Beets is the media library management system. Prior to version 2.10.0, the bundled web UI uses Underscore template interpolation mode <%= ... %> for untrusted metadata fields. In this runtime, <%= ... %> is raw insertion and HTML escaping is only performed by <%- ... %>. Rendered output is then inserted with .html(...), allowing attacker-controlled markup to become active DOM. This issue has been patched in version 2.10.0.
Exploitability
AV:NAC:LAT:PPR:NUI:PVulnerable System
VC:NVI:HVA:NSubsequent System
SC:NSI:NSA:N6/CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:H/VA:N/SC:N/SI:N/SA:NInjection