# LFI2RCE via Eternal waiting
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## Basic Information By default when a file is uploaded to PHP (even if it isn't expecting it), it will generate a temporary file in `/tmp` with a name such as **`php[a-zA-Z0-9]{6}`**, although I have seen some docker images where the generated files don't contain digits. In a local file inclusion, **if you manage to include that uploaded file, you will get RCE**. Note that by default **PHP only allows to upload 20 files in a single request** (set in `/etc/php//apache2/php.ini`): ``` ; Maximum number of files that can be uploaded via a single request max_file_uploads = 20 ``` Also, the **number of potential filenames are 62\*62\*62\*62\*62\*62 = 56800235584** ### Other techniques Other techniques relies in attacking PHP protocols (you won't be able if you only control the last part of the path), disclosing the path of the file, abusing expected files, or **making PHP suffer a segmentation fault so uploaded temporary files aren't deleted**.\ This technique is **very similar to the last one but without needed to find a zero day**. ### Eternal wait technique In this technique **we only need to control a relative path**. If we manage to upload files and make the **LFI never end**, we will have "enough time" to **brute-force uploaded files** and **find** any of the ones uploaded. **Pros of this technique**: * You just need to control a relative path inside an include * Doesn't require nginx or unexpected level of access to log files * Doesn't require a 0 day to cause a segmentation fault * Doesn't require a path disclosure The **main problems** of this technique are: * Need a specific file(s) to be present (there might be more) * The **insane** amount of potential file names: **56800235584** * If the server **isn't using digits** the total potential amount is: **19770609664** * By default **only 20 files** can be uploaded in a **single request**. * The **max number of parallel workers** of the used server. * This limit with the previous ones can make this attack last too much * **Timeout for a PHP request**. Ideally this should be eternal or should kill the PHP process without deleting the temp uploaded files, if not, this will also be a pain So, how can you **make a PHP include never end**? Just by including the file **`/sys/kernel/security/apparmor/revision`** (**not available in Docker containers** unfortunately...). Try it just calling: ```bash php -a # open php cli include("/sys/kernel/security/apparmor/revision"); ``` ## Apache2 By default, Apache support **150 concurrent connections**, following [https://ubiq.co/tech-blog/increase-max-connections-apache/](https://ubiq.co/tech-blog/increase-max-connections-apache/) it's possible to upgrade this number up to 8000. Follow this to use PHP with that module: [https://www.digitalocean.com/community/tutorials/how-to-configure-apache-http-with-mpm-event-and-php-fpm-on-ubuntu-18-04](https://www.digitalocean.com/community/tutorials/how-to-configure-apache-http-with-mpm-event-and-php-fpm-on-ubuntu-18-04). By default, (as I can see in my tests), a **PHP process can last eternally**. Let's do some maths: * We can use **149 connections** to generate **149 \* 20 = 2980 temp files** with our webshell. * Then, use the **last connection** to **brute-force** potential files. * At a speed of **10 requests/s** the times are: * 56800235584 / 2980 / 10 / 3600 \~= **530 hours** (50% chance in 265h) * (without digits) 19770609664 / 2980 / 10 / 3600 \~= 185h (50% chance in 93h) {% hint style="warning" %} Note that in the previous example we are **completely DoSing other clients**! {% endhint %} If the Apache server is improved and we could abuse **4000 connections** (half way to the max number). We could create `3999*20 = 79980` **files** and the **number** would be **reduced** to around **19.7h** or **6.9h** (10h, 3.5h 50% chance). ## PHP-FMP If instead of using the regular php mod for apache to run PHP scripts the **web page is using** **PHP-FMP** (this improves the efficiency of the web page, so it's common to find it), there is something else that can be done to improve the technique. PHP-FMP allow to **configure** the **parameter** **`request_terminate_timeout`** in **`/etc/php//fpm/pool.d/www.conf`**.\ This parameter indicates the maximum amount of seconds **when** **request to PHP must terminate** (infinite by default, but **30s if the param is uncommented**). When a request is being processed by PHP the indicated number of seconds, it's **killed**. This means, that if the request was uploading temporary files, because the **php processing was stopped**, those **files aren't going to be deleted**. Therefore, if you can make a request last that time, you can **generate thousands of temporary files** that won't be deleted, which will **speed up the process of finding them** and reduces the probability of a DoS to the platform by consuming all connections. So, to **avoid DoS** lets suppose that an **attacker will be using only 100 connections** at the same time and php max processing time by **php-fmp** (`request_terminate_timeout`**)** is **30s**. Therefore, the number of **temp files** that can be generated **by second** is `100*20/30 = 66.67`. Then, to generate **10000 files** an attacker would need: **`10000/66.67 = 150s`** (to generate **100000 files** the time would be **25min**). Then, the attacker could use those **100 connections** to perform a **search brute-force**. \*\*\*\* Supposing a speed of 300 req/s the time needed to exploit this is the following: * 56800235584 / 10000 / 300 / 3600 \~= **5.25 hours** (50% chance in 2.63h) * (with 100000 files) 56800235584 / 100000 / 300 / 3600 \~= **0.525 hours** (50% chance in 0.263h) Yes, it's possible to generate 100000 temporary files in an EC2 medium size instance:
{% hint style="warning" %} Note that in order to trigger the timeout it would be **enough to include the vulnerable LFI page**, so it enters in an eternal include loop. {% endhint %} ## Nginx It looks like by default Nginx supports **512 parallel connections** at the same time (and this number can be improved).
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