Server-Side Request Forgery (SSRF): A Dangerous Web Vulnerability You Should Know About

Server-side request forgery (SSRF) is a critical web application vulnerability that enables attackers to exploit an application’s functionality to gain unauthorised access to internal systems. This vulnerability can result in sensitive data exposure, unauthorised system access, and, in severe cases, remote code execution.

How Does a Server-Side Request Forgery Attack Work?

When developing networked applications, it is common to send requests to other servers for external resources, retrieve metadata, or interact with APIs. While these interactions are usually safe, improper user input handling when constructing requests can introduce SSRF vulnerabilities.

SSRF occurs when an application accepts user-controlled input to build a request without proper validation. Attackers exploit this by modifying parameters to force the application’s server to send requests to unintended destinations, including:

• Internal Services – Accessing cloud metadata APIs, database endpoints, or internal admin panels.
• Arbitrary External Servers – Exfiltrating data to attacker-controlled endpoints.
• Self-Targeting Requests – Exploiting recursive calls for denial-of-service (DoS) attacks.

SSRF vulnerabilities can affect any networked software, regardless of the programming language or platform. While web applications are the most common targets, SSRF can also affect server software and other networked systems.

Types of SSRF Attacks

SSRF is a broad category of vulnerabilities that can manifest in different ways depending on how an application processes user-controlled requests. Each type of SSRF presents distinct risks and requires tailored mitigation strategies.

Basic SSRF

This is the most straightforward form of SSRF, where an attacker directly manipulates a URL to access internal resources or external services. For example, attackers may modify requests to access sensitive internal server data or interact with unauthorised external endpoints.

Blind SSRF

Unlike basic SSRF, blind SSRF does not return a direct response to the attacker. Instead, attackers infer the success of their exploitation through side effects, such as delays in the application’s response, error messages, or logs generated on the target system. This makes detection more challenging.

SSRF with Authentication Bypass

Some internal resources require authentication to prevent unauthorised access. However, an attacker may exploit SSRF to bypass authentication mechanisms, gaining unauthorised access to sensitive data, administrative functionalities, or restricted APIs.

SSRF to Local File Inclusion

This attack involves manipulating SSRF to access local files on the server’s file system. By crafting specific requests, an attacker could retrieve configuration files, application source code, or sensitive credentials, potentially leading to further system compromise.

SSRF to Port Scanning

Attackers can use SSRF to scan internal network services by sending requests to different ports. This technique allows them to map internal infrastructure, identify open services, and discover exploitable vulnerabilities within the network.

Tips to Prevent SSRF Attacks

Preventing SSRF attacks requires a combination of industry best practices and security configurations. Here are the best practices to mitigate SSRF risks:

Input Validation

Proper validation and sanitisation of user-supplied input are critical in preventing SSRF attacks. Implement a safe list of trusted domains and IP addresses to restrict external requests. Additionally, parse and normalise URLs to eliminate ambiguous representations and sanitise input to remove or escape potentially harmful characters. While prohibiting known malicious sources may offer some protection, it remains the more robust approach due to the evolving nature of threats.

Network Segmentation

Internal systems and services should be isolated from the public internet whenever possible. By restricting access to internal resources, the attack surface is minimised, reducing the likelihood of unauthorised access even if an SSRF vulnerability is exploited.

Principle of Least Privilege

Adhering to the principle of least privilege ensures that applications operate with only the minimum permissions necessary. This restricts unauthorised access to internal services and limits the potential impact of an SSRF attack.

Web Application Firewalls

Deploying a Web Application Firewall (WAF) provides an additional layer of defence by monitoring and filtering incoming traffic. WAFs can detect and block suspicious patterns in requests, including those indicative of SSRF attempts.

Restrict Outbound Requests

Configure firewall rules, access control lists, or cloud security policies to limit the destinations that the application can access. This ensures that only trusted external services can be reached, minimising the risk of unauthorised internal access.

Regular Security Testing

Conducting regular security assessments, including penetration testing and code reviews, is essential for identifying and addressing SSRF vulnerabilities before they can be exploited. Security testing should be an ongoing practice to adapt to emerging threats and vulnerabilities.

Conclusion

Server-side request forgery is a serious security vulnerability that can be exploited to access internal systems, sensitive data, and cloud-based infrastructure. A proactive and multi-layered security strategy is essential to mitigate these risks effectively. As cyber threats continue to evolve, regular security assessments, continuous monitoring, and adherence to best practices are crucial in preventing SSRF vulnerabilities. A well-secured infrastructure not only protects sensitive data but also strengthens overall system resilience against emerging attack vectors.

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