What Is Server-Side Request Forgery?

To understand server-side request forgery, let’s consider a scenario where a corporate network is protected by a firewall. Within this network, there may be various devices, including computers, personal devices, and servers. However, a web server is connected to both the corporate network and the internet, acting as a bridge between the two.

An attacker can exploit SSRF by leveraging the web server or a cloud server as a pivot point to bypass the firewall and gain access to internal corporate network resources. This unauthorized access can involve activities like file reads, port scanning, or even remote code execution.

For instance, an API request designed to retrieve avatars from a file server could be manipulated by an attacker to access sensitive business or financial documents stored in a different directory. Similarly, in a cloud environment, an attacker could potentially access cloud resources, such as EC2 credentials, by exploiting server-side request forgery.

User input and parameters play a critical role in enabling SSRF attacks. Often, SSRF vulnerabilities arise when an API accepts unvalidated user input or improperly handles parameters in HTTP requests. Attackers can manipulate these parameters to send requests to unintended destinations, such as internal resources or other sensitive parts of the corporate network. Properly validating user input and sanitizing parameters can significantly reduce the risk of SSRF, preventing attackers from exploiting these weaknesses to access internal resources.

While APIs are often considered safe by default, it is crucial to remember that they are essentially web applications. Therefore, any vulnerability that affects a web application can also impact an API. The act of fetching external resources through APIs introduces a significant risk, making SSRF a critical concern for API security professionals.

One of the most effective ways to mitigate SSRF is by securing the communication between the API and back-end services. This includes implementing strict access control measures, ensuring that internal resources are not exposed to unauthorized requests, and carefully configuring DNS settings to prevent attackers from resolving internal IP addresses. Additionally, the use of encrypted channels and ensuring that API endpoints only communicate with trusted back-end services can greatly reduce the risk of SSRF attacks. Regularly updating security protocols and ensuring proper API design aligned with the OWASP API Security Top 10 best practices are also essential.

To mitigate the risk of SSRF, several measures can be implemented:

• Allowlists: Implement allowlists to restrict the resources that can be fetched through APIs. By specifying certain resources, organizations can make sure that only authorized and safe resources are accessed.
• Isolate web servers: Avoid placing web servers directly on the internal corporate network. Instead, consider deploying them in a separate network segment or on the cloud to minimize the potential for SSRF attacks.
• Evaluate resource necessity: Assess whether fetching external resources is truly necessary for the API’s functionality. Minimizing the reliance on external resources can reduce the attack surface and mitigate the risk of server-side request forgery.
• Firewall protection: Always use a firewall on the corporate network to provide an additional layer of defense against unauthorized access and potential SSRF attacks.
• Follow API design recommendations: Stay updated with API design recommendations that provide best practices for developing APIs that are resilient against SSRF attacks.