Securing Self-Hosted Stirling-PDF: Integrating OAuth2 via Authelia
Learn how to implement robust self-hosted Stirling-PDF authentication by integrating Authelia as an OAuth2 identity provider via a reverse proxy.
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When you deploy powerful document processing tools like Stirling-PDF in your home lab or private infrastructure, the default configuration often lacks granular access control. While Stirling-PDF provides some built-in features for basic file management, relying on a dedicated identity provider (IdP) is the gold standard for managing access. By integrating Authelia, you can enforce OAuth2-based authentication, ensuring that your document processing pipeline remains private and protected from unauthorized access. This guide walks you through the architecture and implementation of securing your self-hosted Stirling-PDF instance.
Understanding the Authentication Architecture
To secure a containerized application, we use a reverse proxy as the gatekeeper. The workflow functions as follows: a user requests access to the Stirling-PDF URL; the reverse proxy intercepts the request and checks with Authelia if the user is authenticated. If they are not, Authelia redirects the user to its own login page. Once the user provides valid credentials—and potentially a second factor like TOTP—Authelia issues a session cookie. The reverse proxy then validates this session and forwards the request to the Stirling-PDF container.
This approach is superior to application-level authentication because it centralizes your security policy. If you decide to add more services to your infrastructure, you can reuse the same Authelia instance without modifying the underlying application code. If you are looking for a reliable platform to host your reverse proxy or other services, consider checking out Hostinger Web Hosting. Disclosure: this article contains an affiliate link. DotExe may earn a commission if you purchase through it, at no extra cost to you. Any discount is subject to Hostinger's current offers (20% OFF) and eligibility terms.
Prerequisites and Assumptions
Before starting, ensure you have a working Docker environment. This guide assumes you have:
- A running instance of Stirling-PDF reachable via a reverse proxy (Nginx Proxy Manager, Traefik, or Caddy).
- A functional Authelia instance configured with a user database (e.g., File or LDAP).
- A domain name pointing to your server.
- Basic familiarity with Docker Compose.
Configuring the Reverse Proxy for Authentication
We will use Nginx Proxy Manager (NPM) as our primary example, as it is the most common choice for self-hosted setups. However, the logic remains identical for Caddy or Traefik: you must configure the proxy to pass the Authorization header or use a forward-auth middleware.
Step 1: Define the Forward Auth Endpoint
In your reverse proxy configuration, you need to point the authentication request to your Authelia container. If you are using Nginx, you would typically define a location block that handles the /api/verify endpoint of Authelia. This endpoint tells the proxy whether the user is authorized to proceed. The proxy must be configured to pass the original request URL and headers to Authelia so that the IdP can determine if the user is authorized for that specific resource.
Step 2: Applying the Access List
Within your proxy host settings for Stirling-PDF, enable the "Access List" feature. Point this list to your Authelia instance. When a request hits the proxy, it will pause and query Authelia. If the user is unauthenticated, they are redirected to the Authelia login portal. After a successful login, Authelia sets a session cookie, and the proxy allows the request to reach the Stirling-PDF container. This flow ensures that even if Stirling-PDF itself has no authentication enabled, it is effectively shielded by the proxy layer.
Hardening the Stirling-PDF Container
Even with an authentication layer, you should minimize the attack surface of the container itself. Ensure that your Stirling-PDF instance is not exposed directly to the host network. Use a dedicated Docker network for your internal services to prevent cross-container communication unless explicitly permitted.
networks:
proxy_net:
external: true
services:
stirling-pdf:
image: stirlingpdf/stirling-pdf:latest
networks:
- proxy_net
environment:
- DOCKER_ENABLE_SECURITY=false
# ... other configurations
Always pin your image versions to a specific tag rather than using latest to ensure consistent behavior across deployments. By using specific versions, you avoid unexpected breaking changes in the application's internal security handling. Additionally, ensure the container runs as a non-root user where possible, and mount only the necessary volumes to limit file system access.
Troubleshooting Authentication Failures
If you find yourself stuck in a redirect loop, the most common culprit is a mismatch between the domain names and the cookie domain settings in Authelia. Ensure that the session configuration in your configuration.yml for Authelia matches the domain you are using for Stirling-PDF. If your Stirling-PDF is at pdf.example.com, your cookie domain should be example.com or pdf.example.com.
