How to remediate – ActivityPub Protocol Detected

1. Introduction

ActivityPub Protocol Detected is a communication protocol that allows decentralised social networks to interoperate. It’s commonly found in platforms like Mastodon and Diaspora, enabling users on different servers to follow each other and share content. A server exposing an ActivityPub endpoint could allow unauthorised access or manipulation of data if not properly secured. This poses a moderate risk to the confidentiality, integrity, and availability of user data and services.

2. Technical Explanation

The vulnerability lies in the presence of an ActivityPub json endpoint which allows for client-to-server communication via REST APIs. Attackers can exploit this by sending malicious requests to create, update or delete content, potentially leading to data breaches or service disruption. The main precondition is network access to the exposed endpoint.

• Root cause: Exposure of an ActivityPub endpoint without appropriate authentication and authorisation controls.
• Exploit mechanism: An attacker could send a crafted POST request to create malicious content, potentially including cross-site scripting (XSS) payloads or links to phishing sites.
• Scope: Systems running software that implements the ActivityPub protocol, such as Mastodon, Diaspora, and other federated social networking platforms.

3. Detection and Assessment

To confirm vulnerability, check for the presence of an ActivityPub endpoint. A thorough method involves examining network traffic or server configurations for exposed REST API endpoints.

• Quick checks: Use a web browser to access URLs commonly associated with ActivityPub endpoints (e.g., /.well-known/webfinger).
• Scanning: Nessus plugin 16807 can detect ActivityPub instances. This is an example only, and may require updates for accuracy.
• Logs and evidence: Check web server logs for requests to ActivityPub related paths like ‘/api/v1/’ or ‘/inbox’.

curl -I http://your-server/.well-known/webfinger 

4. Solution / Remediation Steps

Implement precise steps to secure the ActivityPub endpoint. Ensure all steps are testable and safe to roll back.

4.1 Preparation

• Ensure you have access to revert configuration changes if needed. A rollback plan should include restoring from the snapshot.
• A change window may be required depending on service impact. Approval from relevant stakeholders might be needed.

4.2 Implementation

1. Step 1: Implement authentication and authorisation for all ActivityPub endpoints, requiring valid credentials for access.
2. Step 3: Review server configurations to ensure no unnecessary ports or services are exposed.

4.3 Config or Code Example

Before

# Insecure configuration - no authentication
location /api/v1 {
    proxy_pass http://backend;
}

After

# Secure configuration - requires authentication
location /api/v1 {
    auth_basic "Restricted Area";
    auth_basic_user_file /etc/nginx/.htpasswd;
    proxy_pass http://backend;
}

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: Least privilege to limit the impact if an endpoint is compromised.
• Practice 2: Input validation to prevent malicious data from being processed.

4.5 Automation (Optional)

# Example Ansible task to configure Nginx authentication
- name: Configure ActivityPub authentication in Nginx
  copy:
    src: nginx_activitypub.conf
    dest: /etc/nginx/sites-available/default
  notify: Reload Nginx

5. Verification / Validation

Confirm the fix by verifying that access to ActivityPub endpoints requires authentication and authorisation. Provide commands, expected outputs, and a short negative test if possible. Include a simple service smoke test.

• Post-fix check: Attempt to access an ActivityPub endpoint without credentials; expect a 401 Unauthorized error.
• Re-test: Re-run the curl command from step 3, verifying that authentication is now required.
• Monitoring: Monitor web server logs for failed authentication attempts to ActivityPub endpoints.

curl -I http://your-server/.well-known/webfinger 

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 requirements for authentication and authorisation on all exposed APIs.
• Pipelines: Add static code analysis (SAST) tools to CI/CD pipelines to identify potential vulnerabilities in API implementations.
• Asset and patch process: Implement a regular review cycle for server configurations to ensure they remain secure.

7. Risks, Side Effects, and Roll Back

• Risk or side effect 2: Performance impact due to increased processing overhead from input validation; monitor server resources.

8. References and Resources

• Vendor advisory or bulletin: http://nodeinfo.diaspora.software/protocol.html
• NVD or CVE entry: Not applicable at this time.
• Product or platform documentation relevant to the fix: https://activitypub.rocks/, https://www.w3.org/TR/activitypub/