How to remediate – RTMP Server Detection

1. Introduction

RTMP Server Detection identifies a Flash media server listening on a remote host. This indicates an older service is running which may be exposed to the internet, creating a potential attack surface. Systems commonly affected are web servers and streaming platforms that previously used Adobe Flash technology. A successful exploit could allow attackers to gain control of the server or intercept sensitive data. Confidentiality, integrity, and availability may all be impacted.

2. Technical Explanation

The service supports Real Time Messaging Protocol (RTMP), a protocol designed for streaming audio, video, and objects using a binary connection with Flash Player. The vulnerability arises from the continued presence of this server listening on a network port. Attackers can attempt to exploit known vulnerabilities in the RTMP service itself or use it as an entry point into the system.

• Root cause: The service is running and accessible, despite being associated with outdated technology.
• Exploit mechanism: An attacker could connect to the RTMP server and attempt to exploit known vulnerabilities in Flash Player or the server software itself. For example, they might send a crafted packet designed to trigger a buffer overflow.
• Scope: Affected platforms are those running an RTMP server service, typically Linux-based web servers or streaming media infrastructure. Specific versions depend on the installed server software.

3. Detection and Assessment

Confirming whether a system is vulnerable involves checking for the presence of the listening RTMP service. A quick check can identify if the port is open, while more thorough methods involve identifying the specific server software running.

• Quick checks: Use netstat -tulnp or ss -tulnp to list listening ports and associated processes. Look for a process bound to port 1935 (the default RTMP port).
• Scanning: Nessus plugin ID 10428 can detect RTMP servers. OpenVAS also has relevant checks, but results may vary.
• Logs and evidence: Check system logs for messages related to the RTMP server software. Event IDs will depend on the specific server implementation.

netstat -tulnp | grep 1935

4. Solution / Remediation Steps

Fixing this issue involves limiting incoming traffic to the port or removing the RTMP service entirely. These steps should be performed carefully to avoid disrupting legitimate services.

4.1 Preparation

• Dependencies: Identify any applications relying on the RTMP server. A roll back plan involves restoring the snapshot or reinstalling the service.
• Change window: This change may require a maintenance window, depending on service dependencies and impact. Approval from the application owner is recommended.

4.2 Implementation

1. Step 1: Block incoming traffic to port 1935 using a firewall rule. For example, with iptables: iptables -A INPUT -p tcp --dport 1935 -j DROP.
2. Step 2: If the service is not required, uninstall it completely. The command will vary depending on the Linux distribution (e.g., apt remove rtmp-server or yum remove rtmp-server).

4.3 Config or Code Example

Before

# iptables -L INPUT (showing port 1935 open)
Chain INPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     tcp  --  anywhere             anywhere            state RELATED,ESTABLISHED
...
ACCEPT     tcp  --  anywhere             anywhere            dport:1935

After

# iptables -L INPUT (showing port 1935 blocked)
Chain INPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     tcp  --  anywhere             anywhere            state RELATED,ESTABLISHED
...
DROP       tcp  --  anywhere             anywhere            dport:1935

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue and reduce the overall attack surface. These include least privilege, regular patching, and secure network configurations.

• Practice 1: Least privilege – only run services that are absolutely necessary.
• Practice 2: Patch cadence – regularly update all software to address known vulnerabilities.

4.5 Automation (Optional)

#!/bin/bash
# Script to block port 1935 using iptables on multiple servers
for server in server1 server2 server3; do
  ssh $server "sudo iptables -A INPUT -p tcp --dport 1935 -j DROP"
  echo "Blocked port 1935 on $server"
done

5. Verification / Validation

Confirming the fix involves verifying that the RTMP service is no longer accessible and that legitimate services are still functioning correctly.

• Post-fix check: Run netstat -tulnp | grep 1935. The output should be empty, indicating the port is no longer listening.
• Re-test: Re-run the initial scan (Nessus or OpenVAS) to confirm that the vulnerability is no longer detected.
• Smoke test: Verify any dependent applications still function as expected. For example, if a streaming service relies on RTMP, ensure it can still play content.
• Monitoring: Monitor system logs for unexpected errors related to port 1935 or the removed service. Example query: grep “rtmp” /var/log/syslog.

netstat -tulnp | grep 1935

6. Preventive Measures and Monitoring

Preventing similar issues involves updating security baselines, incorporating checks into CI/CD pipelines, and establishing a robust asset and patch management process. For example, ensure that CIS benchmarks include rules to disable unnecessary services.

• Baselines: Update your security baseline or policy to explicitly prohibit running outdated technologies like RTMP servers unless absolutely necessary.
• Pipelines: Add checks in your CI/CD pipeline to scan for open ports and vulnerable software during deployment.
• Asset and patch process: Implement a regular patch review cycle to ensure all systems are updated with the latest security fixes.

7. Risks, Side Effects, and Roll Back

• Roll back: If blocking port 1935 causes issues, remove the iptables rule using iptables -D INPUT -p tcp --dport 1935 -j DROP. If the service was uninstalled, reinstall it using your package manager (e.g., apt install rtmp-server or yum install rtmp-server).

8. References and Resources