How to remediate – Network Block Device Server Detection

1. Introduction

The remote host is running a Network Block Device (NBD) server, which allows one Linux host to use another as a block device. This means another system can directly access storage on this machine. It matters because unauthorised access could lead to data theft or modification. Systems commonly affected are Linux servers offering block-level storage services. Likely impact is medium for confidentiality and integrity, and low for availability if the service is disrupted.

2. Technical Explanation

The vulnerability occurs when a Network Block Device server is running without sufficient access controls or authentication. An attacker can connect to the NBD server and gain direct block-level access to the storage it exposes. Preconditions include network connectivity to the server, and no firewall rules blocking access on the relevant port (typically TCP 10809). There isn’t a specific CVE associated with simply running an NBD server; risk comes from misconfiguration.

• Root cause: Lack of authentication or authorisation for accessing block devices.
• Exploit mechanism: An attacker connects to the NBD server using a client tool, bypassing any intended access restrictions if they are absent or weak. For example, an attacker could use `qemu-nbd` to connect and read/write directly to disks on the target system.
• Scope: Linux systems running NBD servers. Affected versions depend on the specific NBD implementation used (e.g., kernel module, user space server).

3. Detection and Assessment

You can confirm if a system is vulnerable by checking for the presence of an NBD server process or listening port. A thorough method involves examining the configuration files to assess access controls.

• Quick checks: Use `ss -tulnp | grep nbd` to see if any processes are listening on TCP 10809 (the default port).
• Scanning: Nessus plugin ID 16374 can identify running NBD servers. This is an example only, and may require updates for accuracy.
• Logs and evidence: Check system logs (/var/log/syslog or /var/log/messages) for messages related to the NBD server startup or connections.

ss -tulnp | grep nbd

4. Solution / Remediation Steps

The following steps detail how to fix this issue by disabling the NBD service, or securing it with appropriate access controls.

4.1 Preparation

• Ensure you have console access in case remote access is lost. A roll back plan involves restarting the NBD service if it’s required for other functionality.
• A change window may be needed depending on the impact of stopping the service. Approval from a system owner might be necessary.

4.2 Implementation

1. Step 1: Stop the NBD service using `systemctl stop nbd`.
2. Step 2: Disable the NBD service to prevent it starting automatically on boot with `systemctl disable nbd`.
3. Step 3: If disabling is not possible, configure firewall rules to restrict access to TCP port 10809 to only trusted hosts.

4.3 Config or Code Example

Before

# No firewall rules for port 10809

After

iptables -A INPUT -p tcp --dport 10809 -s  -j ACCEPT
iptables -A INPUT -p tcp --dport 10809 -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 services to run, and restrict their access as much as possible.
• Practice 2: Network segmentation – isolate sensitive systems from the wider network to limit the impact of compromise.

4.5 Automation (Optional)

#!/bin/bash
# Stop and disable NBD service on remote hosts via SSH
for host in $(cat /path/to/hostlist); do
  ssh $host "sudo systemctl stop nbd && sudo systemctl disable nbd"
done

5. Verification / Validation

Confirm the fix by checking that the NBD service is stopped and no longer listening on TCP 10809.

• Post-fix check: Run `ss -tulnp | grep nbd`. The output should be empty.
• Re-test: Re-run the initial detection command (`ss -tulnp | grep nbd`) to confirm that the NBD server is no longer listening.
• Monitoring: Monitor system logs for unexpected connections attempts on TCP port 10809 as an example of a regression.

ss -tulnp | grep nbd

6. Preventive Measures and Monitoring

Update security baselines to prevent this issue.

• Baselines: Update your system baseline or configuration management policy to include a rule that disables the NBD service by default, or enforces strict access controls if it’s required.
• Pipelines: Include checks in your CI/CD pipeline to scan for unnecessary services running on deployed systems.
• Asset and patch process: Review system configurations regularly as part of an asset management process.

7. Risks, Side Effects, and Roll Back

Disabling the NBD service may impact applications that rely on it.

• Risk or side effect 1: Disabling the NBD service could break functionality if other systems depend on it for block storage access.
• Risk or side effect 2: Incorrect firewall rules could inadvertently block legitimate traffic.
• Roll back: Step 1: Start the NBD service using `systemctl start nbd`. Step 2: Enable the NBD service with `systemctl enable nbd`. Step 3: Remove any restrictive firewall rules if they caused issues.

8. References and Resources

• Vendor advisory or bulletin: https://nbd.sourceforge.io/