SNMP

Simple Network Management Protocol (SNMP) was created to monitor network devices. In addition, this protocol can also be used to handle configuration tasks and change settings remotely. SNMP-enabled hardware includes routers, switches, servers, IoT devices, and many other devices that can also be queried and controlled using this standard protocol. Thus, it is a protocol for monitoring and managing network devices. In addition, configuration tasks can be handled, and settings can be made remotely using this standard. The current version is SNMPv3, which increases the security of SNMP in particular, but also the complexity of using this protocol.

Communication

In addition to the pure exchange of information, SNMP also transmits control commands using agents over UDP port 161. The client can set specific values in the device and change options and settings with these commands. While in classical communication, it is always the client who actively requests information from the server, SNMP also enables the use of so-called traps over UDP port 162. These are data packets sent from the SNMP server to the client without being explicitly requested. If a device is configured accordingly, an SNMP trap is sent to the client once a specific event occurs on the server-side.

For the SNMP client and server to exchange the respective values, the available SNMP objects must have unique addresses known on both sides. This addressing mechanism is an absolute prerequisite for successfully transmitting data and network monitoring using SNMP.

MIB (Management Information Base)

To ensure that SNMP access works across manufacturers and with different client-server combinations, the Management Information Base (MIB) was created. MIB is an independent format for storing device information. A MIB is a text file in which all queryable SNMP objects of a device are listed in a standardized tree hierarchy. It contains at least one Object Identifier (OID), which, in addition to the necessary unique address and a name, also provides information about the type, access rights, and a description of the respective object. MIB files are written in the Abstract Syntax Notation One (ASN.1) based ASCII text format. The MIBs do not contain data, but they explain where to find which information and what it looks like, which returns values for the specific OID, or which data type is used.

OID (Object Identifier)

An OID represents a node in a hierarchical namespace. A sequence of numbers uniquely identifies each node, allowing the node’s position in the tree to be determined. The longer the chain, the more specific the information. Many nodes in the OID tree contain nothing except references to those below them. The OIDs consist of integers and are usually concatenated by dot notation. We can look up many MIBs for the associated OIDs in the Object Identifier Registry.

SNMP Versions

SNMPv1

SNMP version 1 (SNMPv1) is used for network management and monitoring. SNMPv1 is the first version of the protocol and is still in use in many small networks. It supports the retrieval of information from network devices, allows for the configuration of devices, and provides traps, which are notifications of events. However, SNMPv1 has no built-in authentication mechanism, meaning anyone accessing the network can read and modify network data. Another main flaw of SNMPv1 is that it does not support encryption, meaning that all data is sent in plain text and can be easily intercepted.

SNMPv2

SNMPv2 existed in different versions. The version still exists today is v2c, and the extension c means community-based SNMP. Regarding security, SNMPv2 is on par with SNMPv1 and has been extended with additional functions from the party-based SNMP no longer in use. However, a significant problem with the initial execution of the SNMP protocol is that the community string that provides security is only transmitted in plain text, meaning it has no built-in encryption.

SNMPv3

The security has been increased enormously for SNMPv3 by security features such as authentication using username and password and transmission encryption (via pre-shared key) of the data.

Security Considerations

In the case of a misconfiguration, we would get approximately the same results from snmpwalk as just shown above. Once we know the community string and the SNMP service that does not require authentication (versions 1, 2c), we can query internal system information like in the previous example.

Here we recognize some Python packages that have been installed on the system. If we do not know the community string, we can use onesixtyone and SecLists wordlists to identify these community strings.

Often, when certain community strings are bound to specific IP addresses, they are named with the hostname of the host, and sometimes even symbols are added to these names to make them more challenging to identify. However, if we imagine an extensive network with over 100 different servers managed using SNMP, the labels, in that case, will have some pattern to them. Therefore, we can use different rules to guess them. We can use the tool crunch to create custom wordlists. Creating custom wordlists is not an essential part of this module, but more details can be found in the module Cracking Passwords With Hashcat.

Once we know a community string, we can use it with braa to brute-force the individual OIDs and enumerate the information behind them.

Once again, we would like to point out that the independent configuration of the SNMP service will bring us a great variety of different experiences that no tutorial can replace. Therefore, we highly recommend setting up a VM with SNMP, experimenting with it, and trying different configurations. SNMP can be a boon for an I.T. systems administrator as well as a curse for Security analysts and managers alike.

Tools

OneSixtyOne

OneSixtyOne is a tool used to brute-force SNMP community strings:

[!bash!]$ sudo apt install onesixtyone
[!bash!]$ onesixtyone -c /opt/useful/seclists/Discovery/SNMP/snmp.txt 10.129.14.128

Scanning 1 hosts, 3220 communities
10.129.14.128 [public] Linux htb 5.11.0-37-generic #41~20.04.2-Ubuntu SMP Fri Sep 24 09:06:38 UTC 2021 x86_64

Braa

Braa is a tool used to brute-force SNMP OIDs once a community string is known:

[!bash!]$ sudo apt install braa
[!bash!]$ braa <community string>@<IP>:.1.3.6.*   # Syntax
[!bash!]$ braa public@10.129.14.128:.1.3.6.*

10.129.14.128:20ms:.1.3.6.1.2.1.1.1.0:Linux htb 5.11.0-34-generic #36~20.04.1-Ubuntu SMP Fri Aug 27 08:06:32 UTC 2021 x86_64
10.129.14.128:20ms:.1.3.6.1.2.1.1.2.0:.1.3.6.1.4.1.8072.3.2.10
10.129.14.128:20ms:.1.3.6.1.2.1.1.3.0:548
10.129.14.128:20ms:.1.3.6.1.2.1.1.4.0:mrb3n@inlanefreight.htb
10.129.14.128:20ms:.1.3.6.1.2.1.1.5.0:htb
10.129.14.128:20ms:.1.3.6.1.2.1.1.6.0:US
10.129.14.128:20ms:.1.3.6.1.2.1.1.7.0:78

