We have recently observed that the Mirai-variant FBot (detected by Trend Micro as IoT.Linux.MIRAI.DLET), also known as Satori, has resurfaced with a brand-new obfuscation technique and a tweaked brute-force attack routine. After analysis, we discovered that this FBot malware uses a peculiar combination of XOR encryption and a simple substitution cipher, which has not been previously used by other IoT malware variants. We have also observed that for its brute-force mechanism, the credentials this FBot sample uses are not located within the executable binary — instead, they are received from a command-and-control (C&C) server.
Our honeypots have been under attack by different IP addresses in an attempt to distribute the FBot malware since mid-November. After analyzing the FBot sample, we discovered that the malware obfuscates the strings it uses for its operation. It decrypts its payload in a host machine using the “0x59” XOR encryption.
Figure 1. XOR-encrypted data embedded in the FBot sample
We have also observed that after the payload has been decrypted, there are two strings used in FBot’s simple substitution cipher:
|Cipher Alphabet||Plaintext Alphabet|
By replacing each character in the encrypted string with the corresponding letter in the plaintext alphabet (i.e., M=A, L=B), the following string was found:
/proc/self/exe/proc//maps/comm/cwd/var/tmp/varpizzadongshelperOfHelpingslavefartedlolfgtwolfexecbintelnetonPLSDIEPOST /cdn-cgi//proc/net/tcpGET / HTTP/1.0uc-httpd 1.0.0re the chicken and the melon tasty enough
It should be noted, though, that some of FBot’s strings were encrypted using the “0x07” XOR encryption.
We have also observed that for its brute-force attacks, this FBot variant uses credentials that are located in its C&C server (5[.]206[.]227[.]65:7685). It accesses the server, downloads a list of usernames and passwords, and attempts to gain access to vulnerable devices.
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Indicators of Compromise
With insights from Augusto Remillano II
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