How to remediate – SSH Compression Error Checking

1. Introduction

SSH Compression Error Checking affects systems running SSH software that supports compression algorithms but experiences connection failures when attempting to use them. This can lead to denial of service, as legitimate users may be unable to connect. Systems commonly affected are Linux servers, network devices and any device using OpenSSH. The vulnerability has a low impact on confidentiality, integrity, and availability due to the nature of the error causing connection failure rather than data compromise.

2. Technical Explanation

The remote host advertises support for SSH compression algorithms but fails when an attempt is made to establish a compressed connection. This typically occurs because of a mismatch between the client and server’s implementation or configuration of these algorithms, leading to errors during negotiation. There is no known CVE associated with this specific issue as it often represents a configuration problem rather than a software flaw. An attacker could potentially cause a denial-of-service condition by repeatedly attempting to connect using compression, exhausting resources on the SSH server.

• Root cause: The SSH server advertises compression support but cannot handle compressed connections due to an internal error or configuration issue.
• Exploit mechanism: An attacker initiates an SSH connection and attempts to negotiate a compressed session. This causes the server to fail, potentially leading to resource exhaustion if repeated.
• Scope: Affected platforms include systems running OpenSSH versions where compression is enabled by default or explicitly configured.

3. Detection and Assessment

• Quick checks: Use the following command to view the SSH server configuration file for compression-related options.
• Scanning: Nessus plugin ID 16849 may identify this issue as an informational finding. This is provided as an example only.
• Logs and evidence: Examine SSH server logs (typically located in /var/log/auth.log or similar) for errors related to compression negotiation failures. Look for messages indicating problems with zlib or other compression libraries.

grep -i Compression /etc/ssh/sshd_config

4. Solution / Remediation Steps

The following steps provide a precise, ordered method to fix the issue by disabling SSH compression on the server. These steps are small and testable with safe rollback options.

4.1 Preparation

• Ensure you have a method for restoring the original configuration file in case of issues. A simple rollback plan involves reverting the modified configuration and restarting the SSH service.
• A change window may be required depending on your organisation’s policies, with approval from the system owner.

4.2 Implementation

1. Step 1: Edit the SSH server configuration file (/etc/ssh/sshd_config) using a text editor (e.g., nano or vim).
2. Step 2: Locate the line containing “Compression” and either comment it out by adding a # at the beginning of the line, or set its value to “no”.
3. Step 3: Save the changes to the configuration file.
4. Step 4: Restart the SSH service to apply the new configuration. For example: `sudo systemctl restart sshd`.

4.3 Config or Code Example

Before

Compression yes

After

# Compression yes

4.4 Security Practices Relevant to This Vulnerability

Several security practices can directly address this vulnerability type. Least privilege reduces the impact if exploited, while safe defaults minimise unnecessary features and potential attack surfaces.

• Practice 1: Least privilege – limiting SSH access to only authorized users and systems reduces the potential impact of a denial-of-service attack.
• Practice 2: Safe defaults – disabling unnecessary features like compression by default reduces the attack surface and simplifies configuration.

4.5 Automation (Optional)

# Example Ansible snippet to disable compression
- name: Disable SSH Compression
  lineinfile:
    path: /etc/ssh/sshd_config
    regexp: '^Compression'
    state: absent
  become: true
  notify: Restart sshd

5. Verification / Validation

Confirm the fix by checking the SSH server configuration and attempting to establish a compressed connection again. A post-fix check verifies the compression setting, while retesting confirms the issue is resolved.

• Post-fix check: Run `grep -i Compression /etc/ssh/sshd_config` and confirm that no lines containing “Compression yes” are present.
• Smoke test: Verify that standard SSH connections without compression still function correctly. For example, attempt a login from a known client.

grep -i Compression /etc/ssh/sshd_config

6. Preventive Measures and Monitoring

Update security baselines or policies to prevent this issue by including a requirement to disable SSH compression. Add checks in CI or deployment pipelines to ensure the configuration remains secure.

• Baselines: Update your security baseline (for example, CIS control 5.1) to include disabling SSH compression as a standard setting.
• Pipelines: Incorporate SAST tools into your CI pipeline to scan for insecure configurations like enabled compression in SSH configuration files.
• Asset and patch process: Implement a regular review cycle of SSH server configurations (e.g., quarterly) to identify and address any deviations from the security baseline.

7. Risks, Side Effects, and Roll Back

• Risk or side effect 1: Minor performance impact due to disabling compression. Mitigation: Monitor connection times and revert the change if significant issues are observed.
• Roll back: Restore the original SSH configuration file from backup, then restart the SSH service.

8. References and Resources

• Vendor advisory or bulletin: OpenSSH Website
• NVD or CVE entry: No specific CVE is associated with this configuration issue.
• Product or platform documentation relevant to the fix: OpenSSH Configuration Manual