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161,162,10161,10162/udp - Pentesting SNMP

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Basic Information

SNMP - Simple Network Management Protocol is a protocol used to monitor different devices in the network (like routers, switches, printers, IoTs...).

PORT    STATE SERVICE REASON                 VERSION
161/udp open  snmp    udp-response ttl 244   ciscoSystems SNMPv3 server (public)

{% hint style="info" %} SNMP also uses the port 162/UDP for traps. These are data packets sent from the SNMP server to the client without being explicitly requested. {% endhint %}

MIB

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.

OIDs

Object Identifiers (OIDs) play a crucial role. These unique identifiers are designed to manage objects within a Management Information Base (MIB).

The highest levels of MIB object IDs, or OIDs, are allocated to diverse standard-setting organizations. It is within these top levels that the framework for global management practices and standards is established.

Furthermore, vendors are granted the liberty to establish private branches. Within these branches, they have the autonomy to include managed objects pertinent to their own product lines. This system ensures that there is a structured and organized method for identifying and managing a wide array of objects across different vendors and standards.

You can navigate through an OID tree from the web here: http://www.oid-info.com/cgi-bin/display?tree=#focus or see what a OID means (like 1.3.6.1.2.1.1) accessing http://oid-info.com/get/1.3.6.1.2.1.1.
There are some well-known OIDs like the ones inside 1.3.6.1.2.1 that references MIB-2 defined Simple Network Management Protocol (SNMP) variables. And from the OIDs pending from this one you can obtain some interesting host data (system data, network data, processes data...)

OID Example

Example from here:

1 . 3 . 6 . 1 . 4 . 1 . 1452 . 1 . 2 . 5 . 1 . 3. 21 . 1 . 4 . 7

Here is a breakdown of this address.

1 – this is called the ISO and it establishes that this is an OID. This is why all OIDs start with “1”
3 – this is called ORG and it is used to specify the organization that built the device.
6 – this is the dod or the Department of Defense which is the organization that established the Internet first.
1 – this is the value of the internet to denote that all communications will happen through the Internet.
4 – this value determines that this device is made by a private organization and not a government one.
1 – this value denotes that the device is made by an enterprise or a business entity.

These first six values tend to be the same for all devices and they give you the basic information about them. This sequence of numbers will be the same for all OIDs, except when the device is made by the government.

Moving on to the next set of numbers.

1452 – gives the name of the organization that manufactured this device.
1 – explains the type of device. In this case, it is an alarm clock.
2 – determines that this device is a remote terminal unit.

The rest of the values give specific information about the device.

5 – denotes a discrete alarm point.
1 – specific point in the device
3 – port
21 – address of the port
1 – display for the port
4 – point number
7 – state of the point

SNMP Versions

There are 2 important versions of SNMP:

SNMPv1: Main one, it is still the most frequent, the authentication is based on a string (community string) that travels in plain-text (all the information travels in plain text). Version 2 and 2c send the traffic in plain text also and uses a community string as authentication.
SNMPv3: Uses a better authentication form and the information travels encrypted using (dictionary attack could be performed but would be much harder to find the correct creds than in SNMPv1 and v2).

Community Strings

As mentioned before, in order to access the information saved on the MIB you need to know the community string on versions 1 and 2/2c and the credentials on version 3.
The are 2 types of community strings:

public mainly read only functions
private Read/Write in general

Note that the writability of an OID depends on the community string used, so even if you find that "public" is being used, you could be able to write some values. Also, there may exist objects which are always "Read Only".
If you try to write an object a noSuchName or readOnly error is received**.**

In versions 1 and 2/2c if you to use a bad community string the server wont respond. So, if it responds, a valid community strings was used.

Ports

From Wikipedia:

The SNMP agent receives requests on UDP port 161.
The manager receives notifications (Traps and InformRequests) on port 162.
When used with Transport Layer Security or Datagram Transport Layer Security, requests are received on port 10161 and notifications are sent to port 10162.

Brute-Force Community String (v1 and v2c)

To guess the community string you could perform a dictionary attack. Check here different ways to perform a brute-force attack against SNMP. A frequently used community string is public.

Enumerating SNMP

It is recommanded to install the following to see whats does mean each OID gathered from the device:

apt-get install snmp-mibs-downloader
download-mibs
# Finally comment the line saying "mibs :" in /etc/snmp/snmp.conf
sudo vi /etc/snmp/snmp.conf

If you know a valid community string, you can access the data using SNMPWalk or SNMP-Check:

snmpbulkwalk -c [COMM_STRING] -v [VERSION] [IP] . #Don't forget the final dot
snmpbulkwalk -c public -v2c 10.10.11.136 .

