Hi Security Team,
Summary:

There is no limit to the number of characters in the display name, which allows a DoS attack. The DoS attack affects server-side.
Description

On the input form of Username in nextcloud.com/settings/user there’s no Input validation using this you can send more payload and may cause of Denial of service or error code 500 Internal Server Error/Internal Error
Proof of Concept

1.Go and login to your account
2. Now go to setting and Deck —> Add Boards section
3.Insert name and intercept it
4. Send to repeater replace it with payload the response code on the server side is 500 Internal Server Error

Impact

Impact

Remediation:

+Implementing input validation
+Validating free-form Unicode text
+Define the allowed set of characters to be accepted.
+Minimum and maximum value range
Impact

Attacker can perform a DOS because of lack of input validation