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Cross-site Scripting occurrences in Doorkeeper
Doorkeeper version 2.1.0 through 4.2.5 contains a Cross Site Scripting (XSS) vulnerability in web view's OAuth app form, user authorization prompt web view that can result in Stored XSS on the OAuth Client's name will cause users interacting with it will execute payload. This attack appear to be exploitable via The victim must be tricked to click an opaque link to the web view that runs the XSS payload. A malicious version virtually indistinguishable from a normal link.. This vulnerability appears to have been fixed in 4.2.6, 4.3.0. (2018-03-13, CVE-2018-1000088)
Why Cross-site Scripting can be dangerous
Cross site scripting is an attack where a web page executes code that is injected by an adversary. It usually appears, when users input is presented. This attack can be used to impersonate a user, take over control of the session, or even steal API keys.
The attack can be executed e.g. when you application injects the request parameter directly into the HTML code of the page returned to the user:
https://server.com/confirmation?message=Transaction+Complete
what results in:
<span>Confirmation: Transaction Complete</span>
In that case the message can be modified to become a valid Javascript code, e.g.:
https://server.com/confirmation?message=<script>dangerous javascript code here</script>
and it will be executed locally by the user's browser with full access to the user's personal application/browser data:
<span>Confirmation: <script>dangerous javascript code here</script></span>