I. VULNERABILITY
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WebUntis 2020.12.1 - (Authenticated) Cross Site Scripting

II. BACKGROUND
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WebUntis is a tool for schools and universities to deliver electronic timetables to their students. Depending from the activated modules it does also contain sensitive information within the integrated class-register and grade-book. Furthermore it supports private messaging.  

III. DESCRIPTION
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The private messaging component contains a persistent XSS vulnerability within the message body which allows the execution of arbitrary JavaScript in the context of the victim user's browser.

IV. History
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The issue has been reported back in December 2019 to Untis GmbH. Against my advisories the vendor tried to fix the issue by implementing a Cross-Site-Scripting filter. Public (full) disclosure was on 22.03.2020. It turned out that the filter is not sufficient and can be easily bypassed.

V. PROOF OF CONCEPT
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Send a new private message to any user within WebUntis containing the following message body:

<img src="test.jpg" / onerror="alert('XSS')">

Reading the message either on senders or on recipients account will cause the script to execute.

VI. BUSINESS IMPACT
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The attacker is able to execute any JavaScript in the logged in users context. PoCs to manipulate grades and to steal API/OTP Tokens for full access using the mobile app exist.

VII. SYSTEMS AFFECTED
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WebUntis <= 2020.12.1 (currently unfixed)

VIII. SOLUTION
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Avoid using private messages.

IX. REFERENCES
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https://robin.meis.space/ (German articles)

X. CREDITS
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This vulnerability has been discovered and reported by Robin Meis (disclosure@meis.space)

XI. DISCLOSURE TIMELINE
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11.12.2019 - Report of CSRF and XSS vulnerabilities to Untis GmbH
02.01.2020 - First Response (restored Mail from spam folder)
27.01.2020 - Offer of Bug-Bounty against NDA
 meanwhile - Implementation and tests of a XSS filter by vendor
           - Advised vendor to do proper HTML encoding
           - Report of further XSS vulnerabilities
11.03.2020 - Limited Disclosure, one XSS vulnerability remains unfixed
12.03.2020 - Remaining issue fixed
22.03.2020 - Full Disclosure
07.06.2020 - PoC for filter bypass, Full Disclosure