Google Paid $150K for THIS SSRF Bug – Learn How to Find It

Did you know Google once paid nearly $150k to a security researcher for discovering a single SSRF vulnerability? That vulnerability was Server-Side Request Forgery (SSRF), a potentially devastating flaw that can allow attackers to manipulate web servers. If you’re interested in ethical hacking, bug bounties, or simply understanding modern web security threats, understanding SSRF is crucial.

Let’s break down what SSRF is, why it’s dangerous, and how attackers find and exploit it.

When you visit a website, your browser sends a request (a client request) to a web server. We think of this server as just sending back files like HTML, images, and executing code to generate the page. However, modern web applications are often much more complex. A single web server frequently needs to communicate with other servers to function correctly. These communications from the web server to other servers are called server-side requests. They might happen over the internet to cloud services or within the same internal network between different machines.

Learn how to discover and exploit SSRF (Server-side Request Forgery) vulnerabilities.

Introducing the “Forgery”: Manipulating Server-Side Requests

Now, what happens if an attacker can influence where or how the web server makes these server-side requests? That’s the core of Server-Side Request Forgery.

Think of it like this: we previously learned about Cross-Site Request Forgery (CSRF), where an attacker tricks a user’s browser (the client) into making an unwanted request. In SSRF, the attacker tricks the web server itself into making an unintended request.

An attacker sends a carefully crafted, forged request to the vulnerable web server. This forged request manipulates the server, causing it to send a new request to a destination chosen by the attacker, not the one originally intended by the application developers.

Why is SSRF So Dangerous? The Potential Impact

The consequences of an SSRF vulnerability can range from minor information leaks to complete system compromise, depending on what the server can be forced to do:

1. Bypassing Firewalls and Access Controls: Often, internal servers (like the user database example) are protected and won’t accept connections from the public internet. However, they might trust requests coming from the main web server. An SSRF vulnerability can allow an attacker to proxy their requests through the trusted web server, potentially accessing sensitive internal data or administrative interfaces that should be off-limits.
2. Internal Network Scanning: Attackers can force the server to send requests to internal IP addresses (like localhost or other machines on the local network). By analyzing the responses or errors, they can map the internal network, discover running services, and identify open ports – reconnaissance for further attacks.
3. Interacting with Internal or External Services: The manipulated server could be forced to interact with databases, internal APIs, or even external third-party services as if it were the legitimate server. This could lead to data exfiltration or unauthorized actions.
4. Masking Attacks: If the vulnerable server is forced to attack another target, logs on the target machine will show the attack originating from the vulnerable server, potentially hiding the true attacker.
5. Remote Code Execution (RCE): In some severe cases, SSRF can be chained with other vulnerabilities or interact with insecure internal services, potentially leading to the attacker gaining full control over the server.

Spotting and Exploiting SSRF: A Practical Example

Imagine an online shop with a feature to “Check Stock” for a product in different store locations. An ethical hacker testing this feature might intercept the request sent when they click “Check Stock.”

They notice something interesting. The request includes a parameter, perhaps called stockApi, whose value looks like a URL-encoded web address (e.g., http%3A%2F%2Fstock.internal.api%2Fproduct%3Fid%3D123%26store%3Dparis). Decoding this reveals it’s a URL pointing to another server that handles stock information: http://stock.internal.api/product?id=123&store=paris.

This is a red flag! The client seems to be able to control the URL that the server will request.

To test for SSRF, the hacker tries changing this URL:

1. Targeting Localhost: They replace the original URL with http://localhost (or http://127.0.0.1), URL-encode it, and send the modified request. If the server responds by showing content from its own root web directory (perhaps the shop’s homepage embedded within the stock check result), it confirms the SSRF. The server was tricked into requesting a resource from itself.
2. Accessing Hidden Admin Panels: While probing with http://localhost, the hacker notices an /admin link appears in the response that isn’t normally visible. Trying to access http://yourshop.com/admin directly fails – perhaps it’s blocked unless requested from localhost or by a logged-in admin.
3. Exploiting via SSRF: The hacker uses the SSRF vulnerability again. They modify the stockApi parameter to the URL-encoded version of http://localhost/admin. This time, the server makes the request to itself for the admin page. Since the request originates from localhost, the access controls are bypassed, and the admin panel content is returned to the hacker within the stock check response!
4. Performing Admin Actions: Even with the admin panel visible, clicking links (like “Delete User”) might still fail if attempted directly. The hacker must continue using the SSRF. They copy the link for the action (e.g., /admin/delete?user=carlos), construct the full localhost URL (http://localhost/admin/delete?user=carlos), URL-encode it, and send it via the vulnerable stockApi parameter. The server executes the request on its own behalf, deleting the user.

Through this process, the hacker leveraged the SSRF to view restricted content and perform privileged actions, demonstrating a critical security flaw.

Conclusion

SSRF is a serious vulnerability arising from improper handling of user-supplied input that influences server-side requests. As shown, it can allow attackers to bypass security controls, access internal systems, and perform unauthorized actions by forcing a trusted server to act as their proxy. For developers, robust input validation and avoiding direct user control over request destinations are key defenses. For ethical hackers and bug bounty hunters, understanding how to spot and exploit SSRF remains a valuable skill, sometimes yielding significant rewards.

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⚠️ This video is made for educational purposes only, we only test devices and systems that we own or have permission to test, you should not test the security of devices that you do not own or do not have permission to test. ⚠️

If you’re interested in learning more about Ethical Hacking you should check out more related articles here: Hacking & Security Posts!