SecureFlag

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Last revision (yyyy-mm-dd): 2015-12-14
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Overview

The secure flag is an option that can be set by the application server when sending a new cookie to the user within an HTTP Response. The purpose of the secure flag is to prevent cookies from being observed by unauthorized parties due to the transmission of a the cookie in clear text.

To accomplish this goal, browsers which support the secure flag will only send cookies with the secure flag when the request is going to a HTTPS page. Said in another way, the browser will not send a cookie with the secure flag set over an unencrypted HTTP request.

By setting the secure flag, the browser will prevent the transmission of a cookie over an unencrypted channel.

Setting the Secure Flag

Following sections describes setting the Secure Flag in respective technologies.

Java

Servlet 3.0 (Java EE 6)

Sun Java EE supports secure flag in Cookie interface since version 6 (Servlet class version 3)[1], also for session cookies (JSESSIONID)[2]. Methods setSecure and isSecure can be used to set and check for secure value in cookies.

web.xml

Servlet 3.0 (Java EE 6) introduced a standard way to configure secure attribute for the session cookie, this can be done by applying the following configuration in web.xml

<session-config>
 <cookie-config>
  <secure>true</secure>
 </cookie-config>
</session-config>

Tomcat

In Tomcat 6 if the first request for session is using https then it automatically sets secure attribute on session cookie.

Setting it as a custom header

For older versions the workaround is to rewrite JSESSIONID value using and setting it as a custom header. The drawback is that servers can be configured to use a different session identifier than JSESSIONID.

String sessionid = request.getSession().getId();
response.setHeader("SET-COOKIE", "JSESSIONID=" + sessionid + "; secure");

Environment consideration

With this flag always set, sessions won't work in environments(development/test/etc.) that may use http. SessionCookieConfig [3] interface or setting custom header[4] trick can be leveraged to configure setting of this flag differently for each environment and can be driven by application configuration.

ASP.NET

Set the following in Web.config: <httpCookies requireSSL="true" />

For some objects that have a requireSSL property, like the forms Authentication Cookie, set the requireSSL="true" flag in the web.config for that specific element. For example:

<authentication mode="Forms">
  <forms loginUrl="member_login.aspx"
         cookieless="UseCookies"
         requireSSL="true"
         path="/MyApplication" />
</authentication>  

Which will enable the secure flag on the Forms Authentication cookie, as well as checking that the http request is coming to the server over SSL/TLS connection. Note that in case TLS is offloaded to a load balancer, the requireSSL solution wouldn't work.

Alternatively, the cookies can be set to secure programmatically using the following code by adding a EndRequest event handler to the Global.asax.cs file:

protected void Application_EndRequest(Object sender, EventArgs e) {
   // Iterate through any cookies found in the Response object.
   foreach (string cookieName in Response.Cookies.AllKeys) {
       Response.Cookies[cookieName]?.Secure = true;
   }
} 

PHP

For session cookies managed by PHP, the flag is set either permanently in php.ini PHP manual on SecureFlag through the parameter:

session.cookie_secure = True

or in and during a script via the function [5]:

void session_set_cookie_params  ( int $lifetime  [, string $path  [, string $domain  
                                  [, bool $secure= false  [, bool $httponly= false  ]]]] )

For application cookies a parameter in setcookie() sets Secure flag [6]:

bool setcookie  ( string $name  [, string $value  [, int $expire= 0  [, string $path  
                 [, string $domain  [, bool $secure= false  [, bool $httponly= false  ]]]]]] )

Testing for the Secure Flag

Verifying that a web site sets this flag on any particular cookie is easy. Using an intercepting proxy, like ZAP, you can capture each response from the server and examine any Set-Cookie headers it includes to see if the secure flag is set on the cookie.

Related Articles

• Testing for Cookie Attributes
• http://www.troyhunt.com/2011/11/owasp-top-10-for-net-developers-part-9.html