For an attacker with pre-existing access to send a signal to a workflow, the attacker can make the signal name a script that executes when a victim views that signal. The XSS is in the timeline page displaying the workflow execution details of the workflow that was sent the crafted signal.
Access to send a signal to a workflow is determined by how you configured the authorizer on your server. This includes any entity with permission to directly call SignalWorkflowExecution or SignalWithStartWorkflowExecution, or any entity can deploy a worker that has access to call workflow progress APIs (specifically RespondWorkflowTaskCompleted).

References

• https://nvd.nist.gov/vuln/detail/CVE-2024-2435
• https://github.com/temporalio/ui-server/releases/tag/v2.25.0