When a cookie is configured with the HttpOnly attribute set to true, the browser guaranties that no client-side script will be able to read it. In most cases, when a cookie is created, the default value of HttpOnly is false and it’s up to the developer to decide whether or not the content of the cookie can be read by the client-side script. As a majority of Cross-Site Scripting (XSS) attacks target the theft of session-cookies, the HttpOnly attribute can help to reduce their impact as it won’t be possible to exploit the XSS vulnerability to steal session-cookies.

Ask Yourself Whether

There is a risk if you answered yes to any of those questions.

Recommended Secure Coding Practices

Sensitive Code Example

When the HttpCookie.HttpOnly property is set to false then the cookie can be accessed by client side code:

HttpCookie myCookie = new HttpCookie("Sensitive cookie");
myCookie.HttpOnly = false; // Sensitive: this cookie is created with the httponly flag set to false and so it can be stolen easily in case of XSS vulnerability

The default value of HttpOnly flag is false, unless overwritten by an application’s configuration file:

HttpCookie myCookie = new HttpCookie("Sensitive cookie");
// Sensitive: this cookie is created without the httponly flag  (by default set to false) and so it can be stolen easily in case of XSS vulnerability

Compliant Solution

Set the HttpCookie.HttpOnly property to true:

HttpCookie myCookie = new HttpCookie("Sensitive cookie");
myCookie.HttpOnly = true; // Compliant: the sensitive cookie is protected against theft thanks to the HttpOnly property set to true (HttpOnly = true)

Or change the default flag values for the whole application by editing the Web.config configuration file:

<httpCookies httpOnlyCookies="true" requireSSL="true" />

See