FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 272301
Description This article describes how to troubleshoot the SNMPv3 debug error 'USM decryption error'.
Scope FortiGate.

Debugging SNMP:


diagnose debug reset
diagnose debug application snmpd -1
diagnose debug console timestamp enable
diagnose debug enable


After enabling the above commands, initiate connectivity from the SNMP manager/server.


To stop the debugging, run the following command:

diagnose debug disable


Sample error message in debug:


snmpd: v3 recv: parse failed. errno=-48 (USM decryption error)

2023-08-26 14:26:20 snmpd: </msg> 0

2023-08-26 14:26:25 snmpd: checking if community "snmpcom" is valid
2023-08-26 14:26:25 snmpd: failed to match community "snmpcom"


Capture the SNMPv3 packets:


diagnose sniffer packet any "host and (port 161 or port 162)" 6 0 l


After enabling the above commands, initiate connectivity from the SNMP manager/server.


Decrypt the SNMPv3 packets:


From the decrypted packets, validate the status of both authentication and privacy parameters.

If any errors are observed as below, make sure the authentication and privacy algorithms and the respective passwords are correctly configured on both ends.

If the connectivity is still down afterwards, validate the supported encryption types on both devices.



In this example, the authentication is successful. However, the privacy parameters show a warning even though the packets are decrypted with the correct algorithm and password.


The error 'USM decryption error' occurs when the SNMP server uses a different method from what FortiGate supports to extend the size of the localized Key of the SNMPv3 encryption protocol.


Configure the correct algorithm supported on both devices.


An example output of a successful communication:


SNMPV3 success.png

Related article:

Technical Tip: How to decrypt SNMPv3 packets.