Firewall Wizards mailing list archives
RE: Firewall Rules for NT Server and PDC
From: "Dawes, Rogan (ZA - Johannesburg)" <rdawes () deloitte co za>
Date: Wed, 11 Jul 2001 14:04:34 +0200
I'm not suggesting that this could simply be used without thought or effort. The serious implication (for those who don't realise what a problem protocol NetBIOS is already) is that if one is allowing netbios traffic through a firewall, intending to allow only authentication perhaps, apart from the obvious file and print access possibilities, the range of exploits is actually infinite, based on what commands can be executed using psexec. The effort referred to above could be looking for existing credentials on the origin server, brute forcing domain or server passwords on the target server, etc. The other thing that makes your "bogus" statement bogus itself, is that rexec has its own port assigned to it. The functionality is well known, and defined. Being able to actually execute arbitrary commands over the NetBIOS protocol, which it seems was not designed for this, IS new, at least to me, and I'm sure, to many other readers of this list. I'm 100% in agreement that firewalls are not the entire solution. The simplest attack is to ride on the traffic already permitted through the firewall, whether it be NetBIOS, database access (xp_cmdshell, anyone?) or IIS and MDAC/Unicode/.Printer. The least one CAN do is limit the possibilities that can be exploited using the protocols allowed. Allowing NetBIOS through under the impression that all that can potentially be done is auth, file and print, event log, and registry access is not actually limiting the possibilities (depending on your access controls over those various functions on the target server - I'm not sure exactly how psexec works) Rogan -----Original Message----- From: Benjamin P. Grubin [mailto:bgrubin () pobox com] Sent: 11 July 2001 03:50 To: 'Dawes, Rogan (ZA - Johannesburg)' Cc: firewall-wizards () nfr com Subject: RE: [fw-wiz] Firewall Rules for NT Server and PDC I would never try and convince anyone that NBT or windows networking is safe to pass through a firewall, but this example is bogus. psexec does nothing sexy, it is equivalent to rexec on the un*x platform, which has existed for eons. In order to make use of a tool like this, a trust relationship would have to be exploited. Allowing this trust relationship to exist between something like an IIS web server in a DMZ to a PDC, member server, or workstation on a LAN is the *real* security issue, not the existence of tools like psexec OR allowing NBT through a firewall. Compromising a domain member server SHOULD NOT compromise your domain. When dealing with a DMZ environment that requires domain connectivity, the trust relationships should be carefully considered. The optimal way to do this (imho) is to have a DMZ PDC, which is seperate from the LAN domain. Carefully architecting the appropriate trust relationships with the user domain should be straightforward (if you need them at all). Remember, firewalls are not the complete solution. Proper security practice dictates that you must evaluate the trusts and the application interconnectivity between each security zone in your network and ensure that they can not be used to exploit each other to cross those zones, regardless of infrastructure connectivity. Cheers, Ben ---- Benjamin P. Grubin bgrubin () pobox com PGP Fingerprint: EDE9 A88F 3BCC 514A F310 FEFB 7109 2380
-----Original Message----- From: firewall-wizards-admin () nfr com [mailto:firewall-wizards-admin () nfr com]On Behalf Of Dawes, Rogan (ZA - Johannesburg) Sent: Monday, July 09, 2001 2:21 AM To: 'Bjørnar B. Larsen'; 'Ernest Opoku-Agyemang' Cc: 'firewall-wizards () nfr com' Subject: RE: [fw-wiz] Firewall Rules for NT Server and PDC I saw a tool fairly recently called "psexec", which is able to execute arbitrary commands via nbt services. For this reason, I (where I have the say-so) refuse to allow NBT through a firewall. The example that scares me immediately is: psexec \\computer cmd.exe Instant telnet, if the account on one PC is permitted to access the other computer. Otherwise, start guessing passwords. Rogan C:\>psexec PsExec v1.11 - execute processes remotely Copyright (C) 