Checking Logs
Check the logs of both your reverse proxy and Authelia:
docker logs -f authelia
docker logs -f nginx-proxy-manager
Look for 401 Unauthorized errors in the proxy logs. If you see these, the proxy is correctly rejecting the request, but Authelia is not successfully redirecting the user or the user is failing to authenticate. Verify that your access_control rules in Authelia include the domain for Stirling-PDF. Also, ensure the time on your server is synchronized via NTP, as session tokens and TOTP codes are highly sensitive to time drift.
Operational Considerations
When you secure your tools with OAuth2, you introduce a single point of failure: the identity provider. If Authelia goes down, you lose access to all protected services. To mitigate this, ensure your Authelia instance is backed up and, if possible, run it in a high-availability configuration. For smaller home labs, simply ensuring the container restarts automatically on failure is often sufficient. Use restart: unless-stopped in your Docker Compose files to manage lifecycle events.
Furthermore, consider the impact on API usage. If you use external scripts to interact with the Stirling-PDF API, they will now fail because they lack the session cookie. You will need to configure bypass rules in Authelia for specific API endpoints or provide a way for your scripts to authenticate via an API key or a service account token that the proxy can validate. If you are building automated pipelines, consider using a dedicated service account for your scripts rather than a personal user account.
Advanced Security Patterns
Beyond basic authentication, consider implementing rate limiting at the reverse proxy level. Stirling-PDF performs resource-intensive tasks like OCR and PDF conversion, which can be exploited for Denial of Service (DoS) attacks. By limiting the number of requests per IP address, you protect your server's CPU and memory resources from being exhausted by malicious or accidental heavy usage.
Additionally, examine your internal network traffic. If you use a tool like Nginx Proxy Manager, you can restrict access to the Stirling-PDF container so that it only accepts connections from the reverse proxy's IP range. This prevents any bypass attempts that might occur if someone tries to connect directly to the container's exposed port on the host machine. Always ensure that the host's firewall, such as ufw or iptables, is configured to block direct external access to the container's port, allowing only the proxy to communicate with it.
The Role of Persistent Storage
When managing Stirling-PDF in a secure environment, ensure your persistent storage is encrypted at the filesystem level. Since Stirling-PDF processes sensitive documents, the temporary files and uploaded PDFs stored on disk could contain private information. Using encrypted volumes, such as LUKS on Linux, provides an extra layer of protection if the physical storage media or the server's disk image is compromised. Regularly audit your storage directories to ensure that old, processed files are being cleaned up, either by the application's built-in cleanup tasks or by a custom cron job that removes files older than a specific retention period.
Monitoring and Auditing
Security is not a one-time setup; it requires ongoing monitoring. Use tools to log access attempts to your Stirling-PDF instance. If you notice unusual patterns, such as a high volume of failed login attempts, investigate your Authelia logs immediately. Integrating these logs into a centralized system like the ELK stack or Grafana Loki can help you identify threats in real-time. For smaller setups, even simple log rotation and periodic manual review of access logs can provide significant insight into how your services are being used and whether unauthorized actors are attempting to gain access.
Conclusion
Integrating Authelia with Stirling-PDF provides a professional-grade security layer for your document processing needs. By offloading authentication to a dedicated provider, you ensure that your tools remain accessible only to authorized users while maintaining a clean and manageable configuration. Remember to test your authentication flow thoroughly after any changes to your proxy settings to avoid being locked out of your own services. A well-secured instance allows you to leverage the full power of Stirling-PDF without worrying about unauthorized access or data exposure.
Official references
Frequently Asked Questions
Why use Authelia instead of built-in Stirling-PDF authentication?
Using Authelia allows you to centralize your identity management, meaning you can use the same credentials and multi-factor authentication across all your self-hosted services. It provides a more robust and unified security policy compared to managing individual user databases for every single application.
Does this setup work for API requests to Stirling-PDF?
Standard OAuth2 flows via a reverse proxy often break programmatic API access because scripts cannot easily handle the browser-based login redirect. You will need to configure specific bypass rules in your Authelia configuration for the API paths or use an alternative method like header-based authentication for your scripts.
What happens if the Authelia container fails?
If the Authelia container goes down, your reverse proxy will likely be unable to verify session cookies, resulting in a 503 or 401 error for all protected services. It is critical to monitor your identity provider's health and ensure it is configured to restart automatically if the process crashes.
Is this setup secure enough for public exposure?
While this adds a strong layer of authentication, exposing any service to the internet carries inherent risks. Always ensure your Authelia instance is updated, use strong passwords, and enforce multi-factor authentication to protect against credential stuffing or brute-force attacks.