Actually, this issue assumes a wide-open NAT device, like you might find in a "default-configured" RRAS server with IP-IP paths defined for internal hosts. If a real firewall (ISA, BigIP, Checkpoint, whathaveyou) is in place, the traffic between hosts is limited to the definition of the traffic (HTTP, SMTP, etc.) profile requested by the caller (internal or external) and the rules in place for that traffic definition. Assume a properly-configured firewall/proxy: If I receive an email from BadGuy and I'm dumb enough to open the mail, thus executing the link from the mail client I never patched, what happens is that I get what I asked for (literally). He can't make a SQL, SMTP, NetBIOS, anything connection to my box because the rules in effect for this session don't allow it. All he can do is respond to requests that I make; he can't initiate anything, even if he dumps a listener on my internal host (we can discuss firewall client issues separately). If he decides that his trojan should make further HTTP requests to his evil server, they'll be handled according to the rules in effect on the proxy/firewall. * Jim Harrison MCP(NT4/2K), A+, Network+ Services Platform Division The burden of proof is not satisfied by a lack of evidence to the contrary.. -----Original Message----- From: Ian Lyte [mailto:ilyteat_private] Sent: Friday, October 18, 2002 6:58 AM To: vuln-devat_private Subject: HTML email and external embedded links. b0iler recently said ... >Personally, I signed up to this list to get vulnerability devolopment >disscussion. So with this in mind I have decided to post early and get the lists feedback. I've been meaning to post this for some time but haven't done enough research yet. I'm sure that I'm missing something incredibly obvious here but I'm equally sure that there is room for development in this vulnerability. As I understand it, network address translation works like this (very simply): Box A inside the network requests information from host B on the internet. The internal IP of BOX A passes through the router and put in a NAT table. This then goes out to Host B with the IP address of the router. When a response from Host B is routed back to Box A it has the routers IP address. It arrives at the router, the router looks up in the table and say 'hey, box A requested information from port xx on Host B, this packet is from Host B port xx -> route to Box A'. Stop me if I'm being stupid yet. I realise that if you have packet content monitoring or a more complicated NAT table this all falls to pieces but I think that quite a few corporations don't have that! If I send you an HTML email, with an embedded picture, and that picture is stored on www.malware.com/malimage.jpg. When you open the email you will automatically make a connection to that server. Assuming that I am monitoring this server 24 x 7 and see you access that image I know that for a period of time I can send requests to your Box as long as I ensure that the connection appears to originate from port 80 on Host B. I could be wrong but simpler NAT set-ups just map the requests from Box A and not even the originating port. If you know a box A behind the NAT is running (say) an unsecured SQL server could not one assume that the firewall in place would be configured to allow traffic on port 1433. How about NETBIOS - whilst these are usually blocked at a firewall level just simple NAT may let it pass? I can even imagine simple firewalls only blocking incoming 135 when _no_ outgoing attempt has been made to contact the source IP. I really don't know - I'm not an expert in this field - that's your jobs!! But I do believe that there must be some way of exploiting the fact that by sending an html email, with an external embedded link, you can create a connection to the Box that opens that email whilst remaining very inconspicuous. I'm just not sure why it isn't being exploited yet. Maybe you'll tell me ;) Cheers Ian
This archive was generated by hypermail 2b30 : Wed Oct 23 2002 - 10:07:51 PDT