Solarwinds Serv-U 15.2.3 Share URL XSS (CVE-2021-32604)

Sometimes when pen-testing a large network you come across a few exposed web hosts running out-of-the-box software. When the rest of the test can run a little dry, I start attacking these hosts in hopes of finding some undiscovered vulnerabilities. In this example, I found a small, yet interesting vulnerability within the SolarWinds Serv-U FTP Server. Although the initial vector requires authentication, a low privileged user is able to create a publicly accessible URL that triggers an XSS payload when visited.

Discovery

Serv-U contains two features to send and receive files from others, when testing the 'Request Files' feature I noticed that the Sender Email input was not being encoded when being place in a publicly accessible share URL. This meant a discreet share URL could be sent to a victim.

Steps to reproduce

The following steps were tested on version 15.2.3, as the host, I was originally testing was running an older version, I downloaded a trial version to see if it was still applicable. Once authenticated, a user can go to the 'Request Files' tab to generate a file request URL.

Using dummy data and sending the request shows a publicly shareable URL.

Checking the generated URL shows a file upload form. Files uploaded here will be sent back to the link creator's Serv-U folder.

Now if we modify the original link generation request and include an XSS payload such as "'/>%40localhost.local in the 'SenderEmail' field.

And then grab the 'ShareURL' from the response.

We now get XSS on the publicly shareable URL.

Weaponization

I thought of different ways that this could be weaponized, although it seemed that certain application flows prevented the interesting cookie stealing or CSRF fun that might happen when the URL is sent to a logged-in Serv-U user. Unfortunately, the web application for one reason or another does not actually load the shareable links when a user is already logged in......BUT I figured I wouldn't leave you empty-handed.

The next best thing I could think of was a man-in-the-middle attack, intercepting uploaded files and sending them to a 3rd party server without altering the view of the page.

The idea here is that perhaps during a Red Team or other social engineering event, access to a Serv-U account could Serv-U well as the URL's will be on a trusted domain.

It took a little bit of tweaking to get the files to upload correctly to Serv-U and be sent off at the same time due to the Async Ajax calls being used. The script can simply be hosted and then injected via XSS using