SNMP

SNMP Services

Simple Network Management Protocol (SNMP) is a widely used protocol for collecting and organizing information about managed devices on IP networks and for modifying that information to change device behavior. From a penetration testing perspective, SNMP can often provide valuable information about network devices and their configurations.

Protocol Overview

SNMP operates primarily on UDP ports 161 (for general SNMP operations) and 162 (for SNMP traps). The protocol follows a client-server architecture:

  • SNMP Managers: Systems that collect and process information from SNMP agents

  • SNMP Agents: Software components on managed devices that provide information via SNMP

SNMP has evolved through several versions:

  • SNMPv1: The original version with basic functionality but weak security

  • SNMPv2c: Expanded functionality but still using community string authentication

  • SNMPv3: Added security features including authentication and encryption

SNMP Structure and MIB

At the core of SNMP is the Management Information Base (MIB), which holds information about network device parameters in a tree-like hierarchy. Each point in the hierarchy is addressed by an Object Identifier (OID).

Key MIB Trees and OIDs

These OIDs often contain valuable information for penetration testers:

OID
Description

1.3.6.1.2.1.25.1.6.0

System Processes

1.3.6.1.2.1.25.4.2.1.2

Running Programs

1.3.6.1.2.1.25.4.2.1.4

Processes Path

1.3.6.1.2.1.25.2.3.1.4

Storage Units

1.3.6.1.2.1.25.6.3.1.2

Software Name

1.3.6.1.4.1.77.1.2.25

User Accounts

1.3.6.1.2.1.6.13.1.3

TCP Local Ports

1.3.6.1.2.1.25.4.2

Running Processes Parameters

Enumeration Techniques

Port Scanning

# Basic scan
nmap -sU -p161 10.13.13.55

# Service version and script scan
sudo nmap -sU -p161 -sVC --script=snmp* 10.13.13.55

Example output:

PORT    STATE SERVICE VERSION
161/udp open  snmp    SNMPv1 server; net-snmp SNMPv3 server (public)
| snmp-interfaces: 
|   lo
|     IP address: 127.0.0.1  Netmask: 255.0.0.0
|     Type: softwareLoopback  Speed: 10 Mbps
|     Status: up
|   eth0
|     IP address: 10.13.13.55  Netmask: 255.255.255.0
|     MAC address: 00:0c:29:c4:42:38
|     Type: ethernetCsmacd  Speed: 1 Gbps
|     Status: up
| snmp-info: 
|   enterprise: net-snmp
|   engineID: 0x80001f888076c40b110c2978
|   engineBoots: 19
|   engineTime: 16h12m44s
|   authoritativeEngineID: 0x80001f888076c40b110c2978
|   enterpriseID: 8072
|_  engineIDFormat: 1
| snmp-netstat: 
|   TCP  0.0.0.0:22           0.0.0.0:0
|   TCP  0.0.0.0:3306         0.0.0.0:0
|   TCP  127.0.0.1:25         0.0.0.0:0
|   UDP  0.0.0.0:161          *:*
|_  UDP  0.0.0.0:42971        *:*
| snmp-processes: 
|   1: 
|     Name: systemd
|     Path: /sbin/init
|   2: 
|     Name: kthreadd
|   3: 
|     Name: rcu_gp
...

Community String Brute Forcing

SNMP v1 and v2c use community strings for authentication, with "public" and "private" being common defaults.

Using Nmap for Community String Brute Forcing

sudo nmap -sU -p 161 --script snmp-brute --script-args snmp-brute.communitiesdb=wordlist.txt 10.13.13.55

Example output:

PORT    STATE SERVICE
161/udp open  snmp
| snmp-brute: 
|   public - Valid credentials
|_  private - Valid credentials

Using Onesixtyone

# Single target
onesixtyone -c dict.txt 10.13.13.55

# Multiple targets
onesixtyone -c dict.txt -i hosts.txt

Example output:

Scanning 1 hosts, 51 communities
10.13.13.55 [public] Linux server 5.4.0-42-generic #46-Ubuntu SMP
10.13.13.55 [private] Linux server 5.4.0-42-generic #46-Ubuntu SMP

Extracting Information with SNMP

Using snmpwalk

Once a valid community string is identified, snmpwalk can be used to extract information:

# Basic system information
snmpwalk -v2c -c public 10.13.13.55 system

# Hardware information
snmpwalk -v2c -c public 10.13.13.55 hrDevice

# Installed software
snmpwalk -v2c -c public 10.13.13.55 hrSWInstalledName

# Running processes
snmpwalk -v2c -c public 10.13.13.55 hrSWRunName

# Open TCP ports
snmpwalk -v2c -c public 10.13.13.55 tcpLocalPort

# Network interfaces
snmpwalk -v2c -c public 10.13.13.55 interfaces

# User accounts
snmpwalk -v2c -c public 10.13.13.55 1.3.6.1.4.1.77.1.2.25

Example output from system information:

SNMPv2-MIB::sysDescr.0 = STRING: Linux server 5.4.0-42-generic #46-Ubuntu SMP
SNMPv2-MIB::sysObjectID.0 = OID: NET-SNMP-MIB::netSnmpAgentOIDs.10
SNMPv2-MIB::sysUpTime.0 = Timeticks: (584360) 1:37:23.60
SNMPv2-MIB::sysContact.0 = STRING: Admin <admin@example.com>
SNMPv2-MIB::sysName.0 = STRING: server.example.com
SNMPv2-MIB::sysLocation.0 = STRING: Server Room
SNMPv2-MIB::sysServices.0 = INTEGER: 72
SNMPv2-MIB::sysORLastChange.0 = Timeticks: (0) 0:00:00.00

