Caketap, a new Unix rootkit used to siphon ATM banking data

Pierluigi Paganini March 18, 2022

Experts spotted a new Unix rootkit, called Caketap, that was used to steal ATM banking data.

Mandiant researchers discovered a new Unix rootkit named Caketap, which is used to steal ATM banking data, while investigating the activity of the LightBasin cybercrime group (aka UNC1945).

The China-linked hacking group has been active since at least 2016, according to the CrowdStrike researchers it is using a very sophisticated toolset. CrowdStrike researchers reported that at least 13 telecommunication companies were compromised by the group since 2019.

The group hacked mobile telephone networks around the globe and used specialized tools to access calling records and text messages from telecommunications companies.

Now Mandiant researchers documented the activity associated with LightBasin targeting bank customers and focusing on card frauds.

The Caketap rootkit is a Unix kernel module that the UNC2891 threat actor deployed on server running Oracle Solaris. CAKETAP can operate in stealthy mode by hiding network connections, processes, and files. Upon its initialization, the malware removes itself from the loaded modules list and updates the last_module_id with the previously loaded module to delete any trace of its presence.

In order to identify CAKETAP running on a Solaris system, administrators can check for the presence of a hook installed in the ipcl_get_next_conn hook function.

Below is an example command to identify a hooked ipcl_get_next_conn function:

root@solaris:~# echo ‘ipcl_get_next_conn::dis -n 0 ; ::quit’ | mdb -k

The output in a clean SPARC Solaris system would look similar to the following:

ipcl_get_next_conn: save %sp, -0xb0, %sp

Mandiant researchers reported that the rootkit supports the following commands:

EmptyAdd the CAKETAP module back to loaded modules list
MChange the signal string for the getdents64 hook
IAdd a network filter (format <IP>p<PORT>)
iRemove a network filter
PSet the current thread TTY to not be filtered by the getdents64 hook
pSet all TTYs to be filtered by the getdents64 hook
SDisplays the current configuration

Caketap was designed to intercept banking card and PIN verification data from compromised ATM switch servers and carry out unauthorized transactions.

“Memory forensics from one victim’s ATM switch server revealed a variant of CAKETAP with additional network hooking functionality that intercepted specific messages relating to card and pin verification. Evidence suggests that this variant of CAKETAP was used as part of an operation to perform unauthorized transactions using fraudulent bank cards.” reads the analysis published by Mandiant. “This CAKETAP variant targeted specific messages destined for the Payment Hardware Security Module (HSM).”

Caketap allows the manipulation of card verification messages and reply PIN verification messages.

Then the malware saves valid messages that match non-fraudulent PANs (Primary Account Numbers) internally and sends them to the HSM to avoid impacting legitimate customer transactions and arise suspicion.

“Based on Mandiant’s investigation findings, we believe that CAKETAP was leveraged by UNC2891 as part of a larger operation to successfully use fraudulent bank cards to perform unauthorized cash withdrawals from ATM terminals at several banks.” concludes the report. “UNC2891 maintains a high level of OPSEC and employs several techniques to evade detection. The actor uses their skill and experience to take full advantage of the decreased visibility and security measures that are often present in Unix and Linux environments. Mandiant expects that UNC2891 will continue to capitalize on this and perform similar operations for financial gain that target mission critical systems running these operating systems.”

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Pierluigi Paganini

(SecurityAffairs – hacking, Caketap)

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