How to remediate – SSH Rate Limited Device

1. Introduction

The SSH Rate Limited Device vulnerability means a networking device is limiting how many times someone can try to log in via SSH within a certain time. This can cause legitimate users to be locked out, disrupting network access. It usually affects routers, switches, firewalls and other similar devices. Impact on confidentiality is low unless combined with other attacks; integrity is minimal; availability may be affected due to intermittent authentication failures.

2. Technical Explanation

The root cause is a configured rate limit on SSH connections. This limit is intended as a security measure, but can become too restrictive and block valid attempts. An attacker could attempt to trigger the rate limiting by repeatedly sending connection requests, causing a denial of service for legitimate users. Local checks are enabled where possible to identify devices with this configuration.

• Root cause: overly aggressive SSH connection rate limiting configured on the device.
• Exploit mechanism: An attacker sends numerous SSH connection attempts in quick succession, exceeding the configured rate limit and blocking legitimate logins.
• Scope: Networking devices running SSH, including routers, switches, firewalls, and load balancers. Specific versions are not generally applicable as this is a configuration issue.

3. Detection and Assessment

Confirming vulnerability involves checking the device’s SSH configuration for rate limiting settings. A quick check can be done via the device’s web interface or command line. More thorough assessment requires reviewing the full SSH configuration file.

• Quick checks: Use the device’s CLI to show the current SSH configuration. For example, on some Cisco devices `show running-config | include ssh rate limit`.
• Scanning: Nessus plugin ID 16384 may identify this issue as an example only.
• Logs and evidence: Check system logs for messages related to failed login attempts or SSH connection limits being reached. Log file locations vary by device vendor.

show running-config | include ssh rate limit

4. Solution / Remediation Steps

Fixing the issue involves adjusting the SSH rate limiting settings to allow legitimate users to connect without being blocked. These steps should be performed during a maintenance window.

4.1 Preparation

• Dependencies: Access to the device’s command line or web interface is required. Roll back by restoring the backed-up configuration file.
• Change window: A short maintenance window may be needed to avoid disrupting users. Approval from network administrators is recommended.

4.2 Implementation

1. Step 1: Access the device’s command line or web interface.
2. Step 2: Identify the SSH configuration section that controls rate limiting.
3. Step 3: Increase the connection limit to a reasonable value, such as 60 connections per minute.
4. Step 4: Save the changes and restart the SSH service if necessary.

4.3 Config or Code Example

Before

ssh rate limit 10 60

After

ssh rate limit 60 60

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: Secure Defaults – Configure SSH with reasonable default settings to avoid overly restrictive rate limits that can cause denial of service.
• Practice 2: Monitoring – Regularly monitor logs for failed login attempts and connection limit events to identify potential attacks or misconfigurations.

4.5 Automation (Optional)

# Example Ansible snippet - use with caution!
- name: Adjust SSH rate limit
  cisco.ios.ios_config:
    lines:
      - ssh rate limit 60 60
    parents: "line vty 0 15"
  become: true

5. Verification / Validation

Confirm the fix by checking the SSH configuration and verifying that legitimate users can connect without being blocked. Run a negative test to ensure rate limiting still prevents brute-force attacks.

• Post-fix check: Use `show running-config | include ssh rate limit` and confirm the value is now 60 60.
• Re-test: Re-run the earlier detection command to show that the SSH configuration no longer has a restrictive rate limit.
• Smoke test: Attempt to log in via SSH from multiple locations to verify connectivity.
• Monitoring: Monitor system logs for failed login attempts and connection limit events. A spike may indicate an attack attempt.

show running-config | include ssh rate limit

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 recommended SSH rate limiting settings. For example, a CIS control for network device configuration.
• Asset and patch process: Review device configurations regularly as part of an asset management process. A quarterly review cycle is sensible.

7. Risks, Side Effects, and Roll Back

• Roll back: Restore the backed-up device configuration file if issues occur.

8. References and Resources

• Vendor advisory or bulletin: Check your device vendor’s website for specific SSH configuration recommendations.
• NVD or CVE entry: This issue is typically a misconfiguration, so does not have a dedicated CVE.
• Product or platform documentation relevant to the fix: Refer to your device’s official documentation for details on configuring SSH rate limiting.