How to remediate – Discard Service Detection

1. Introduction

A discard service is running on the remote host. This service listens for connections but ignores any data it receives, effectively discarding it. While generally harmless, an unused ‘discard’ service represents a potential attack surface and can consume system resources. A successful exploit could lead to denial of service or information leakage in rare cases. Confidentiality, integrity, and availability are all potentially at low risk.

2. Technical Explanation

The vulnerability stems from the presence of an unnecessary network service listening for connections without performing any meaningful processing. An attacker could connect to the discard service and send data, which is simply ignored. While not directly exploitable in a traditional sense, it can be used as part of reconnaissance or denial-of-service attacks. There are no known CVEs associated with this specific issue; however, it represents a general security best practice violation. An attacker could use the service to probe for open ports and potentially mask malicious activity.

• Root cause: The ‘discard’ service is enabled and listening on a network port.
• Exploit mechanism: An attacker connects to the service and sends data, which is discarded. This can be used for reconnaissance or as part of a larger attack.
• Scope: Unix-based systems (Linux, macOS) and Windows systems running the SimpTCP service are affected.

3. Detection and Assessment

You can confirm if a system is vulnerable by checking for the presence of the ‘discard’ service or its equivalent on Windows. A quick check involves listing listening ports, while a thorough method includes examining configuration files.

• Quick checks: Use netstat -tulnp (Linux) to list listening TCP and UDP ports and identify if ‘discard’ is present. On Windows, use netstat -an | findstr "0.0.0.0:19" to check for the default discard port.
• Scanning: Nessus plugin ID 32865 can detect running discard services on Unix systems.
• Logs and evidence: Examine /etc/inetd.conf (Unix) or Windows Event Logs for entries related to the ‘discard’ service.

netstat -tulnp | grep discard

4. Solution / Remediation Steps

Disable the unused ‘discard’ service to reduce the attack surface and free up system resources. The steps vary depending on the operating system.

4.1 Preparation

• Approval: Changes may require approval from system administrators depending on organizational policies.

4.2 Implementation

1. Step 1: Under Unix systems, open /etc/inetd.conf in a text editor.
2. Step 2: Comment out the line containing ‘discard’ by adding a ‘#’ at the beginning of the line.
3. Step 3: Save the changes to /etc/inetd.conf.
4. Step 4: Restart the inetd process using the command sudo systemctl restart inetd or equivalent for your distribution.
5. Step 5: Under Windows systems, open the Registry Editor (regedit).
6. Step 6: Navigate to HKLMSystemCurrentControlSetServicesSimpTCPParameters.
7. Step 7: Set the value of ‘EnableTcpDiscard’ to 0.
8. Step 8: Launch cmd.exe as an administrator.
9. Step 9: Type net stop simptcp and press Enter.
10. Step 10: Type net start simptcp and press Enter.

4.3 Config or Code Example

Before

# /etc/inetd.conf
discard stream tcp nowait root /usr/sbin/tcpd discard

After

# /etc/inetd.conf
#discard stream tcp nowait root /usr/sbin/tcpd discard

4.4 Security Practices Relevant to This Vulnerability

List only practices that directly address this vulnerability type. Use neutral wording and examples instead of fixed advice. For example: least privilege, input validation, safe defaults, secure headers, patch cadence. If a practice does not apply, do not include it.

• Practice 1: Principle of Least Privilege – disabling unused services reduces the potential attack surface.
• Practice 2: Secure Defaults – ensure unnecessary services are disabled by default during system configuration.

4.5 Automation (Optional)

# Example Bash script to comment out discard in /etc/inetd.conf
#!/bin/bash
sed -i '/discard/s/^/#/' /etc/inetd.conf
systemctl restart inetd

5. Verification / Validation

Confirm the fix by checking that the ‘discard’ service is no longer listening on any ports. Re-run the earlier detection method to verify its removal.

• Post-fix check: Run netstat -tulnp | grep discard (Linux) or netstat -an | findstr "0.0.0.0:19" (Windows). The command should return no output.
• Re-test: Repeat the quick check from Section 3 to confirm that ‘discard’ is no longer listed as a listening service.

netstat -tulnp | grep discard # Expected output: (empty)

6. Preventive Measures and Monitoring

Suggest only measures that are relevant to the vulnerability type. Use “for example” to keep advice conditional, not prescriptive.

• Baselines: Update security baselines or policies to include disabling unused services as a standard configuration setting.
• Pipelines: Incorporate checks in CI/CD pipelines to scan for and flag unnecessary network services during system builds.
• Asset and patch process: Review the list of running services regularly as part of asset management and vulnerability assessment processes.

7. Risks, Side Effects, and Roll Back

• Roll back: Re-enable the ‘discard’ service in /etc/inetd.conf (Unix) and restart inetd, or set ‘EnableTcpDiscard’ to 1 in the Windows Registry and restart SimpTCP.

8. References and Resources

• Vendor advisory or bulletin: No specific vendor advisory for this general issue.
• NVD or CVE entry: No specific CVE associated with the ‘discard’ service itself