XSS can injected in the reference tooltips.
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
Create a release in a package named like <img src=a onerror=alert(1)> and reference this release somewhere else with a cross ref. Going hover the cross ref will trigger the XSS.
References
CWE 79
OWASP Cross-site Scripting