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ASP.NET 1.1+ comes with a feature called Request Validation, preventing the server to accept content containing un-encoded HTML. This
feature comes as a first protection layer against Cross-Site Scripting (XSS) attacks and act as a simple Web Application Firewall (WAF) rejecting
requests potentially containing malicious content.
While this feature is not a silver bullet to prevent all XSS attacks, it helps to catch basic ones. It will for example prevent <script
type="text/javascript" src="https://malicious.domain/payload.js"> to reach your Controller.
Note: Request Validation feature being only available for ASP.NET, no Security Hotspot is raised on ASP.NET Core applications.
Ask Yourself Whether
- the developer doesn’t know the impact to deactivate the Request Validation feature
- the web application accepts user-supplied data
- all user-supplied data are not validated
There is a risk if you answered yes to any of those questions.
Recommended Secure Coding Practices
- Activate the Request Validation feature for all HTTP requests
Sensitive Code Example
At Controller level:
[ValidateInput(false)]
public ActionResult Welcome(string name)
{
...
}
At application level, configured in the Web.config file:
<configuration>
<system.web>
<pages validateRequest="false" />
...
<httpRuntime requestValidationMode="0.0" />
</system.web>
</configuration>
Compliant Solution
At Controller level:
[ValidateInput(true)]
public ActionResult Welcome(string name)
{
...
}
or
public ActionResult Welcome(string name)
{
...
}
At application level, configured in the Web.config file:
<configuration>
<system.web>
<pages validateRequest="true" />
...
<httpRuntime requestValidationMode="4.5" />
</system.web>
</configuration>
See
- OWASP - Top 10 2021 Category A3 - Injection
- HttpRuntimeSection.RequestValidationMode Property
- OWASP ASP.NET Request Validation
- OWASP Cheat Sheet - XSS
Prevention Cheat Sheet
- OWASP - Top 10 2017 Category A7 - Cross-Site Scripting
(XSS)
- CWE - CWE-79 - Improper Neutralization of Input During Web Page Generation ('Cross-site
Scripting')