What is HttpOnly?

According to a daily blog article by Jordan Wiens, “No cookie for you!”, HttpOnly cookies were first implemented in 2002 by Microsoft Internet Explorer developers for Internet Explorer 6 SP1.

What is HttpOnly?

According to the Microsoft Developer Network, HttpOnly is an additional flag included in a Set-Cookie HTTP response header. Using the HttpOnly flag when generating a cookie helps mitigate the risk of client side script accessing the protected cookie (if the browser supports it).

• The example below shows the syntax used within the HTTP response header:

Set-Cookie: <name>=<value>[; <Max-Age>=<age>]
`[; expires=<date>][; domain=<domain_name>]
[; path=<some_path>][; secure][; HttpOnly]

If the HttpOnly flag (optional) is included in the HTTP response header, the cookie cannot be accessed through client side script (again if the browser supports this flag). As a result, even if a cross-site scripting (XSS) flaw exists, and a user accidentally accesses a link that exploits this flaw, the browser (primarily Internet Explorer) will not reveal the cookie to a third party.

If a browser does not support HttpOnly and a website attempts to set an HttpOnly cookie, the HttpOnly flag will be ignored by the browser, thus creating a traditional, script accessible cookie. As a result, the cookie (typically your session cookie) becomes vulnerable to theft or modification by malicious script. Mitigating.

Mitigating the Most Common XSS attack using HttpOnly

According to Michael Howard, Senior Security Program Manager in the Secure Windows Initiative group at Microsoft, the majority of XSS attacks target theft of session cookies. A server could help mitigate this issue by setting the HttpOnly flag on a cookie it creates, indicating the cookie should not be accessible on the client.

If a browser that supports HttpOnly detects a cookie containing the HttpOnly flag, and client side script code attempts to read the cookie, the browser returns an empty string as the result. This causes the attack to fail by preventing the malicious (usually XSS) code from sending the data to an attacker’s website.

Using Java to Set HttpOnly

Since Java Enterprise Edition 6 (JEE 6), which adopted Java Servlet 3.0 technology, it’s programmatically easy to set the HttpOnly flag on a cookie.

In fact setHttpOnly and isHttpOnly methods are available in the Cookie interface JEE 6, JEE 7 and also for session cookies (JSESSIONID) JEE 6, JEE 7 cookie.setHttpOnly(true);

Moreover, since JEE 6 it’s also declaratively easy setting HttpOnly flag in a session cookie by applying the following configuration in the deployment descriptor WEB-INF/web.xml:

<session-config>
   <cookie-config>
    <http-only>true</http-only>
   </cookie-config>
</session-config>

For Java Enterprise Edition versions prior to JEE 6 a common workaround is to overwrite the SET-COOKIE HTTP response header with a session cookie value that explicitly appends the HttpOnly flag:

String sessionid = request.getSession().getId();
// be careful overwriting: JSESSIONID may have been set with other flags
response.setHeader("SET-COOKIE", "JSESSIONID=" + sessionid + "; HttpOnly");

In this context, overwriting, despite appropriate for the HttpOnly flag, is discouraged because the JSESSIONID may have been set with other flags. A better workaround is taking care of the previously set flags or using the ESAPI#Java_EE library: in fact the addCookie method of the SecurityWrapperResponse 3 takes care of previously set flags for us. So we could write a servlet filter as the following one:

public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain) throws IOException, ServletException {
    HttpServletRequest httpServletRequest = (HttpServletRequest) request;
    HttpServletResponse httpServletResponse = (HttpServletResponse) response;
    // if errors exist then create a sanitized cookie header and continue
    SecurityWrapperResponse securityWrapperResponse = new SecurityWrapperResponse(httpServletResponse, "sanitize");
    Cookie[] cookies = httpServletRequest.getCookies();
    if (cookies != null) {
        for (int i = 0; i < cookies.length; i++) {
            Cookie cookie = cookies[i];
            if (cookie != null) {
                // ESAPI.securityConfiguration().getHttpSessionIdName() returns JSESSIONID by default configuration
                if (ESAPI.securityConfiguration().getHttpSessionIdName().equals(cookie.getName())) {
                    securityWrapperResponse.addCookie(cookie);
                }
            }
        }
    }
    filterChain.doFilter(request, response);
}

Some web application servers, that implement JEE 5, and servlet containers that implement Java Servlet 2.5 (part of JEE 5), also allow creating HttpOnly session cookies:

• Tomcat 6 In context.xml set the context tag’s attribute useHttpOnly 4 as follow:

<?xml version="1.0" encoding="UTF-8"?>
<Context path="/myWebApplicationPath" useHttpOnly="true">

• JBoss 5.0.1 and JBOSS EAP 5.0.1 In server <myJBossServerInstance> \deploy\jbossweb.sar\context.xml set the SessionCookie tag 5 as follow:

<Context cookies="true" crossContext="true">
    <SessionCookie secure="true" httpOnly="true" />

• IBM Websphere offer HTTPOnly for session cookies as a configuration option

Using .NET to Set HttpOnly

• By default, .NET 2.0 sets the HttpOnly attribute for

1. Session ID
2. Forms Authentication cookie

In .NET 2.0, HttpOnly can also be set via the HttpCookie object for all custom application cookies

• Via web.config in the system.web/httpCookies element

<httpCookies httpOnlyCookies="true" …> 

• Or programmatically

C# Code:

