Revision of fundamentals of linux privilege escalation to add protections and decetions

1. Fundamentals of
Linux Privilege
Escalation

2. Introduction
❖ Elliott Cutright
❖ Sr. Red Team for a Fortune 10 in Richmond VA
❖ Professional Red Team for 6 years
❖ Linux and Web Applications
❖ Past worked in Threat Intelligence and Systems Admin
and a 24 x 7 x 365 DOD SOC

3. Disclaimer
The views and opinions expressed here are
those of Elliott Cutright only and in no way
represent the views, positions or opinions -
expressed or implied - of my employer or
anyone else.

4. Setup
❖ This is NOT how to get in
❖ How do we go from low privileges to high privileges
❖ Webshells, Stolen SSH Keys, etc
❖ We do not know the user's password
❖ Everything in this talk is something I have done or seen
in the real world on real production machines; This is not
THEORY, it's FACT

5. Method 1:
Exploits

6. Exploits
❖ Most take advantage of a flaw in the Linux Kernel
❖ Easier because reliable exploit code is widely available
❖ Be careful, if unreliable good chance you will crash
system as you might see in the demo
❖ Generally low skill set can achieve grand results

7. Exploits
❖ Identify OS and Kernel Version
❖ Enumerate tools to build exploit (gcc, python, perl, etc)
❖ Get the exploit to the system
❖ Execute Exploit
❖ …
❖ ROOT

8. Exploit - ID System
❖ Determine kernel version
❖ uname -a
❖ Linux ubuntu-demo 3.8.0-19-generic #30-Ubuntu
SMP Wed May 1 16:36:13 UTC 2013 i686 i686 i686
GNU/Linux
❖ Linux cent-demo 2.6.18-8.el5 #1 SMP Thu Mar 15
19:57:35 EDT 2007 i686 i686 i386 GNU/Linux

9. Exploit - ID System
❖ OS Release
❖ Ubuntu - cat /etc/lsb-release
❖ DISTRIB_ID=Ubuntu
❖ DISTRIB_RELEASE=13.04
❖ DISTRIB_CODENAME=raring
❖ DISTRIB_DESCRIPTION="Ubuntu 13.04”
❖ RedHat/CENT - cat /etc/redhat-release
❖ CentOS release 5 (Final)

10. Exploit - Get the file on the
Server
❖ Any means available
❖ curl/wget
❖ NetCat
❖ FTP
❖ SCP/SFTP
❖ SMB
❖ TFTP
❖ Copy/Paste - for source code
❖ DNS TXT Records - for source code

11. Exploit - Where To Hide It?
❖ Directories starting with a ‘.’ are hidden on Linux
Filesystem
❖ /tmp/.nothinghere/exploit.c
❖ /tmp/…/exploit.c
❖ Verify you can run commands from your directory
❖ mount
❖ /dev/vda3 on /tmp type ext4 (rw,noexec)

12. Exploit - ID Build System
❖ gcc -v
❖ Using built-in specs.
❖ COLLECT_GCC=gcc
❖ Target: i686-linux-gnu
❖ Configured with: ../src/configure ……..
❖ gcc version 4.7.3 (Ubuntu/Linaro 4.7.3-1ubuntu1)
❖ python -V
❖ Python 2.4.3

13. Exploit - ID Build System
❖ gcc -v
❖ -bash: gcc: command not found
❖ Common on Servers
❖ python -V
❖ -bash: /usr/bin/python: No such file or directory
❖ RARE

14. Exploit - Building The Exploit
❖ Most exploits have build directions in the headers
❖ Most common method
❖ gcc exploit.c -o exploit
❖ ./exploit

15. Exploit - Build Local
❖ If GCC is not present, build a VM or VPS with the exact
matching kernel and OS (Ex. Ubuntu 13.10 with Kernel
3.8.0-19-generic)
❖ Once build on your local system, move the compiled
exploit to your target system
❖ WARNING: This is not the preferred method and can
have unexpected results…but may work in a pinch

16. CVE-2009-2692 - sock_sendpage() exploit
https://www.youtube.com/watch?v=65w7ROFbdqc
Demo

17. Protect/Detect
❖ Patching
❖ No Really…Install Patches
❖ Limit locations for code execution
❖ GRSecurity, if you are up to it
❖ You need to be really comfortable with Linux for this one
❖ Adds significant overhead to updating as you have to
rebuild for EVERY kernel version

18. Method 2:
File Permissions

19. World Readable/Writeable
❖ These are files that anyone can read or write
❖ Easy to find
❖ find / -perm -2 ! -type l -ls
❖ My Ubuntu box had 1,681 files and folder and its a
basic install of 14.04

20. Dangers
❖ ANYONE can read or write these files
❖ While that is by design for some files, others it adds a
great deal of risk
❖ Config Files
❖ Websites /Application source code
❖ Scripts run by init or cron
❖ Commands/Scripts used by admins

