How vunerable to XSS attacks is Flash?

The reason why I ask is th开发者_运维问答at I'm telling a vendor of ours they have to use the MS AntiXSS library with the ASP.NET UI components they make, but they also work with Flex to build Flash based UIs - and I was wondering if there's an equivalent for Flash (assuming it's vunerable).

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If I understand correctly...

ASP.NEt is used to make web pages, and all UI Components they make will be running in a browser as HTML / JavaScript. Is that correct?

If that is the case, I can understand why preventing cross site scripting would be important in that case.

With Flex (which runs in the Flash Player), everything is compiled down into a binary file, called a SWF. Most of the time, the SWF runs inside the Flash Player, which runs in the browser as a plugin. There would be no way to hack an individual Flex component using XSS.

I don't believe the code you write needs to be protected from cross site scripting. Your biggest fear is player vulnerabilities, which you don't have much control over.

None of this should be a reason not to validate user input.

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The short answer is: the Flash player has a lot of features in place to prevent XSS attacks, but they're built in to the player itself, so there isn't any particular library you need to use. If you don't call any security-related APIs, and don't put config files on your server, then security-wise, you are already using the most restrictive settings available. (Assuming you also pay attention to how you make use of user input.)

More generally, APIs that have the potential to lead to XSS vulnerabilities are as a rule disabled in XSS situations unless you actively enable them. For example, if an HTML page on your site loads in a flash file from another site, and that flash content tries to, say, make javascript calls into your page, those calls will be blocked by default unless you allow them. Similarly, if flash content on your site loads in components from another site, those components will not be able to introspect into their parent unless you call APIs to allow them to. There are also various restrictions on what happens when another site tries to load in Flash content from your site without your having allowed it.

For all the details, I highly recommend this excellent overview:

• Creating more secure SWF web applications

With all that said, since you also asked about sanitizing user inputs, it's worth noting that since AS3 has no equivalent of an eval command there is never any question of user input being executed as script. However, any user input that relates to content being loaded could be a vector of XSS attack. (For example, if you append a user-input string to a URL you then load, the user could cause your site to load in their malicious SWF.) But such a case is no different from a situation where you load in a benign 3rd-party SWF, and someone later replaces it with malicious content. Hence in context of Flash, protecting against XSS attacks is not so much about sanitizing user input as it is about making sure that externally loaded contents are not granted permission to run as if they were locally trusted.

And further, since it's often useful or necessary to relax the default restrictions if you want to do something interesting with 3rd-party content (like flash avatars, components, or even banner ads), in those situations it's important for the site admin to understand what they are allowing, and how to prevent the relaxed restrictions from exposing a vulnerability.