How to remediate – Unprotected memcached

1. Introduction

Unprotected memcached refers to instances of the Memcached object store running without authentication, accessible over the public internet. This means anyone can query and potentially manipulate data stored within it. Businesses face risks including data breaches, denial-of-service attacks, and malicious code execution. Systems commonly affected are web servers using Memcached for caching session data or other frequently accessed information. A successful exploit could lead to loss of confidential data, disruption of service availability, and damage to system integrity.

2. Technical Explanation

The root cause is the lack of built-in security features in Memcached itself; it was designed for performance within a trusted network environment. Attackers can exploit this by directly connecting to the server on port 11211 (default) and sending commands to retrieve or modify data. No authentication is required, making it easy to access.

• Root cause: missing authentication mechanisms in Memcached’s design.
• Exploit mechanism: An attacker connects directly to the memcached server using telnet or netcat and issues commands like ‘stats items’ to enumerate stored data, or ‘set key value’ to inject malicious content. For example: telnet 11211 followed by entering ‘stats items’.
• Scope: All platforms running Memcached versions prior to those with implemented security features are affected. This includes Linux, Windows and macOS systems.

3. Detection and Assessment

Confirming vulnerability involves checking for open port 11211 on public IPs and verifying access without authentication. A quick check is a simple network scan; thorough assessment requires attempting to query the server directly.

• Quick checks: Use `netstat -tulnp | grep memcached` to see if Memcached is listening on port 11211.
• Scanning: Nessus plugin ID 36854 and OpenVAS scanner family “Memcached Information Disclosure” can identify exposed instances. These are examples only, results may vary.
• Logs and evidence: Check firewall logs for connections to port 11211 from external sources. Memcached itself typically does not log connection attempts.

telnet  11211

4. Solution / Remediation Steps

Fixing this issue requires protecting the server with a firewall and restricting access to authorized hosts only. Follow these steps carefully to avoid service disruption.

4.1 Preparation

• Change window: A short maintenance window may be needed for testing and verification. Approval from the system owner is recommended.

4.2 Implementation

1. Step 1: Configure the firewall (e.g., `iptables`, `firewalld`, Windows Firewall) to block incoming connections on port 11211 from all sources except trusted hosts.
2. Step 2: Add rules allowing access only from specific IP addresses or networks that require Memcached access.
3. Step 3: Restart the firewall service to apply changes.

4.3 Config or Code Example

Before

# iptables example - no rules for memcached
iptables -L INPUT

After

# iptables example - allow access from 192.168.1.0/24 only
iptables -A INPUT -p tcp --dport 11211 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 11211 -j DROP

4.4 Security Practices Relevant to This Vulnerability

Practices that directly address this vulnerability include least privilege and network segmentation. Least privilege limits the impact if Memcached is compromised, while network segmentation restricts access from untrusted networks.

• Practice 1: Implement least privilege by only allowing necessary services to communicate with Memcached.
• Practice 2: Network segmentation isolates Memcached within a trusted network zone, reducing exposure to external threats.

4.5 Automation (Optional)

# Example Ansible playbook snippet
- name: Block Memcached port except for trusted networks
  firewalld:
    port: 11211/tcp
    permanent: true
    state: disabled
    immediate: yes
- name: Allow access from trusted network
  firewalld:
    port: 11211/tcp
    permanent: true
    source: 192.168.1.0/24
    state: enabled
    immediate: yes

5. Verification / Validation

Confirm the fix by checking firewall rules and attempting to connect from an untrusted host. A service smoke test ensures legitimate access is still possible.

• Post-fix check: Use `iptables -L INPUT` (or equivalent for your firewall) to verify that only authorized hosts are allowed access on port 11211.
• Re-test: Attempt a telnet connection from an untrusted host; the connection should be refused or timed out.
• Smoke test: Verify that applications relying on Memcached can still connect and cache data successfully.
• Monitoring: Monitor firewall logs for any unexpected attempts to connect to port 11211. Example query: search for blocked connections on port 11211 from unknown IP addresses.

telnet  11211 # Connection should be refused

6. Preventive Measures and Monitoring

Update security baselines to include firewall rules for Memcached, and add checks in deployment pipelines to ensure the port is not exposed. A regular patch or configuration review cycle helps identify new vulnerabilities.

• Baselines: Update your security baseline (e.g., CIS benchmark) to require blocking external access to Memcached by default.
• Pipelines: Add a check in your CI/CD pipeline to scan for open ports and flag any instances of Memcached listening on public interfaces.
• Asset and patch process: Implement a monthly review of server configurations to identify potentially exposed services like Memcached.

7. Risks, Side Effects, and Roll Back

• Risk or side effect 2: Temporary service disruption during rule changes. Mitigation: Implement changes during a maintenance window with a clear roll back plan.
• Roll back: Step 1: Remove the new firewall rules. Step 2: Restart the firewall service to apply the previous configuration.

8. References and Resources

• Vendor advisory or bulletin: http://memcached.org/