to the file is found, it can be loaded and executed in the context of the Samba-server process, using the SambaCry vulnerability. Afterwards the file is deleted in order to hide the traces. From this moment it exists and runs only in the virtual memory.
In our case two files were uploaded and executed in such a way: INAebsGB.so (349d84b3b176bbc9834230351ef3bc2a - Backdoor.Linux.Agent.an) and cblRWuoCc.so (2009af3fed2a4704c224694dfc4b31dc - Trojan-Downloader.Linux.EternalMiner.a).
This file stores the simplest reverse-shell. It connects to the particular port of the IP-address specified by its owner, giving him remote access to the shell (/bin/sh). As a result, the attackers have an ability to execute remotely any shell-commands. They can literally do anything they want, from downloading and running any programs from the Internet, to deleting all the data from the victim's computer.
Listing of INAebsGB.so
It's worth noting that a similar payload can be found in the implementation of the SambaCry exploit in Metasploit.
The main functionality of this file is to download and execute one of the most popular open-source cryptocurrency mining utilities – cpuminer (miderd). It is done by the hardcoded shell-command, shown on the screenshot below.
The main functionality of cblRWuoCc.so
The file minerd64_s (8d8bdb58c5e57c565542040ed1988af9 — RiskTool.Linux.BitCoinMiner.a) downloaded in such a way is stored in /tmp/m on the victim's system.
The interesting part is that the version of cpuminer used is "upgraded", so it can be launched without any parameters to mine currency directly to the hardcoded attackers' wallet. We obviously became interested in this wallet, so we decided to investigate a bit and uncover the balance of the attackers account.
Along with the attackers' wallet number, the pool address (xmr.crypto-pool.fr:3333) can be found in the body of the miner. This pool is created for mining the open-source cryptocurrency – monero. Using all this data we managed to check out the balance on the attackers' wallet and the full log of transactions. Let's have a look:
Balance of the attackers' account on 08.06.2017
Log of transactions with all the attackers' cryptocurrency income
The mining utility is downloaded from the domain registered on April 29th 2017. According to the log of the transactions, the attackers received their first crypto-coins on the very next day, on April 30th. During the first day they gained about 1 XMR (about $55 according to the currency exchange rate for 08.06.2017), but during the last week they gained about 5 XMR per day. This means that the botnet of devices working for the profit of the attackers is growing.
Considering that the world discovered the EternalRed vulnerability only at the end of May, and the attackers had already adopted it, the rate of growth in the number of infected machines has significantly increased. After about a month of mining, the attackers gained 98 XMR, which means they earned about $5,500 according to the currency exchange rate at the time of writing.
As a result, the attacked machine turns into a workhorse on a large farm, mining crypto-currency for the attackers. In addition, through the reverse-shell left in the system, the attackers can change the configuration of a miner already running or infect the victim's computer with other types of malware.
At the moment we don't have any information about the actual scale of the attack. However, this is a great reason for system administrators and ordinary Linux users to update their Samba software to the latest version immediately to prevent future problems.