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Server Side Request Forgery (SSRF)

Updated: Jan 12, 2023

Server-Side Request Forgery (SSRF) is a type of attack that allows an attacker to force a server to make requests to arbitrary destinations on the web. By exploiting this vulnerability, an attacker can gain access to sensitive information, such as internal network resources, or even execute malicious code on the vulnerable server.


What is SSRF?


Server-Side Request Forgery (SSRF) is a type of attack that allows an attacker to force a server to make requests to arbitrary destinations on the web. This can be done by manipulating the target server's request parameters and using them to craft malicious requests.


The attacker can use SSRF to gain access to sensitive information, such as internal network resources, or even execute malicious code on the vulnerable server.


What are the Types of SSRF?


There are several different types of SSRF attacks, each with their own unique characteristics.


1. Open Redirect: This type of attack involves manipulating the target server's request parameters to redirect the user to a malicious website.


2. File Disclosure: This type of attack involves manipulating the target server's request parameters to gain access to sensitive files on the server.


3. Code Execution: This type of attack involves manipulating the target server's request parameters to execute malicious code on the vulnerable server.


4. Internal Port Scanning: This type of attack involves manipulating the target server's request parameters to scan for open ports on the internal network.


What are the Common Causes of SSRF?


The most common cause of SSRF is an application that does not properly validate user input. If an application does not properly validate user input, it can be vulnerable to SSRF attacks.


In addition, applications that use external services can be vulnerable to SSRF attacks. For example, if an application uses an external API to retrieve data, it could be vulnerable to SSRF if the API does not properly validate the user input.


Finally, applications that use third-party libraries can also be vulnerable to SSRF attacks. If a third-party library does not properly validate user input, it can be vulnerable to SSRF attacks.


How to Exploit SSRF?


In order to exploit an SSRF vulnerability, an attacker must first identify a vulnerable application. Once the vulnerable application is identified, the attacker must craft a malicious request that will force the application to make requests to arbitrary destinations.


For example, let's say an attacker wants to exploit an SSRF vulnerability to enumerate a host behind a load balancer. The attacker can craft an HTTP POST request with a SSRF payload that will force the application to make a request to the host behind the load balancer. The request might look something like this:



POST /vulnerable_endpoint HTTP/1.1
Host: vulnerable_host
Content-Type: application/x-www-form-urlencoded

ssrf_payload=http://host_behind_load_balancer


In this example, the attacker is forcing the application to make a request to the host behind the load balancer. The application will then return the response from the host, which can be used to enumerate the host.


What are the Mitigations for SSRF?


The best way to mitigate SSRF vulnerabilities is to properly validate user input. This can be done by ensuring that all user input is sanitized and validated before being used by the application.


In addition, applications should use secure protocols when making requests to external services. This will help to prevent attackers from manipulating the request parameters and using them to craft malicious requests.


Finally, applications should use secure libraries when making requests to external services. This will help to ensure that the libraries are not vulnerable to SSRF attacks.


Conclusion


Server-Side Request Forgery (SSRF) is a type of attack that allows an attacker to force a server to make requests to arbitrary destinations on the web. By exploiting this vulnerability, an attacker can gain access to sensitive information, such as internal network resources, or even execute malicious code on the vulnerable server.


To mitigate SSRF vulnerabilities, it is important to properly validate user input, use secure protocols when making requests to external services, and use secure libraries when making requests to external services. By following these best practices, organizations can help to protect their applications from SSRF attacks.



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