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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)
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.

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

OIDs stands for Object Identifiers. OIDs uniquely identify managed objects in a MIB hierarchy. This can be depicted as a tree, the levels of which are assigned by different organizations. Top level MIB object IDs (OIDs) belong to different standard organizations. Vendors define private branches including managed objects for their own products.
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

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
(Example take from here)

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

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

Settings
Description
rwuser noauth
Provides access to the full OID tree without authentication.
rwcommunity <community string> <IPv4 address>
Provides access to the full OID tree regardless of where the requests were sent from.
rwcommunity6 <community string> <IPv6 address>
Same access as with rwcommunity with the difference of using IPv6.

Cisco

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

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:

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 [email protected]:.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

One of the first things I do is extract the sysDesc .1.3.6.1.2.1.1.1.0 MIB data from each file to determine what devices I have harvested information from. This can easily be done using the following grep command:
grep ".1.3.6.1.2.1.1.1.0" *.snmp

Identify private string

As an example, if I can identify the private community string used by an organization on their Cisco IOS routers, then I could possibly use that community string to extract the running configurations from those routers. The best method for finding such data has often been related to SNMP Trap data. So again, using the following grep we can parse through a lot of MIB data quickly searching for the key word of “trap”:
grep -i "trap" *.snmp

Usernames/passwords

Another area of interest is logs, I have discovered that there are some devices that hold logs within the MIB tables. These logs can also contain failed logon attempts. Think about the last time you logged into a device via Telnet or SSH and inadvertently entered your password as the username. I typically search for key words such as fail, failed or login and examine that data to see if there is anything of value.
grep -i "login\|fail" *.snmp

Emails

grep -E -o "\b[A-Za-z0-9._%+-][email protected][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

HackTricks Automatic Commands

Protocol_Name: SNMP #Protocol Abbreviation if there is one.
Port_Number: 161 #Comma separated if there is more than one.
Protocol_Description: Simple Network Managment Protocol #Protocol Abbreviation Spelled out
Entry_1:
Name: Notes
Description: Notes for SNMP
Note: |
SNMP - Simple Network Management Protocol is a protocol used to monitor different devices in the network (like routers, switches, printers, IoTs...).
https://book.hacktricks.xyz/pentesting/pentesting-snmp
Entry_2:
Name: SNMP Check
Description: Enumerate SNMP
Command: snmp-check {IP}
Entry_3:
Name: OneSixtyOne
Description: Crack SNMP passwords
Command: onesixtyone -c /usr/share/seclists/Discovery/SNMP/common-snmp-community-strings-onesixtyone.txt {IP} -w 100
Entry_4:
Name: Nmap
Description: Nmap snmp (no brute)
Command: nmap --script "snmp* and not snmp-brute" {IP}
Entry_5:
Name: Hydra Brute Force
Description: Need Nothing
Command: hydra -P {Big_Passwordlist} -v {IP} snmp
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