Technical Analysis of "Digging for SSRF in Next.js Apps"

Overview:
AssetNote's detailed blog post uncovers techniques for finding Server-Side Request Forgery (SSRF) vulnerabilities in applications built with the Next.js framework. The write-up lays out instances where SSRF can occur due to the inherent behavior of Next.js features like API routes and image optimization.

Key Technical Details:

1. Introduction to SSRF:
   SSRF vulnerabilities occur when an application can be tricked into making HTTP requests to arbitrary domains, which attackers can control. This can lead to accessing internal systems, cloud metadata services, or other restricted resources.

2. Next.js Overview:
   Next.js is a popular React framework that enables server-side rendering (SSR) and static site generation (SSG). It also includes API routes, which facilitate server-side functionalities within a Next.js application.

3. Identifying SSRF in API Routes:

4. API Routes: Next.js allows for the creation of custom API endpoints within the application using the /pages/api directory. These routes can introduce SSRF vulnerabilities if they handle user inputs to make backend HTTP requests.
5. Example API Route: The write-up provides an example where an API route fetches data from another site based on a URL parameter provided by the user. If the URL is not properly validated, it can be manipulated to fetch data from internal network addresses. js // pages/api/get-data.js export default async (req, res) => { const { url } = req.query; const response = await fetch(url); const data = await response.json(); res.status(200).json(data); }

6. Mitigation Strategies: Always validate and sanitize user inputs to ensure that only trusted or whitelisted domains are allowed. Regex validation or a predefined list of acceptable URLs can mitigate the risk.

7. The Role of Image Optimization:

8. Next.js Image Component: The Next.js <Image> component optimizes images on the fly and supports remote images. This feature can be abused to perform SSRF if the URL of the image is user-controlled and not sufficiently validated.
9. Image URL Handling: The URL provided to the src attribute can fetch images from external servers, leading to potential SSRF if the URL parameter is user-supplied. ```js import Image from 'next/image';

export default function MyImage({ src }) { return <Image src={src} width={500} height={500} /> } ``` - Protection: Implement validation to ensure that the image URLs are trusted and follow proper validation patterns.

1. Practical Exploitation:
2. Lab Environment: The authors describe setting up a lab environment with a Next.js application and intentionally weak URL validation to demonstrate SSRF exploitation.
3. Payload Construction: Example payloads are shown to manipulate image and API route URLs to perform internal scans or reach out to metadata services (e.g., on cloud providers).

4. Real-World Scenario: The write-up highlights how poorly validated URLs can lead to significant security risks on real-world applications, including potential data breaches or infrastructure exposure.

5. Preventing SSRF in Next.js:

6. Input Validation: Always sanitize and validate user inputs, especially those that will be used to form backend requests.
7. URL Whitelisting: Implement strict whitelisting of domains or URL patterns to constrain where the application can fetch resources.
8. Use Built-in Security Features: Leverage security features provided by Next.js and other libraries to enforce security, such as CSP (Content Security Policy).
9. Security Audits and Code Reviews: Regularly conduct security audits and code reviews focused on input handling and backend request logic.

Key Takeaways:

• API Routes are a Major SSRF Risk: Custom API routes in Next.js can easily become SSRF entry points if they make network requests based on user inputs.
• Image Optimization Requires Scrutiny: Features like Next.js's image optimization can inadvertently open doors to SSRF when handling remote images from user-controlled sources.
• Holistic Input Validation: Implement thorough validation and sanitization for all inputs used in backend network requests.
• Security Best Practices: Follow security best practices, including domain whitelisting, regular code reviews, and leveraging existing security mechanisms.

Conclusion:

The write-up by AssetNote serves as a critical reminder of the potential security pitfalls when using frameworks like Next.js. By understanding and mitigating SSRF risks, developers can enhance the security posture of their web applications. For developers and security professionals, remaining vigilant about the ways user-supplied data is handled—especially in network requests—is paramount.

For the full write-up and further technical details, visit the original blog post here.