changedetection.io is a free open source web page change detection tool. A Stored Cross Site Scripting is present in changedetection.io Watch update API in versions prior to 0.50.34 due to insufficient security checks. Two scenarios are possible. In the first, an attacker can insert a new watch with an arbitrary URL which really points to a web page. Once the HTML content is retrieved, the attacker updates the URL with a JavaScript payload. In the second, an attacker substitutes the URL in an existing watch with a new URL that is in reality a JavaScript payload. When the user clicks on Preview and then on the malicious link, the JavaScript malicious code is executed. Version 0.50.34 fixes the issue.