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Basic Info

If you found a Local File Inclusion even if you don't have a session and session.auto_start is Off. If you provide the PHP_SESSION_UPLOAD_PROGRESS in multipart POST data, PHP will enable the session for you.

$ curl http://127.0.0.1/ -H 'Cookie: PHPSESSID=iamorange'
$ ls -a /var/lib/php/sessions/
. ..
$ curl http://127.0.0.1/ -H 'Cookie: PHPSESSID=iamorange' -d 'PHP_SESSION_UPLOAD_PROGRESS=blahblahblah'
$ ls -a /var/lib/php/sessions/
. ..
$ curl http://127.0.0.1/ -H 'Cookie: PHPSESSID=iamorange' -F 'PHP_SESSION_UPLOAD_PROGRESS=blahblahblah'  -F 'file=@/etc/passwd'
$ ls -a /var/lib/php/sessions/
. .. sess_iamorange

In the last example the session will contain the string blahblahblah

Note that with PHP_SESSION_UPLOAD_PROGRESS you can control data inside the session, so if you includes your session file you can include a part you control (a php shellcode for example).

{% hint style="info" %} Although most tutorials on the Internet recommends you to set session.upload_progress.cleanup to Off for debugging purpose. The default session.upload_progress.cleanup in PHP is still On. It means your upload progress in the session will be cleaned as soon as possible. So this will be Race Condition. {% endhint %}

The CTF

In the original CTF where this technique is commented, it wasn't enough to exploit the Race Condition but the content loaded needed to start also with the string @<?php.

Due to the default setting of session.upload_progress.prefix, our SESSION file will start with a annoying prefix upload_progress_ Such as: upload_progress_controlledcontentbyattacker

The trick to remove the initial prefix was to base64encode the payload 3 times and then decode it via convert.base64-decode filters, this is because when base64 decoding PHP will remove the weird characters, so after 3 times only the payload sent by the attacker will remain (and then the attacker can control the initial part).

More information in the original writeup https://blog.orange.tw/2018/10/ and final exploit https://github.com/orangetw/My-CTF-Web-Challenges/blob/master/hitcon-ctf-2018/one-line-php-challenge/exp_for_php.py

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