Is your PRTG server leaking your SNMP community string? Mine was!

I recently had a lengthy discussion with Paessler’s Technical Support Team Manager regarding a leak of my SNMP community string. The conclusion reached was this behavior is actually expected, per the default configuration of PRTG. If you’re not familiar with PRTG, it’s an enterprise network monitoring application created by Paessler AG.

PRTG logo

I became aware of the leak after reviewing my firewall logs and finding the unexpected incoming SNMP traffic from my Remote Probe. I found the traffic occurring every day at the same time, roughly 2:50 PM local time, with three packets sent each time.

Unusual SNMP traffic

I fired up my packet capture machine and re-routed the incoming SNMP traffic to it. Upon inspecting the traffic in Wireshark I found each packet was an SNMP get-next-request which contained my community string for all to see.

Wireshark screenshot

So one might stop at this point and ask, why am I not using SNMPv3 instead of SNMPv2c? This was a calculated choice I made, given that my SNMP traffic is only flowing on a segmented portion of my LAN and never should be traversing the internet.

At this point I contacted Paessler’s Security Team to share my findings. Unfortunately I didn’t make much headway and was soon escalated to Technical Support Team Manager after I sent a follow up to Paessler’s CEO, Dirk Paessler.

After much discussion back and forth it was finally discovered my off-site Remote Probe was sending the SNMP traffic due to it inheriting the default “Advanced Network Analysis — System Information” permissions from my Local Probe (Core Server).

I was a bit dismayed at this fact, since I had diligently turned off the other default settings for “Unusual Detection” and “Similar Sensors Detection” when I configured my PRTG installation.

However the horror didn’t stop there. I found the “System Information” feature was enabled by default for all my devices, due to the permission inheritance. While this may be a useful feature in some cases, I found my SNMP community string had been broadcast daily to every device I monitored. This included external websites, public DNS servers, and other devices outside my LAN.

So how can this be prevented? I recommend always turning off Unusual Detection, Similar Sensors Detection, and now System Information as well when configuring PRTG. These settings are found under Advanced Network Analysis section and can be configured at the “Root” level, as shown below.

PRTG configuration

If any of these features are desired, they can be enabled at the group and/or individual device level.

Per my recommendations, Paessler has updated their documentation regarding the System Information feature, found here. The following note is now included:

Note: The feature System Information is enabled by default. To retrieve the data, PRTG will automatically use Credentials for Windows Systems and Credentials for SNMP Devices as defined in the Device Settings or as inherited from a parent object like the Root group. Please consider this when you monitor devices outside the local network, especially when using SNMP v1 or v2c that do not provide encryption.

Is your PRTG installation leaking?

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