2001 Mark Russinovich www.sysinternals.com PsExec executes a program on a remote system, where remotely executed console applications execute interactively. Usage: psexec \\computer [-u user [-p psswd]] [-s] [-c] [-d] program [arguments] -u Specifies optional user name for login to remote computer. -p Specifies optional password for user name. If you omit this you will be prompted to enter a hidden password. -s Run the remote process in the System account. -c Copy the specified program to the remote system for execution. If you omit this option the application must be in the system path on the remote system. -d Don't wait for process to terminate (non-interactive). program Name of application to execute. arguments Arguments to pass (note that file paths must be absolute paths on the target system). You can enclose applications that have spaces in their name with quotation marks e.g. psexec \\marklap "c:\long name app.exe". Input is only passed to the remote system when you press the enter key, and typing Ctrl-C terminates the remote process. If you omit a user name the process will run in the context of your account on the remote system, but will not have access to network resources (because it is impersonating). Specify a valid user name in the Domain\User syntax if the remote process requires access to network resources or to run in a different account. Note that the password is transmitted in clear text to the remote system. C:\> -----Original Message----- From: Bjørnar B. Larsen [mailto:Bjornar.B.Larsen () ementor no] Sent: 05 July 2001 07:30 To: 'Ernest Opoku-Agyemang' Cc: 'firewall-wizards () nfr com' Subject: Re: [fw-wiz] Firewall Rules for NT Server and PDC "Volker Tanger" <volker.tanger () detewe de> wrote:The connection NT-webserver and PDC necessarily is symmetrical. You will probably need to open both tcp & udp 135, 137-139 and 1024+ in both directions with no questions asked.What you need is to allow udp137, udp138 and tcp139 (often called the NBT ports). Open them exclusively between the web-server and the PDC. There's no need for the high ports. (Tested with NT4SP6a on both servers.)But with doing that you will grant the web server and thus all hackers attacking it (seen the latest IIS exploits yet?) all access to your internal system(s).Assuming the web server is on its own interface in the firewall like this INET---FW---WEB | | LAN and assuming you've made sure nothing but HTTP(S) can reach your web-server(s) from the Internet: Attackers need to gain control over the web-server by cracking the web-service through HTTP, then crack the PDC through NBT (typically password-cracking or -sniffing). That's when they're finally in and can do everything imaginable to your internal net. You obviously want to make sure both the PDC and the web-server are locked down tight and patched, and that the developers of your webserver make their scripts/appliations secure. :) Bjørnar _______________________________________________ firewall-wizards mailing list firewall-wizards () nfr com http://www.nfr.com/mailman/listinfo/firewall-wizards _______________________________________________ firewall-wizards mailing list firewall-wizards () nfr com http://www.nfr.com/mailman/listinfo/firewall-wizards
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Current thread:
- Firewall Rules for NT Server and PDC Ernest Opoku-Agyemang (Jul 03)
- Re: Firewall Rules for NT Server and PDC Volker Tanger (Jul 05)
- <Possible follow-ups>
- Re: Firewall Rules for NT Server and PDC Bjørnar B . Larsen (Jul 07)
- RE: Firewall Rules for NT Server and PDC Dawes, Rogan (ZA - Johannesburg) (Jul 09)
- RE: Firewall Rules for NT Server and PDC Benjamin P. Grubin (Jul 11)
- RE: Firewall Rules for NT Server and PDC Scott, Richard (Jul 11)
- Re: Firewall Rules for NT Server and PDC Volker Tanger (Jul 12)
- RE: Firewall Rules for NT Server and PDC Dawes, Rogan (ZA - Johannesburg) (Jul 11)
- Re: Firewall Rules for NT Server and PDC Patrick Giagnocavo (Jul 12)
- RE: Firewall Rules for NT Server and PDC Benjamin P. Grubin (Jul 13)
- RE: Firewall Rules for NT Server and PDC Ben Nagy (Jul 12)
- RE: Firewall Rules for NT Server and PDC Benjamin P. Grubin (Jul 13)
- RE: Firewall Rules for NT Server and PDC Jeroen Veeren (Jul 13)