Server-side request forgery (SSRF) is a type of web application vulnerability that allows attackers to make requests to internal network resources that should not be accessible from the internet. This vulnerability can be exploited to gain access to sensitive data, execute arbitrary code, or launch attacks against other vulnerable systems on the network. In this blog post, we will look at how to test for and find SSRF vulnerabilities using Burp Suite.
What is Burp Suite?
Burp Suite is a web application security testing tool that is widely used by security professionals to identify and exploit vulnerabilities in web applications. It comes with a range of features that make it a valuable tool for security testing, including a proxy server, a web application scanner, and an intercepting proxy.
Setting up Burp Suite for SSRF testing
Before we dive into how to use Burp Suite for SSRF testing, it’s essential to set it up correctly. Here are the steps to follow:
Step 1: Install and launch Burp Suite
You can download Burp Suite from the official website and install it on your computer. Once installed, you can launch Burp Suite and start a new project.
Step 2: Configure your web browser to use Burp Suite as a proxy
Burp Suite acts as a proxy between your web browser and the target website. To use Burp Suite for SSRF testing, you need to configure your web browser to use Burp Suite as a proxy. You can do this by going to your web browser settings and setting the proxy address to localhost and the port number to the port number that Burp Suite is listening on.
Step 3: Configure Burp Suite to intercept requests
Burp Suite allows you to intercept and modify requests sent to the target website. To intercept requests, you need to configure Burp Suite to act as an intercepting proxy. You can do this by going to the “Proxy” tab and clicking on the “Intercept is On” button.
Step 4: Enable the “Match and Replace” rule
The “Match and Replace” rule is a feature in Burp Suite that allows you to modify requests on the fly. To enable this rule, go to the “Proxy” tab and click on the “Options” button. In the options menu, click on the “Match and Replace” tab and select the “Enable” checkbox.
Testing for SSRF vulnerabilities using Burp Suite
Once you have set up Burp Suite for SSRF testing, you can start testing for vulnerabilities. Here are the steps to follow:
Step 1: Identify input fields that can be used for SSRF attacks
The first step in testing for SSRF vulnerabilities is to identify input fields that can be used for SSRF attacks. These fields can include parameters in URLs, form fields, and cookies. Look for fields that allow you to enter a URL or IP address.
Step 2: Craft a payload for the input field
Once you have identified an input field that can be used for SSRF attacks, you need to craft a payload. The payload should contain a URL or IP address that points to an internal network resource. You can use the following payload to test for SSRF vulnerabilities:
This payload points to the local SSH service on the target machine.
Step 3: Send the payload using Burp Suite
To send the payload using Burp Suite, enter the payload in the input field and submit the form. Burp Suite will intercept the request, and you can modify the payload using the “Match and Replace” rule. You can modify the payload to point to other internal network resources to test for vulnerabilities.
Step 4: Analyze the response
After you have sent the payload, Burp Suite will intercept the response from the target website. Analyzing the response is an essential step in testing for SSRF vulnerabilities. Here are some things to look out for:
- Response status code: If the response status code is 200, it indicates that the request was successful, and the target website was able to access the internal network resource. This is a clear indication of an SSRF vulnerability.
- Error messages: Some web applications may display error messages that provide useful information about the internal network. Look out for error messages that contain IP addresses or hostnames.
- Response time: If the response time is significantly longer than other requests, it may indicate that the target website is making requests to internal network resources. This is a sign of an SSRF vulnerability.
- Response content: If the response contains information that is not available on the public-facing website, it may indicate that the target website is accessing internal network resources. Look out for content that contains sensitive information such as passwords, usernames, or other confidential data.
- Requests in Burp history: Another way to identify SSRF vulnerabilities is to look at the requests in Burp history. If there are requests to internal network resources that were not initiated by you, it indicates that the target website is making requests to internal network resources.
Mitigating SSRF vulnerabilities
If you find an SSRF vulnerability using Burp Suite, it’s essential to take action to mitigate the vulnerability. Here are some steps you can take:
- Restrict access to the internal network: Configure the firewall to restrict access to internal network resources. This will prevent attackers from accessing internal network resources even if they exploit an SSRF vulnerability.
- Validate user input: Validate all user input to ensure that it does not contain URLs or IP addresses that point to internal network resources.
- Use a whitelist approach: Whitelist the URLs and IP addresses that are allowed to access internal network resources. This approach ensures that only authorized users can access internal network resources.
- Monitor logs: Monitor logs for any suspicious activity that may indicate an SSRF attack.
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