Restful services are concerned with the integration of software systems using HTTP as base. Research studies addressing security assessments over JAX-RS are scarce, even more in Cross Site Scripting (XSS), which is a sort of attack that consists of stealing data or phishing. Thus, the aim of this paper is to present an assessment of the vulnerabilities over JAX-RS implementations when a XSS attack is involved. The assessment comprises: (1) selection of attack methods, (2) programming and assessing of attacks throughout dynamic programming and recursive methods; (3) identifying the vulnerabilities by means of a mathematical model, which determines the level of security of implementations. As a proof of concept, a prototype is implemented to demonstrate how the guideline is applied. Additionally, controls are proposed for every vulnerability identified.