In Eclipse Theia versions up to and including 0.16.0, in the notification messages there is no HTML escaping, so Javascript code can run.
Exploitability
AV:NAC:LPR:NUI:RScope
S:CImpact
C:LI:LA:N6.1/CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:NOther