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SNMP on Windows servers

Shut off SNMP access to your server, or, understand what can happen if you don't.


SNMP on Windows servers
Tom Lancaster

You may have heard the commotion recently about certain SNMP vulnerabilities that were discovered in the products of over 100 different vendors. Many security researchers have described these vulnerabilities as far more dangerous than even the Code Red worm, and they're not exaggerating.

If you have Windows servers, you should turn SNMP off, if at all possible. If you are actually using SNMP for something important, you should restrict SNMP access to your server by allowing only packets from certain IP addresses.

To configure this in Windows 2000, open Computer Manager and click Services under "Services and Applications". Scroll down the pane on the right until you find a service named SNMP Service, and SNMP Trap Service. Right-click these services one at a time, and select Properties from the menu.

If you want to disable SNMP, from the General tab, make sure the "Service status" is "Stopped". If it isn't, click the Stop button.

If you need to run SNMP, click the Security tab and select the radio-button "Accept SNMP packets from these hosts". Then you can use the Add button to put the IP address of any legitimate sources of SNMP traffic, such as your network management station.

However, even if you restrict access, it is important to understand what you have done, and that you are still vulnerable. What you have done is instructed Windows to inspect the Source IP Address field in the IP header for any packets destined to UDP port 161. If Windows doesn't find the source address in the list you configured, it will reject the packet.

This is a good idea and solid security practice, but you're not quite out of the woods yet. The problem is that spoofing an IP address is a trivial exercise. An attacker can still send an SNMP packet to your server from his unauthorized workstation, but if he puts the IP address of your network management workstation in the Source Address field of his packet, then your Windows server will have no way of knowing the packet did not actually originate on your network management workstation.

For this reason, many people also recommend ingress and egress filtering on your network, which prevents spoofing. It is important to understand that a combination of ingress filtering and restricting the IP addresses that your Windows server will accept, will prevent an attacker from the Internet from taking advantage of these vulnerabilities, but they will do nothing to protect you from internal attackers, because the ingress filtering only happens on the perimeter of your network.

While we'd all like to trust our co-workers, remember that most computer crimes originate inside the network, and according to the FBI, the average cost of an inside job is $2.4 million, while an attack from the Internet costs on average $27k.

Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.

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