HttpCookie myCookie = new HttpCookie("myCookie");
myCookie.HttpOnly = true;
Response.AppendCookie(myCookie);

VB.NET Code:

Dim myCookie As HttpCookie = new HttpCookie("myCookie")
myCookie.HttpOnly = True
Response.AppendCookie(myCookie)

• However, in .NET 1.1, you would have to do this manually, e.g.,

Response.Cookies[cookie].Path += ";HttpOnly";

Using Python (cherryPy) to Set HttpOnly

Python Code (cherryPy): To use HTTP-Only cookies with Cherrypy sessions just add the following line in your configuration file: tools.sessions.httponly = True If you use SLL you can also make your cookies secure (encrypted) to avoid “manipulator-in-the-middle” cookies reading with: tools.sessions.secure = True

Using PHP to set HttpOnly

PHP supports setting the HttpOnly flag since version 5.2.0 (November 2006).

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

session.cookie_httponly = True

or in and during a script via the function6:

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

For application cookies last parameter in setcookie() sets HttpOnly flag7:

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

Web Application Firewalls

If code changes are infeasible, web application firewalls can be used to add HttpOnly to session cookies:

• Mod_security – using SecRule and Header directives8
• ESAPI WAF9 using add-http-only-flag directive10

Browsers Supporting HttpOnly

Using WebGoat’s HttpOnly lesson, the following web browsers have been tested for HttpOnly support. If the browsers enforces HttpOnly, a client side script will be unable to read or write the session cookie. However, there is currently no prevention of reading or writing the session cookie via a XMLHTTPRequest.

Note: These results may be out of date as this page is not well maintained. A great page that is focused on keeping up with the status of browsers is at: Browserscope. Just look at the HttpOnly column. The Browserscope site does not provide as much detail on HttpOnly as this page, but provides lots of other details this page does not.

Our results as of Feb 2009 are listed below in table 1.

Table 1: Browsers Supporting HttpOnly

* An attacker could still read the session cookie in a response to an **XmlHttpRequest.

As of 2011, 99% of browsers and most web application frameworks support HttpOnly[1].

Using WebGoat to Test for HttpOnly Support

The goal of this section is to provide a step-by-step example of testing your browser for HttpOnly support.

WARNING

The OWASP WEBGOAT HttpOnly lab is broken and does not show IE 8 Beta 2 with ms08-069 as complete in terms of HttpOnly XMLHTTPRequest header leakage protection. This error is being tracked via Issue 18.

Getting Started

Assuming you have installed and launched WebGoat, begin by navigating to the ‘HttpOnly Test’ lesson located within the Cross-Site Scripting (XSS) category. After loading the ‘HttpOnly Test’ lesson, as shown in figure 1, you are now able to begin testing web browsers supporting HttpOnly.

Lesson Goal

If the HttpOnly flag is set, then your browser should not allow a client-side script to access the session cookie. Unfortunately, since the attribute is relatively new, several browsers may neglect to handle the new attribute properly.

The purpose of this lesson is to test whether your browser supports the HttpOnly cookie flag. Note the value of the unique2u cookie. If your browser supports HttpOnly, and you enable it for a cookie, a client-side script should NOT be able to read OR write to that cookie, but the browser can still send its value to the server. However, some browsers only prevent client side read access, but do not prevent write access.

Testing Web Browsers for HttpOnly Support

The following test was performed on two browsers, Internet Explorer 7 and Opera 9.22, to demonstrate the results when the HttpOnly flag is enforced properly. As you will see, IE7 properly enforces the HttpOnly flag, whereas Opera does not properly enforce the HttpOnly flag.

Disabling HttpOnly

1) Select the option to turn HttpOnly off as shown below in Figure 2.

2) After turning HttpOnly off, select the “Read Cookie” button.

• An alert dialog box will display on the screen notifying you that since HttpOnly was not enabled, the ‘unique2u’ cookie was successfully read as shown below in figure 3.

3) With HttpOnly remaining disabled, select the “Write Cookie” button.

• An alert dialog box will display on the screen notifying you that since HttpOnly was not enabled, the ‘unique2u’ cookie was successfully modified on the client side as shown below in figure 4.

• As you have seen thus far, browsing without HttpOnly on is a potential threat. Next, we will enable HttpOnly to demonstrate how this flag protects the cookie.

Enabling HttpOnly

4) Select the radio button to enable HttpOnly as shown below in figure 5.

5) After enabling HttpOnly, select the “Read Cookie” button.

• If the browser enforces the HttpOnly flag properly, an alert dialog box will display only the session ID rather than the contents of the ‘unique2u’ cookie as shown below in figure 6.

• However, if the browser does not enforce the HttpOnly flag properly, an alert dialog box will display both the ‘unique2u’ cookie and session ID as shown below in figure 7.

• Finally, we will test if the browser allows write access to the cookie with HttpOnly enabled.

6) Select the “Write Cookie” button.

• If the browser enforces the HttpOnly flag properly, client side modification will be unsuccessful in writing to the ‘unique2u’ cookie and an alert dialog box will display only containing the session ID as shown below in figure 8.

• However, if the browser does not enforce the write protection property of HttpOnly flag for the ‘unique2u’ cookie, the cookie will be successfully modified to HACKED on the client side as shown below in figure 9.