Example SNMP Walk Output

Example output from snmpwalk showing various OIDs and their values:

iso.3.6.1.2.1.1.9.1.3.1 = STRING: "The MIB module for SNMPv2 entities"
iso.3.6.1.2.1.1.9.1.3.2 = STRING: "The MIB module for managing IP and ICMP implementations"
iso.3.6.1.2.1.1.9.1.3.3 = STRING: "The MIB module for managing TCP implementations"
iso.3.6.1.2.1.1.9.1.3.4 = STRING: "The MIB module for managing UDP implementations"
iso.3.6.1.2.1.1.9.1.3.5 = STRING: "The MIB modules for managing SNMP Notification, plus filtering."
iso.3.6.1.2.1.1.9.1.3.6 = STRING: "The MIB module for managing TCP implementations"
iso.3.6.1.2.1.1.9.1.3.7 = STRING: "The MIB module for managing IP and ICMP implementations"
iso.3.6.1.2.1.1.9.1.3.8 = STRING: "The MIB module for managing UDP implementations"
iso.3.6.1.2.1.1.9.1.3.9 = STRING: "The MIB modules for managing SNMP Notification, plus filtering."
iso.3.6.1.2.1.1.9.1.3.10 = STRING: "The MIB module for logging SNMP Notifications."
iso.3.6.1.2.1.1.9.1.4.1 = Timeticks: (0) 0:00:00.00
iso.3.6.1.2.1.1.9.1.4.2 = Timeticks: (0) 0:00:00.00
iso.3.6.1.2.1.1.9.1.4.3 = Timeticks: (0) 0:00:00.00
iso.3.6.1.2.1.1.9.1.4.4 = Timeticks: (0) 0:00:00.00
iso.3.6.1.2.1.1.9.1.4.5 = Timeticks: (0) 0:00:00.00
iso.3.6.1.2.1.1.9.1.4.6 = Timeticks: (0) 0:00:00.00
iso.3.6.1.2.1.1.9.1.4.7 = Timeticks: (0) 0:00:00.00
iso.3.6.1.2.1.1.9.1.4.8 = Timeticks: (0) 0:00:00.00
iso.3.6.1.2.1.1.9.1.4.9 = Timeticks: (0) 0:00:00.00
iso.3.6.1.2.1.1.9.1.4.10 = Timeticks: (0) 0:00:00.00
iso.3.6.1.2.1.25.1.1.0 = Timeticks: (3676678) 10:12:46.78
iso.3.6.1.2.1.25.1.2.0 = Hex-STRING: 07 E5 09 14 0E 2B 2D 00 2B 02 00 
iso.3.6.1.2.1.25.1.3.0 = INTEGER: 393216
iso.3.6.1.2.1.25.1.4.0 = STRING: "BOOT_IMAGE=/boot/vmlinuz-5.11.0-34-generic root=UUID=9a6a5c52-f92a-42ea-8ddf-940d7e0f4223 ro quiet splash"
iso.3.6.1.2.1.25.1.5.0 = Gauge32: 3
iso.3.6.1.2.1.25.1.6.0 = Gauge32: 411
iso.3.6.1.2.1.25.1.7.0 = INTEGER: 0
iso.3.6.1.2.1.25.1.7.0 = No more variables left in this MIB View (It is past the end of the MIB tree)

...SNIP...

iso.3.6.1.2.1.25.6.3.1.2.1232 = STRING: "printer-driver-sag-gdi_0.1-7_all"
iso.3.6.1.2.1.25.6.3.1.2.1233 = STRING: "printer-driver-splix_2.0.0+svn315-7fakesync1build1_amd64"
iso.3.6.1.2.1.25.6.3.1.2.1234 = STRING: "procps_2:3.3.16-1ubuntu2.3_amd64"
iso.3.6.1.2.1.25.6.3.1.2.1235 = STRING: "proftpd-basic_1.3.6c-2_amd64"
iso.3.6.1.2.1.25.6.3.1.2.1236 = STRING: "proftpd-doc_1.3.6c-2_all"
iso.3.6.1.2.1.25.6.3.1.2.1237 = STRING: "psmisc_23.3-1_amd64"
iso.3.6.1.2.1.25.6.3.1.2.1238 = STRING: "publicsuffix_20200303.0012-1_all"
iso.3.6.1.2.1.25.6.3.1.2.1239 = STRING: "pulseaudio_1:13.99.1-1ubuntu3.12_amd64"
iso.3.6.1.2.1.25.6.3.1.2.1240 = STRING: "pulseaudio-module-bluetooth_1:13.99.1-1ubuntu3.12_amd64"
iso.3.6.1.2.1.25.6.3.1.2.1241 = STRING: "pulseaudio-utils_1:13.99.1-1ubuntu3.12_amd64"
iso.3.6.1.2.1.25.6.3.1.2.1242 = STRING: "python-apt-common_2.0.0ubuntu0.20.04.6_all"
iso.3.6.1.2.1.25.6.3.1.2.1243 = STRING: "python3_3.8.2-0ubuntu2_amd64"
iso.3.6.1.2.1.25.6.3.1.2.1244 = STRING: "python3-acme_1.1.0-1_all"
iso.3.6.1.2.1.25.6.3.1.2.1245 = STRING: "python3-apport_2.20.11-0ubuntu27.21_all"
iso.3.6.1.2.1.25.6.3.1.2.1246 = STRING: "python3-apt_2.0.0ubuntu0.20.04.6_amd64" 

...SNIP...