Ian McKellar has a blog post entitled Insecurity is Ruby on Rails Best Practice where he points out that by default the Ruby on Rails framework makes sites vulnerable to a certain class of exploits. Specifically, he discusses the vulnerabilities in two Ruby on Rails applications, 37 Signals Highrise and Magnolia, then proposes solutions. He writes

Cross Site Request Forgery
CSRF is the new bad guy in web application security. Everyone has worked out how to protect their SQL database from malicious input, and RoR saves you from ever having to worry about this. Cross site scripting attacks are dying and the web community even managed to nip most JSON data leaks in the bud.

Cross Site Request Forgery is very simple. A malicious site asks the user's browser to carry out an action on a site that the user has an active session on and the victim site carries out that action believing that the user intended that action to occur. In other words the problem arises when a web application relies purely on session cookies to authenticate requests.
Easy Solutions
There aren't any good easy solutions to this. A first step is to do referrer checking on every request and block GET requests in form actions. Simply checking the domain on the referrer may not be enough security if there's a chance that HTML could be posted somewhere in the domain by an attacker the application would be vulnerable again.

Better Solutions
Ideally we want a shared secret between the HTML that contains the form and the rails code in the action. We don't want this to be accessible to third parties so serving as JavaScript isn't an option. The way other platforms like Drupal achieve this is by inserting a hidden form field into every form that's generated that contains a secret token, either unique to the current user's current session or (for the more paranoid) also unique to the action. The action then has to check that the hidden token is correct before allowing processing to continue.

Incidents of Cross Site Request Forgery have become more popular with the rise of AJAX and this is likely to become as endemic as SQL injection attacks until the majority of Web frameworks take this into account in their out of the box experience. 

At Microsoft, the Web teams at MSN and Windows Live have given the folks in Developer Division the virtue of their experience building Web apps which has helped in making sure our Web frameworks like ASP.NET Ajax (formerly codenamed Atlas) avoid this issue in their default configuration. Scott Guthrie outlines the safeguards against this class of issues in his post JSON Hijacking and How ASP.NET AJAX 1.0 Avoids these Attacks where he writes

Recently some reports have been issued by security researchers describing ways hackers can use the JSON wire format used by most popular AJAX frameworks to try and exploit cross domain scripts within browsers. Specifically, these attacks use HTTP GET requests invoked via an HTML include element to circumvent the "same origin policy" enforced by browsers (which limits JavaScript objects like XmlHttpRequest to only calling URLs on the same domain that the page was loaded from), and then look for ways to exploit the JSON payload content.

ASP.NET AJAX 1.0 includes a number of default settings and built-in features that prevent it from being susceptible to these types of JSON hijacking attacks.
ASP.NET AJAX 1.0 by default only allows the HTTP POST verb to be used when invoking web methods using JSON, which means you can't inadvertently allow browsers to invoke methods via HTTP GET.

ASP.NET AJAX 1.0 requires a Content-Type header to be set to "application/json" for both GET and POST invocations to AJAX web services. JSON requests that do not contain this header will be rejected by an ASP.NET server. This means you cannot invoke an ASP.NET AJAX web method via a include because browsers do not allow append custom content-type headers when requesting a JavaScript file like this.

These mitigations would solve the issues that Ian McKellar pointed out in 37 Signals Highrise and Magnolia because HTML forms hosted on a malicious site cannot set the Content-Type header so that exploit is blocked. However neither this approach nor referrer checking to see if the requests come from your domain is enough if the malicious party finds a way to upload HTML or script onto your site. 

To completely mitigate against this attack, the shared secret approach is most secure and is what is used by most large websites. In this approach each page that can submit a request has a canary value (i.e. hidden form key) which must be returned with the request. If the form key is not provided, is invalid or expired then the request fails. This functionality is provided out of the box in ASP.NET by setting the Page.ViewStateUserKey property. Unfortunately, this feature is not on by default. On the positive side, it is a simple one line code change to get this functionality which needs to be rolled by hand on a number of other Web platforms today.