Targeted OID Queries

For more specific information, target individual OIDs:

# System processes
snmpwalk -v2c -c public 10.13.13.55 1.3.6.1.2.1.25.1.6.0

# Running software
snmpwalk -v2c -c public 10.13.13.55 1.3.6.1.2.1.25.4.2.1.2

# Process paths
snmpwalk -v2c -c public 10.13.13.55 1.3.6.1.2.1.25.4.2.1.4

# Storage information
snmpwalk -v2c -c public 10.13.13.55 1.3.6.1.2.1.25.2.3.1.4

# Memory information
snmpwalk -v2c -c public 10.13.13.55 hrMemorySize

SNMPv3 Enumeration

SNMPv3 uses username-based security with authentication and privacy features:

# Enumerate SNMPv3 users
nmap -sU -p 161 --script=snmp-info 10.13.13.55

# Testing SNMPv3 with known credentials
snmpwalk -v3 -l authPriv -u username -a SHA -A authpassword -x DES -X privpassword 10.13.13.55 system

Common SNMP Vulnerabilities

Weak Community Strings

Default or weak community strings (e.g., "public", "private") are common and easily guessed.

Information Disclosure

SNMP often reveals sensitive information about:

  • Device configurations

  • Network topology

  • User accounts

  • Running services

  • Software versions

SNMP Write Access

If write access is enabled (using "private" or other community strings), attackers may be able to modify device configurations:

# Testing write access
snmpset -v2c -c private 10.13.13.55 SNMPv2-MIB::sysContact.0 s "Hacked"

Denial of Service

Some SNMP implementations are vulnerable to DoS attacks from malformed packets.

SNMP Attack Scenarios

Network Reconnaissance

SNMP can provide comprehensive information about network infrastructure:

  1. Scan the network for SNMP-enabled devices

  2. Brute force community strings

  3. Extract information about network interfaces, routing tables, and ARP caches

  4. Map the network topology

Credential Harvesting

User account information may be exposed through SNMP:

# Extract usernames
snmpwalk -v2c -c public 10.13.13.55 1.3.6.1.4.1.77.1.2.25

Service Enumeration

Identify running services and open ports:

# List running processes
snmpwalk -v2c -c public 10.13.13.55 hrSWRunName

# Check listening TCP ports
snmpwalk -v2c -c public 10.13.13.55 tcpLocalPort

Defensive Measures

When testing SNMP services, consider these security recommendations:

  1. Restrict SNMP Access: Use firewall rules to limit access to SNMP ports

  2. Use Strong Community Strings: Avoid defaults and use complex strings

  3. Migrate to SNMPv3: Use authentication and encryption features

  4. Implement ACLs: Restrict SNMP access to specific IP addresses

  5. Use Read-Only Community Strings: Avoid write access where possible

  6. Regular Auditing: Monitor SNMP access and configuration changes

  7. Update SNMP Software: Keep SNMP implementations patched and updated

SNMP Penetration Testing Methodology

When testing SNMP services, follow these steps:

  1. Discovery: Identify systems with SNMP enabled

  2. Version Detection: Determine SNMP version(s) in use

  3. Community String Testing: Test for default and weak community strings

  4. Information Gathering: Extract system, network, and configuration details

  5. Write Access Testing: Test for modifiable OIDs

  6. SNMPv3 Testing: Test authentication and encryption mechanisms

  7. Reporting: Document findings and suggest security improvements

Practical Testing Scripts

Automating SNMP Discovery

#!/bin/bash
# Simple SNMP discovery script
for ip in $(seq 1 254); do
    host="192.168.1.$ip"
    (snmpwalk -v2c -c public $host system 2>/dev/null | grep "Desc" && echo "SNMP found on $host") &
done
wait

Community String Tester

#!/bin/bash
# Test multiple community strings against a target
TARGET=$1
COMMUNITYLIST=$2

for community in $(cat $COMMUNITYLIST); do
    echo "Testing community string: $community"
    result=$(snmpwalk -v2c -c $community $TARGET system 2>&1)
    if ! echo "$result" | grep -q "Timeout\|cannot get"; then
        echo "[+] Valid community string found: $community"
        echo "$result"
    fi
done

SNMP Command Reference

# Basic snmpwalk commands
snmpwalk -v2c -c public TARGET system
snmpwalk -v2c -c public TARGET interfaces
snmpwalk -v2c -c public TARGET ip
snmpwalk -v2c -c public TARGET tcp
snmpwalk -v2c -c public TARGET udp
snmpwalk -v2c -c public TARGET hrSWRunName
snmpwalk -v2c -c public TARGET hrSWInstalledName
snmpwalk -v2c -c public TARGET hrStorageDescr
snmpwalk -v2c -c public TARGET hrMemorySize

# snmpset (write) command
snmpset -v2c -c private TARGET OID s|i|a VALUE

# snmptrap command (sending trap)
snmptrap -v2c -c public TARGET '' NET-SNMP-EXAMPLES-MIB::netSnmpExampleHeartbeatNotification netSnmpExampleHeartbeatRate i 123456

# SNMPv3 commands
snmpwalk -v3 -l authPriv -u username -a SHA -A authpass -x AES -X privpass TARGET system

By understanding SNMP services and their security implications, penetration testers can effectively identify vulnerabilities and provide valuable recommendations for securing network management infrastructure.

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