21. Protect/Detect
❖ World Read/Write is normal part of the filesystem
❖ Issues arise when users/admins/scripts start changing
permissions
❖ stop using `chmod 777` please
❖ Audit on a semi-regular basis for overly permissive files
and folders

22. SetUID and SetGID
❖ SetUID - SET User ID upon execution
❖ SetGUID - SET Group ID upon execution
❖ Allows you to run programs as another user upon
execution
❖ Generally executed as elevated privilege user (root)

23. SetUID Risks
❖ Binaries run with elevated privileges can access
privileged information
❖ SetUID on ‘ls’ will allow you to list directories you
otherwise wouldn’t have rights to
❖ SetUID on ‘vim’ will allow you to edit files you
otherwise wouldn’t have rights to

24. SetUID Risks
❖ Buffer overflow exploits or command injection flaws in
SetUID applications will result in the attacker running
code with the elevated privileges

25. Find SetUID
❖ ls -l /bin/ls
❖ -rwxr-xr-x 1 root root 108708 Jan 17 2013 /bin/ls
❖ dir:owner:group:world
❖ ls -al /bin/ping
❖ -rwsr-sr-x 1 root root 34780 Oct 2 2012 /bin/ping

26. Find SetUID
❖ sudo find / -xdev ( -perm -4000 ) -type f -print0 -exec ls
-l {} ;
❖ note: sudo is not required, you just wont be able to
check directories you don't have permissions to

27. Exploiting SetUID
❖ Use the functionality of the tool in unintended ways for
elevated privileges (more on this idea later)
❖ Find an application that has public exploit or start fuzzing
on your own
❖ Command Injection

28. Protect/Detect
❖ While setUID is 100% required under normal operations
we see admins overusing it
❖ It is not a fix all
❖ Understand the Risk vs Reward when setting setUID on
an application; Do audits for these apps

29. Method 3:
Permissive
SUDO

30. SUDO
❖ su do
❖ note: `su` does not mean SuperUser, it is Substitute
User
❖ Allows you to run commands as elevated user with your
user password rather than a shared root (BAD!)
password

31. /etc/sudoers
❖ Config file for sudo
❖ Limits what users and groups can run what commands
❖ ex:
❖ rootALL=(ALL:ALL) ALL
❖ %sudo ALL=(ALL) NOPASSWD:ALL

32. /etc/sudoers
❖ Can allow for very granular configurations
❖ User_Alias FULLTIMERS = millert, mikef, dowdy
❖ Host_Alias SERVERS = master, mail, www, ns
❖ Cmnd_Alias SHUTDOWN = /usr/sbin/shutdown
❖ Cmnd_Alias REBOOT = /usr/sbin/reboot
❖ FULLTIMERS ALL = NOPASSWD: ALL
❖ mikef ALL, !SERVERS = ALL

33. Concerns
❖ With great power, comes great responsibility
❖ sudo will allow you to shoot yourself in the foot
❖ THINK about the commands you allow via sudo

34. Problems?
❖ Why are these commands an issue?
❖ vi/vim
❖ more/less/cat
❖ echo
❖ nmap

35. Find Exec
Demo

36. Protect/Detect
❖ Again, Risk vs Reward of allowing sudo
❖ The more specific you can be in config, the better
❖ Know what the application you are allowing CAN do

37. Method 4:
PATH issues

38. Linux PATH
❖ An environment variable that contains the location of
executables
❖ printenv
❖ PATH=/usr/local/rvm/gems/ruby-1.9.3-
p448/bin:/usr/local/rvm/gems/ruby-1.9.3-
p448@global/bin:/usr/local/rvm/rubies/ruby-1.9.3-
p448/bin:/usr/local/rvm/bin:/usr/local/sbin:/usr/local/bin
:/usr/sbin:/usr/bin:/sbin:/bin

39. Linux PATH
❖ ruby -v
❖ ruby 1.9.3p448 (2013-06-27 revision 41675) [i686-
linux]
❖ which ruby
❖ /usr/local/rvm/rubies/ruby-1.9.3-p448/bin/ruby

40. Linux PATH Issues
❖ What would happen if the ‘.’ was prepended to the path?
❖ Where would it look for ruby first?
❖ What if a script was calling ruby?
❖ As root…….

41. Attack Path Example
❖ Sysadmin has ‘.’ in his path
❖ Email and say you can’t list the files in your home dir
❖ Make bash script called ‘ls’ that sends a reverse shell
and hides itself from the admin
❖ Admin logs in as root
❖ Goes to your home dir and runs ls
❖ Shell

42. ls reverse shell
Demo

43. Protect/Detect
❖ Don't put ‘.’ in your path….just don't
❖ No Risk vs Reward here, Risk will almost always
outweigh the reward

44. Questions? e: elliott.cutright@gmail.com
t: @nullthreat