How to remediate – ICMP Netmask Request Information Disclosure

1. Introduction

The ICMP Netmask Request Information Disclosure vulnerability affects systems that respond to ICMP_MASKREQ queries. This allows an attacker to discover network configuration details, potentially aiding in bypassing security filters. Systems running standard networking stacks are typically affected. This may lead to a low impact on confidentiality due to information disclosure, with no direct impact on integrity or availability.

2. Technical Explanation

The remote host answers ICMP_MASKREQ queries (type 17) and responds with its netmask. An attacker can send these queries to gather network topology information. This is a legacy feature that provides little benefit but reveals internal network details. The vulnerability is tracked as CVE-1999-0524, CWE-200.

• Root cause: The host responds to ICMP_MASKREQ packets by default.
• Exploit mechanism: An attacker sends an ICMP_MASKREQ packet and analyses the response for netmask information. For example, using `ping -I eth0 -t 17 ` on a Linux system.
• Scope: Affected platforms include systems running TCP/IP stacks that respond to ICMP type 17 requests.

3. Detection and Assessment

• Quick checks: Use `ping -I eth0 -t 17 ` on Linux to check for responses.
• Scanning: Nessus plugin ID 19864 can detect this vulnerability. This is an example only.
• Logs and evidence: Network captures will show ICMP type 17 replies from the target host.

ping -I eth0 -t 17 

4. Solution / Remediation Steps

The following steps provide a precise way to fix this issue.

4.1 Preparation

• Backups are not usually needed for this change, but a network snapshot is recommended. No services need stopping.
• Dependencies: Ensure you have access to configure the firewall or networking stack. Roll back by reverting the firewall rules or re-enabling ICMP responses.
• Change window: This change can be made during normal business hours with minimal risk, but approval may be needed depending on your organisation’s policies.

4.2 Implementation

1. Step 1: Configure the firewall to deny incoming ICMP packets of type 17.
2. Step 2: Verify the configuration change.

4.3 Config or Code Example

Before

# No specific ICMP filtering configured

After

iptables -A INPUT -p icmp --icmp-type 17 -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 traffic, reducing the attack surface.
• Practice 2: Secure defaults – configure systems with restrictive default settings and disable unnecessary features.

4.5 Automation (Optional)

#!/bin/bash
# This script adds an iptables rule to block ICMP type 17 packets.
iptables -A INPUT -p icmp --icmp-type 17 -j DROP
service iptables save # Or equivalent command for your distribution

5. Verification / Validation

Confirm the fix by checking that the host no longer responds to ICMP_MASKREQ queries.

• Post-fix check: Run `ping -I eth0 -t 17 ` and confirm there is no response.
• Re-test: Repeat the quick check from Section 3, confirming no responses are received.
• Smoke test: Ensure basic network connectivity (e.g., ping to a known good host) remains functional.
• Monitoring: Monitor firewall logs for dropped ICMP type 17 packets as an example of regression detection.

ping -I eth0 -t 17 

6. Preventive Measures and Monitoring

Update security baselines to include this configuration.

• Baselines: Update your network security baseline or policy to explicitly deny ICMP type 17 requests.
• Pipelines: Consider using infrastructure-as-code tools to enforce firewall rules consistently across all systems.
• Asset and patch process: Review network configurations regularly as part of a vulnerability management program.

7. Risks, Side Effects, and Roll Back

Blocking ICMP type 17 packets should not cause service disruption.

• Roll back: Remove the iptables rule using `iptables -D INPUT -p icmp –icmp-type 17 -j DROP` and save the firewall configuration.

8. References and Resources

Link only to sources that match this exact vulnerability.

• Vendor advisory or bulletin: No specific vendor advisory is available for this generic issue.
• NVD or CVE entry: CVE-1999-0524
• Product or platform documentation relevant to the fix: Refer to your operating system’s firewall documentation for specific configuration instructions.