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Failed PCI Compliance scan - SNMP parameters

I have a client of mine that recently failed a PCI-compliance network scan by Trustwave. In a nutshell, the scan turned up a Guessable SNMP Community String. I have all ports blocked from the WAN, and I've disabled ports 161 and 162 on the LAN. They continue to fail the test, and need to remedy this or they will start incurring fines from their credit card company. What do I do? I know what SNMP is, but I'm not an SNMP expert. Help me Experts Exchange!

CVE-1999-0517  udp/161 Guessable SNMP Community String

Severity: High
PCI Status: Fail
Description: SNMP is a protocol used for remote monitoring and configuration of network devices and servers. The community string (essentially, the password) for your SNMP service was easily guessed. Although only the "read" (monitoring) string was tested, this probably means that the "write" (configuration) string is also guessable. An attacker who knows the community strings for this device will be able to monitor or reconfigure the device, potentially leading to a serious denial of service to your system or network.
Remediation: At a minimum, you should change your read and write community strings to something that is hard to guess. If SNMP is not required, you should disable it. Also, SNMP (UDP/161) should not be generally accessible from the Internet.
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giltjr
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Try using nmap to scan your IP's to find out open ports.

Are you trying to scan servers or routers or firewalls?

Thanks
Rajan
It would seem reasonable to ask them what IP they are seeing the community string on; I have seen cases where ISPs leave *their own routers* open for SNMP/ReadOnly so their customers/billing dept can gather performance metrics, so it might not even be your device.

Other than that, once you find where it goes to, see what else runs on that node, and if it doesn't need SNMP, just disable it entirely.
Most likely they are finding devices with "public" or "private" as the SNMP community strings. These are the defaults in almost every device ever made that has SNMP enabled and are easily guessed.
@Lrmoore: Yup. but when given what looks suspiciously like the output of a automated scan, first step is usually to ask what IP the issue is with; often external pentest evals do an entire IP range, and can include devices upstream of the client, devices shared with other clients etc etc. Being able to say a given IP is "out of scope" because it isn't owned by the client is often easier than fixing an issue.
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ASKER

Sorry for not maintaining this feed. The answer was amazingly simple and silly. Because of extreme pressure from the client to solve this, I overlooked the obvious.

The scanning agency (Trustwave) was scanning the known external IP address of my client's network. But the ISP had changed their IP address (dynamic IP user) on the router. Trustwave had no way to know this. To quote Indiana Jones and Sallah, "They're digging in the wrong place!"

The takeaway from this is worthy to be remembered: Don't overlook the basics, especially when the client provides a self-diagnosis of the problem. They are calling you because they can't figure it out!
I've requested that this question be closed as follows:

Accepted answer: 0 points for oregonfinn's comment #a38664524

for the following reason:

Words of Wisdom in dealing with annoying customers.
The answer: Verify the IP they are scanning. Duh.