CVE-2019 – 18345 Reflected Cross-Site Scripting (XSS) vulnerability in DAViCal CalDAV Server

All three vulnerabilities exist in the web-based management pages that come with DAViCal. We have written three separate advisories to describe the vulnerabilities:

1. CVE-2019-18345 — (this advisory) Reflected Cross-Site Scripting
2. CVE-2019-18346 – Cross-Site Request Forgery
3. CVE-2019-18347 – Persistent Cross-Site Scripting

About DAViCal

DAViCal is a server for calendar sharing. It is an implementation of the CalDAV protocol which is designed for storing calendaring resources on a remote shared server. It can be used by various e‑mail and calendaring clients to centrally store and share calendars.

It includes a web-based management application. It was in these pages that we discovered this vulnerability.

Affected systems

DAViCal CalDAV Server 1.1.8 and prior

Overview

A reflected cross-site scripting (XSS) vulnerability was found in multiple pages of the DAViCal CalDAV Server. This is possible because the application echoes user supplied input without encoding.

POC URL: http://davical.host/admin.php?action=edit&t="><script>alert()</script>&id=1

Impact

If a user visits an attacker-supplied link, the attacker can view all data the attacked user can view, as well as perform all actions in the name of the user. If the user is administrator, the attacker can for example change the password of the user to take over the account and gain full access to the application.

Solution

Update to version 1.1.9.2

Technical solution details

XSS vulnerabilities are a problem with dynamically generated websites that use user input. If user input is not correctly sanitized you could very well end up with a user pushing some Javascript to your frontend.

XSS isn’t a vulnerability that’s hard to grasp or circumvent but it’s awfully easy to make a mistake like that. One thing you’ll hear over and over again is never to trust user input. Always sanitize it when it comes in and it’s best to still not trust it then. Characters like <, > and " should never be rawly echoed to the frontend. The use cases for echoing user input back to the frontend are endless. From a simple Greetings, $username to editing personal user information with the form having all the fields already filled in. So when someone has a quote in their name, you shouldn’t echo the raw quote but &­quot;.

These days web frameworks handle a lot of sanitation for us. Laravel for example uses simple brackets to echo variables to the user all these variables are escaped first: {{ $username }}. Twig does something similar by using a pipe like syntax: {{ $username | escape}}.

These days when developing your application you need to make sure you sanitize everything you output to the user. But since DAViCal is an established project it’s not doable to sift through the code to look for functions that output text to the frontend. Another problem was that DAViCal dynamically adds GET parameters to echoed urls. This is why I chose to sanitize both incoming variables and their names. In the DAViCal always.php I added a function that loops through the $_GET and $_POST array recursively (as arrays can contain arrays and so forth) and run the names and variables through htmlspecialchars() except for the password field which of course should be able to have special characters in them.

The reason you don’t do it this way in new applications is because now if for some reason someone has another way of interacting with your application (by API calls for example) you’d have to sanitize your input on both sides. Moreover, APIs that pass JSON objects around for example, don’t need to have script tags encoded as it means nothing to them and JSON objects are encoded in a different way. In this case however, DAViCal doesn’t have other entry points which you can use to insert data in the database. So sanitizing all input once will suffice!

Responsible Disclosure timeline

Useful links

• DAViCal 1.1.9.2 Release Notes
• DAViCal on Gitlab
• This issue in the Common Vulnerabilities and Exposures (CVE) database
• This issue in the National Vulnerability Database (NVD)
• This advisory on fulldisclosure
• Debian security advisory