Jenkins form controls include an expandable textbox that can transform from a single-line text box to a multi-line text area. The implementation of this transformation interpreted the text content of the form field as HTML. This resulted in a cross-site scripting vulnerability exploitable by attackers able to control the contents of such f:expandableTextbox form controls.
References:
https://jenkins.io/security/advisory/2019-09-25/#SECURITY-1498