snmpwalk -v [VERSION_SNMP] -c [COMM_STRING] [DIR_IP]
snmpwalk -v [VERSION_SNMP] -c [COMM_STRING] [DIR_IP] 1.3.6.1.2.1.4.34.1.3 #Get IPv6, needed dec2hex
snmpwalk -v [VERSION_SNMP] -c [COMM_STRING] [DIR_IP] NET-SNMP-EXTEND-MIB::nsExtendObjects #get extended
snmpwalk -v [VERSION_SNMP] -c [COMM_STRING] [DIR_IP] .1 #Enum all

snmp-check [DIR_IP] -p [PORT] -c [COMM_STRING]

nmap --script "snmp* and not snmp-brute" <target>

braa <community string>@<IP>:.1.3.6.* #Bruteforce specific OID

Thanks to extended queries (download-mibs), it is possible to enumerate even more about the system with the following command :

snmpwalk -v X -c public <IP> NET-SNMP-EXTEND-MIB::nsExtendOutputFull

SNMP has a lot of information about the host and things that you may find interesting are: Network interfaces (IPv4 and IPv6 address), Usernames, Uptime, Server/OS version, and processes

running (may contain passwords)....

Dangerous Settings

In the realm of network management, certain configurations and parameters are key to ensuring comprehensive monitoring and control.

Access Settings

Two main settings enable access to the full OID tree, which is a crucial component in network management:

rwuser noauth is set to permit full access to the OID tree without the need for authentication. This setting is straightforward and allows for unrestricted access.
For more specific control, access can be granted using:
- rwcommunity for IPv4 addresses, and
- rwcommunity6 for IPv6 addresses.

Both commands require a community string and the relevant IP address, offering full access irrespective of the request's origin.

SNMP Parameters for Microsoft Windows

A series of Management Information Base (MIB) values are utilized to monitor various aspects of a Windows system through SNMP:

System Processes: Accessed via 1.3.6.1.2.1.25.1.6.0, this parameter allows for the monitoring of active processes within the system.
Running Programs: The 1.3.6.1.2.1.25.4.2.1.2 value is designated for tracking currently running programs.
Processes Path: To determine where a process is running from, the 1.3.6.1.2.1.25.4.2.1.4 MIB value is used.
Storage Units: The monitoring of storage units is facilitated by 1.3.6.1.2.1.25.2.3.1.4.
Software Name: To identify the software installed on a system, 1.3.6.1.2.1.25.6.3.1.2 is employed.
User Accounts: The 1.3.6.1.4.1.77.1.2.25 value allows for the tracking of user accounts.
TCP Local Ports: Finally, 1.3.6.1.2.1.6.13.1.3 is designated for monitoring TCP local ports, providing insight into active network connections.

Cisco

Take a look to this page if you are Cisco equipment:

{% content-ref url="cisco-snmp.md" %} cisco-snmp.md {% endcontent-ref %}

From SNMP to RCE

If you have the string that allows you to write values inside the SNMP service, you may be able to abuse it to execute commands:

{% content-ref url="snmp-rce.md" %} snmp-rce.md {% endcontent-ref %}

Massive SNMP

Braa is a mass SNMP scanner. The intended usage of such a tool is, of course, making SNMP queries – but unlike snmpwalk from net-snmp, it is able to query dozens or hundreds of hosts simultaneously, and in a single process. Thus, it consumes very few system resources and does the scanning VERY fast.

Braa implements its OWN snmp stack, so it does NOT need any SNMP libraries like net-snmp.

Syntax: braa [Community-string]@[IP of SNMP server]:[iso id]

braa ignite123@192.168.1.125:.1.3.6.*

This can extract a lot MB of information that you cannot process manually.

So, lets look for the most interesting information (from https://blog.rapid7.com/2016/05/05/snmp-data-harvesting-during-penetration-testing/):

Devices

The process begins with the extraction of sysDesc MIB data (1.3.6.1.2.1.1.1.0) from each file to identify the devices. This is accomplished through the use of a grep command:

grep ".1.3.6.1.2.1.1.1.0" *.snmp

Identify Private String

A crucial step involves identifying the private community string used by organizations, particularly on Cisco IOS routers. This string enables the extraction of running configurations from routers. The identification often relies on analyzing SNMP Trap data for the word "trap" with a grep command:

grep -i "trap" *.snmp

Usernames/Passwords

Logs stored within MIB tables are examined for failed logon attempts, which might accidentally include passwords entered as usernames. Keywords such as fail, failed, or login are searched to find valuable data:

grep -i "login\|fail" *.snmp

Emails

Finally, to extract email addresses from the data, a grep command with a regular expression is used, focusing on patterns that match email formats:

grep -E -o "\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,6}\b" *.snmp

Modifying SNMP values

You can use NetScanTools to modify values. You will need to know the private string in order to do so.

Spoofing

If there is an ACL that only allows some IPs to query the SMNP service, you can spoof one of this addresses inside the UDP packet an sniff the traffic.

Examine SNMP Configuration files

snmp.conf
snmpd.conf
snmp-config.xml