WebUntis 2020.12.1 Cross Site Scripting

Credit: Robin Meis
Risk: Low
Local: No
Remote: Yes

I. VULNERABILITY ------------------------- WebUntis 2020.12.1 - (Authenticated) Cross Site Scripting II. BACKGROUND ------------------------- 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 ------------------------- 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 ------------------------- 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 ------------------------- 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 ------------------------- 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 ------------------------- WebUntis <= 2020.12.1 (currently unfixed) VIII. SOLUTION ------------------------- Avoid using private messages. IX. REFERENCES ------------------------- https://robin.meis.space/ (German articles) X. CREDITS ------------------------- This vulnerability has been discovered and reported by Robin Meis (disclosure@meis.space) XI. DISCLOSURE TIMELINE ------------------------- 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

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