How to remediate – ASG-Sentry SNMP Agent Detection

1. Introduction

The ASG-Sentry SNMP Agent Detection vulnerability indicates an SNMP agent is listening on a remote host that is part of the ASG-Sentry web-based network management system. This could allow attackers to gather information about your network devices and potentially modify configurations. Systems running ASG-Sentry are usually affected, particularly those with exposed SNMP services. A successful exploit could lead to loss of confidentiality, integrity, and availability of network data and device settings.

2. Technical Explanation

The vulnerability occurs because the ASG-Sentry SNMP agent is running and accessible. Attackers can query this agent for information about the network. The preconditions needed for exploitation are network connectivity to the affected host and an open port allowing SNMP traffic (typically UDP 161). While there isn’t a specific CVE associated with the detection of the agent itself, it represents a potential attack vector. An attacker could use standard SNMP tools to enumerate devices, read configuration data, or potentially write commands if community strings are weak or default.

• Root cause: The ASG-Sentry SNMP agent is enabled and listening for connections.
• Exploit mechanism: An attacker uses SNMP queries (e.g., using snmpwalk) to gather information from the agent. If default or weak community strings are used, they may be able to modify device configurations.
• Scope: ASG-Sentry installations with exposed SNMP services on UDP port 161.

3. Detection and Assessment

To confirm whether a system is vulnerable, you can check for the presence of an open SNMP service and identify if it’s associated with ASG-Sentry. A quick check involves using netstat or similar tools to see if port 161 is listening.

• Quick checks: Use `netstat -tulnp | grep 161` to see if anything is listening on UDP port 161.
• Scanning: Nessus plugin ID 80935 can detect the ASG-Sentry SNMP agent.
• Logs and evidence: Check system logs for SNMP traffic originating from or destined to the affected host.

netstat -tulnp | grep 161

4. Solution / Remediation Steps

To fix this issue, filter incoming traffic to the port if desired.

4.1 Preparation

• Dependencies: Ensure firewall rules do not disrupt legitimate network management activities. Roll back plan: Revert the firewall rule change to allow traffic again.
• Change window needs: Coordinate with network administrators for approval, especially in production environments.

4.2 Implementation

1. Step 1: Configure your firewall to block incoming SNMP traffic on UDP port 161 unless explicitly required.
2. Step 2: If SNMP is needed, restrict access to specific trusted IP addresses or networks.

4.3 Config or Code Example

Before

# No firewall rule blocking SNMP traffic (example using iptables)
# iptables -L INPUT | grep 161  (will likely show nothing specific)

After

# Block all incoming SNMP traffic except from trusted IPs (example using iptables)
iptables -A INPUT -p udp --dport 161 -j DROP
# Or allow only specific IP addresses:
iptables -A INPUT -s /32 -p udp --dport 161 -j ACCEPT
iptables -A INPUT -p udp --dport 161 -j DROP

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue.

• Practice 1: Least privilege – only allow necessary network access and services.
• Practice 2: Network segmentation – isolate sensitive systems from the wider network.

4.5 Automation (Optional)

# Example using Ansible to block SNMP traffic on UDP port 161
- name: Block incoming SNMP traffic
  firewalld:
    port: 161/udp
    permanent: true
    state: disabled
    immediate: yes

5. Verification / Validation

To confirm the fix worked, check that port 161 is no longer accessible from untrusted networks.

• Post-fix check: Run `netstat -tulnp | grep 161` again; it should show nothing listening on UDP port 161 or only trusted IPs.
• Re-test: Use an SNMP scanner (e.g., snmpwalk) from an untrusted network to verify that the port is blocked.
• Smoke test: Ensure any legitimate SNMP monitoring tools still function correctly if access has been restricted.
• Monitoring: Monitor firewall logs for dropped SNMP packets, which can indicate attempted unauthorized access.

netstat -tulnp | grep 161

6. Preventive Measures and Monitoring

Update security baselines to include restrictions on unnecessary network services.

• Baselines: Update your security baseline or policy to require disabling unused SNMP services.
• Pipelines: Add checks in CI/CD pipelines to ensure that only necessary ports are open and exposed.
• Asset and patch process: Regularly review asset inventories to identify systems with potentially vulnerable services like SNMP.

7. Risks, Side Effects, and Roll Back

Blocking SNMP traffic could disrupt legitimate network monitoring tools if not configured carefully.

• Risk or side effect 1: Disruption of network management systems – ensure that any allowed IPs are correctly configured.
• Roll back: Remove the firewall rule blocking UDP port 161 to restore access.

8. References and Resources

Links only to sources that match this exact vulnerability.

• Vendor advisory or bulletin: http://www.nessus.org/u?0da8508b