1. Introduction
The “Service Detection: 3 ASCII Digit Code Responses” vulnerability involves a plugin identifying services responding with three-digit codes, like FTP or SMTP. This matters because it can reveal running services which attackers could then target. Affected systems are typically those undergoing network discovery scans. A successful exploit could lead to information disclosure and potential remote access. Impact on confidentiality is likely medium, integrity low, and availability low.
2. Technical Explanation
This plugin complements the find_service1.nasl script by identifying services that return 3 ASCII digit codes in their responses. This can indicate the presence of specific network services. An attacker could use this information to map a target network and identify potential entry points for further exploitation. There is no known CVE associated with this detection method itself, but it highlights potentially exposed services. For example, an attacker discovering an FTP service might attempt brute-force login attacks or exploit known vulnerabilities in that specific FTP implementation.
- Root cause: The plugin identifies services responding to a standard set of probes.
- Exploit mechanism: An attacker uses the information gathered by this detection method to target identified services with appropriate exploits.
- Scope: Any system running network services that respond with 3-digit codes (FTP, SMTP, NNTP, etc.).
3. Detection and Assessment
Confirming vulnerability involves identifying systems responding to service probes with three-digit codes. A quick check is reviewing the output of network scans. Thorough assessment requires analysing scan results for these responses.
- Quick checks: Review network scan reports for services listed with 3-digit response codes.
- Scanning: Nessus plugin ID 64852 can identify this condition as an informational finding.
- Logs and evidence: Network traffic captures may show the three-digit responses from vulnerable services.
nmap -p 21,25,110 4. Solution / Remediation Steps
Fixing this issue involves reducing unnecessary service exposure and hardening identified services.
4.1 Preparation
- Dependencies: Ensure no critical applications rely on the services being modified. Roll back plan: Restore configuration backups or restart stopped services.
- Change window: Coordinate with application owners for any service downtime.
4.2 Implementation
- Step 1: Disable unnecessary network services that respond with three-digit codes.
- Step 2: Configure firewalls to restrict access to essential services from untrusted networks.
- Step 3: Update identified services to the latest patched versions.
4.3 Config or Code Example
Before
# /etc/services (example - FTP enabled)
ftp 21/tcp # Standard FTP service
After
# /etc/services (example - FTP disabled, commented out)
# ftp 21/tcp # Standard FTP service
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.
- Practice 1: Least privilege – Only run services absolutely necessary for business function.
- Practice 2: Network segmentation – Isolate sensitive services from untrusted networks.
4.5 Automation (Optional)
# Example Bash script to disable FTP service on multiple systems (use with caution!)
for host in ; do
ssh $host "sudo systemctl stop vsftpd"
done
5. Verification / Validation
Confirm the fix by verifying that unnecessary services are disabled and firewalls restrict access. Re-run network scans to confirm no three-digit responses are present.
- Post-fix check: Run `nmap -p 21,25,110
` and verify no open ports or service detection. - Re-test: Re-run the Nessus plugin ID 64852 to confirm it no longer reports three-digit responses.
- Monitoring: Monitor firewall logs for blocked connection attempts to previously exposed ports.
nmap -p 21,25,110 # Expected output should show no open ports or service detection. 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 to include a list of approved network services.
- Pipelines: Implement automated scanning in CI/CD pipelines to detect unnecessary service exposure.
- Asset and patch process: Establish a regular review cycle for running services and associated patches.
7. Risks, Side Effects, and Roll Back
- Risk or side effect 1: Disabling a required service can cause application downtime; verify dependencies first.
- Roll back: 1) Restore configuration backups. 2) Restart stopped services using `sudo systemctl start
`. 3) Revert firewall rule changes.
8. References and Resources
- Vendor advisory or bulletin: N/A – This is a detection method, not a specific vendor issue.
- NVD or CVE entry: N/A – This is a detection method, not a specific vulnerability with a CVE.
- Product or platform documentation relevant to the fix: Documentation for your specific network service (e.g., vsftpd configuration guide).