Good job! You paid tens of thousands of dollars for that shiny new name-brand VPN, and then decided to deploy its web VPN functionality because, well, it was just easier than deploying software clients.
What does this mean for you? If your implementation allows dynamic URL rewriting (i.e., end users can put in any URL and have the web VPN fetch it) it’s GAME OVER, since every website a user views through that service appears to come from the same domain – your trusted VPN server. This is worst-case, but there are many other scenarios where an attacker could set up shop to exploit the session, especially if the end user is on a public network where DNS is compromised. There are a bunch of ways to exploit this, especially in multi-step attacks when the bad guy can get on the internal network (easy enough with malware). Don’t be surprised if this shows up in BeEF (a comprehensive tool for exploiting browser vulnerabilities) soon.
Friends don’t let friends connect clientless – fix it the right way. Read the US-CERT vulnerability note for more detailed information. You can mitigate many of the potential problems by only authorizing the SSL VPN to manage traffic for trusted domains, and avoid tunneling to random destinations. If it’s a full SSL VPN product with a re-browsing feature, turn that capability off!
Oh, not to add to the confusion, but Sun’s JRE is also recently vulnerable to same origin policy violations as well.