Friday, December 02, 2005

Security and the Dynamic Script Request API

Possible Security Issue

Dynamically adding SCRIPT SRC tags to pull in data as JavaScript is nice since it gets around cross-domain issues, but it could also be problematic if you are providing user data, data that required the user to authenticate in some way.

If the User A authenticates for using some JavaScript data APIs via SCRIPT SRC tags, it is possible that Hacker B could guide User A to Hacker B's page that includes a data source that uses User A's credentials.

User A authentication could have been through setting cookies that will travel to the data API domain. The API could also require that User A have an "API key" to use the API (User A registers with the data service provider, and receives a API key text string that must be passed to any data calls. Google Maps is an example).

Hacker B could find out User A's data API key, and if the API required authentication cookies to be set, Hacker B just needs to be sure User A authenticates before coming to Hacker B's page.

Protection Measures

Data Service Registration
  • The user must register for an API key. To obtain the API key, the user must authenticate with the data service provider to prove their identity.
  • As part of this registration, the user must specify which domains are allows to use the API key, and possibly which user names are authorized to use the API Key.
  • Since the user is authenticated, that user name can use the API. Any other user names that are added to the API key must receive an email notification. The other user names must authenticate and grant permission to be on the allowed users for the API key.
Script SRC requests
  • In addition to sending the API key, there must be a timestamp as part of the request. These parameters should be querystring parameters (like
Data Service Server
  • The server for the data service should not allow caching of the data request. They should set the appropriate HTTP headers to expire the results of the request immediately, to avoid Hacker B taking advantage of the browser cache (that is why query string params are used on the URL too).
  • The server will reject any request that does not have a registered domain in the Referrer HTTP header.
  • The data service domain should only have data services. It should not allow any user-generated web pages under that domain. Ideally it should be unrelated to any domains that host user web page content -- no subdomains should match to prevent document.domain tricks from working.
Test scenarios

Even with these measures in place, are there other holes? I want to make some tests to verify the following:
  • Does authenticating with the data service pass the cookies along properly for SCRIPT SRC requests?
  • Does the Referrer field get set correctly for SCRIPT SRC requests? In all browsers?
  • How can the Referrer field be spoofed? Use XMLHTTPRequest. But hopefully since only data services are allowed on the data service domain that can't be used. Hopefully the document.domain protection above will help too. Is there some way a server proxy running on Hacker B's site be used? Hopefully the User's authentication cookies can't travel to that domain.
  • Even if the server sets expiration headers, will the browser still cache? Hopefully since a timestamp is sent and the URL uses query string params, Hacker B's URL won't be the same anyway, and therefore a different browser cache entry?
These are just first thoughts. I haven't checked yet to see if someone else has already worked this out. And it would be good to do some testing too, think of other scenarios that might break.

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