IdentityServer IdentityServer4 1.x before 1.5.3 and 2.x before 2.1.3 does not encode the redirect URI on the authorization response page, which might lead to XSS in some configurations.
IdentityServer IdentityServer4 through 2.4 has stored XSS via the httpContext to the host/Extensions/RequestLoggerMiddleware.cs LogForErrorContext method, which can be triggered by viewing a log.