Typestack Class-validator Cross-site Scripting

In TypeStack class-validator 0.10.2, validate() input validation can be bypassed because certain internal attributes can be overwritten via a conflicting name. Even though there is an optional forbidUnknownValues parameter that can be used to reduce the risk of this bypass, this option is not documented and thus most developers configure input validation in the vulnerable default manner. With this vulnerability, attackers can launch SQL Injection or XSS attacks by injecting arbitrary malicious input. NOTE: a software maintainer agrees with the "is not documented" finding but suggests that much of the responsibility for the risk lies in a different product. (2019-10-24, CVE-2019-18413)

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>