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
- the cookie is sensitive, used to authenticate the user, for instance a session-cookie
- the
HttpOnly attribute offer an additional protection (not the case for an XSRF-TOKEN cookie / CSRF token for example)
There is a risk if you answered yes to any of those questions.
Recommended Secure Coding Practices
- By default the
HttpOnly flag should be set to true for most of the cookies and it’s mandatory for session /
sensitive-security cookies.
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" />
- the
requireSSL attribute corresponds programmatically to the Secure field.
- the
httpOnlyCookies attribute corresponds programmatically to the httpOnly field.
See
