A XSS can injected in the responses of project and user autocompleters.
Impact
An attacker could use this vulnerability to force a victim to execute uncontrolled code.
CVSSv3 score: 5.4 (CVSS:3.0/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N)
Exploitation
Set the real name of a user to something like <img src=a onerror=alert(1)> and then access to the page https://<tuleap_instance>/user/autocomplete.php?return_type=json_for_select_2&name=<query_to_match_your_user>
References
CWE 79
OWASP Cross-site Scripting