Understanding Server Side Request Forgery (SSRF): Owasp API6 | 2023

Exploring the Working Principle, Exploitation, and Prevention Techniques of SSRF Vulnerabilities | Karthikeyan Nagaraj

3 min readMay 2, 2023

Server-side request forgery (SSRF) is a common web application vulnerability that allows attackers to send unauthorized requests to internal or external systems.
This vulnerability can be exploited to perform various malicious activities, such as accessing sensitive data, executing arbitrary code, and bypassing security controls.
This article aims to provide a comprehensive guide to SSRF vulnerabilities, including their working principle, exploitation techniques, and prevention strategies.

SSRF is a vulnerability that occurs when a web application allows attackers to specify the URL of a remote resource that the application will access.
The attacker can manipulate the URL to send a request to an internal system, such as a backend server, that is not intended to be accessed by external users.
The attacker can also send a request to an external system, such as a third-party service, using the victim’s credentials or IP address.

To exploit an SSRF vulnerability, an attacker typically needs to find a vulnerable web application that allows them to specify a URL parameter.
The attacker can then manipulate the URL parameter to send a request to an internal or external system.
The attacker can use various techniques, such as IP address manipulation, protocol injection, and URL redirection, to bypass security controls and access sensitive data or execute arbitrary code.
For example, an attacker can send a request to an internal server that is not intended to be accessible from the internet, such as a database server or a file server, to steal sensitive information or execute commands on the server.

To prevent SSRF vulnerabilities, web application developers can implement various security measures, such as input validation, URL whitelisting, and network segmentation.
Input validation involves validating user input to ensure that it meets expected criteria and does not contain malicious code or URLs.
URL whitelisting involves maintaining a list of allowed URLs that the application can access and rejecting requests to unauthorized URLs.
Network segmentation involves separating the internal and external networks and restricting access to internal systems from external sources.
Web application developers can also implement secure coding practices, such as using secure coding libraries and frameworks, and performing regular security assessments and penetration testing to identify and mitigate vulnerabilities.

Exploiting an SSRF vulnerability in a web application can have severe consequences, such as data theft, system compromise, and unauthorized access.
For example, in 2017, a vulnerability in the popular cloud computing platform, Amazon Web Services (AWS), allowed attackers to execute arbitrary code on EC2 instances by sending a request to the AWS metadata endpoint.
In 2019, a vulnerability in the online payment system, PayPal, allowed attackers to access sensitive user data, such as email addresses and home addresses, by sending a request to an internal server.

SSRF vulnerabilities are a significant threat to web application security and can be exploited to perform various malicious activities.
Web application developers should implement appropriate security measures, such as input validation, URL whitelisting, and network segmentation, to prevent SSRF vulnerabilities.
It is also essential to perform regular security assessments and penetration testing to identify and mitigate vulnerabilities before they can be exploited by attackers.