Netsparker detected that it's possible to control the action attribute of form tag in the HTML.
Depending on the purpose of the detected form an attacker can steal the content of the form. To do this the attacker needs to lead the user to this form with a specially crafted URL. When the user submits the form on the page, data on the form will go to the attacker controlled server.
Do not allow user input to control the form tag's attributes. If the dynamic usage is necessary then use whitelisting.