What is Server-Side Request Forgery?

Quick Summary: SSRF can target your crucial server resources and risk confidentiality. It is among the most critical web app vulnerabilities as per OWASP Top Ten list. Get a complete understanding of this security loophole along with ways to prevent it in this article. Keep reading for complete understanding and make prudent decisions.

Web applications are the most favorite targets for attackers, given their widespread use and exposure to the internet. There are numerous strategies used to target web applications including Server-Side Request Forgery (SSRF).

SSRF allows an attacker to manipulate your server and cause disruptions or financial damage. It has been the reason for several security breaches, including the Microsoft Exchange server hack and leak of data at Capital One.

Identifying and preventing such security incidents is the best measure to safeguard your data. It can help you uncover potential weaknesses in your web applications to prevent cyberattacks. Hence, testing web apps for security flaws with a reliable vulnerability scanner is the best bet against such threats.

Let’s dive into the article for an in-depth understanding of Server-Side Request Forgery.

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What is SSRF?

Server-Side Request Forgery (SSRF) is a vulnerability that lets an attacker trick a server into making unintended requests on its behalf. Think of it as an attacker using the server as a puppet to reach places they shouldn't. Due to its high impact, the SSRF attack is one of the most critical web app security vulnerabilities.

Attackers can abuse the server functionality with this vulnerability to send a request to an arbitrary location. In some cases, the attacker can send the request to an unintended outbound location or connect to internal services in other cases. Usually, a server’s internal resources are the main target for SSRF attacks. These are protected behind firewalls but can be bypassed with this attack.

SSRF is also one of the OWASP Top 10 vulnerabilities that cause severe damage to victims. It can be used to launch various attacks like remote code execution, access restricted server resources, and data leaks.

Web applications that read data from URLs are vulnerable to this type of attack. Attackers can manipulate HTTP headers or utilize path traversal exploits to make unauthorized requests to any external URL.

How the SSRF Attack Works (With Example)

An SSRF attack works by abusing the server’s ability to make network requests. The attacker doesn’t reach internal systems directly; they make the server do it for them.

Here’s a step-by-step breakdown of how a typical SSRF attack unfolds:

• An application accepts a URL from the user and uses it to fetch data or perform some action. When this input isn’t properly validated, it becomes an opportunity for abuse.
• An attacker then replaces the expected external URL with one that points to an internal service or a private IP address. This could be something like http://127.0.0.1/admin or a cloud metadata endpoint that normally stays hidden.
• The server follows this crafted URL because it trusts itself, not realizing the request is malicious. It sends the request into its internal network or cloud infrastructure without any restrictions.
• Once the request is made, the attacker may receive the response directly, or they may use the server’s behavior to infer what’s happening internally. This lets them map internal services, discover sensitive endpoints, or identify weak spots.
• In a cloud environment, reaching metadata services can expose access tokens, configuration details, or credentials. That often leads to privilege escalation and deeper access into the system.

Types of Server-Side Request Forgery Attacks

Server-Side Request Forgery attacks can vary based on the characteristics of the underlying vulnerability. Here are the common types of SSRF attacks you need to know about.

Standard SSRF Attack

A standard SSRF attack happens when the attacker supplies a malicious URL that the server blindly follows. Since the response is visible to the attacker, they can directly analyze what the server fetched. This makes it easy to discover internal services, exposed endpoints, or sensitive data. The attacker can use this feedback loop to map internal networks and plan further exploitation. In many cases, this becomes the entry point to access systems not meant to be public.

Blind SSRF Attack

In a blind SSRF attack, the attacker cannot see the server’s response, so they rely on side effects to understand what happened. They submit crafted inputs and observe how the application shows error messages, status codes, or subtle differences in how the app responds. These indirect signals help them determine whether their request reached the internal resource. Even without a direct response, this method still allows attackers to probe internal networks and confirm reachable targets.

Back-End SSRF Attack

A back-end SSRF attack targets internal services that sit behind the main application, such as databases, admin panels, internal APIs, or cloud provider endpoints. The attacker forces the server to interact with these back-end components using crafted URLs. Since these systems often lack strong authentication internally, the server may unintentionally expose sensitive data or perform actions on their behalf. This type of SSRF is especially dangerous because it can lead to privilege escalation and unauthorized access to critical internal systems.

Time-Based Blind SSRF Attack

In a time-based blind SSRF attack, the attacker relies on response delays to learn what’s happening. They send a payload that triggers the server to reach a controlled or slow-responding endpoint and measure how long the application takes to reply. A noticeable delay often indicates the server successfully made the request. By repeating this process with different endpoints, the attacker can slowly map internal services based purely on timing patterns.

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Tactics Used for Server-Side Request Forgery Attacks

The following tactics are used by attackers to manipulate servers and perform a successful cyberattack.

Encoded URL

URL encoding is a tactic used for SSRF. In this situation, an attacker could try to force an application to send a request to another URL that it didn’t intend to by using an encoded URL. Usually, URLs contain characters defined by ASCII. However, when any special characters or characters outside of the ASCII range need to be added, URL encoding is needed. In that case, ASCII code encoded with percent values is used in place of those characters.

IPV6 Address Exploit

If there is an application that allows users to enter a URL to fetch resources, attackers can exploit it to make unintended requests by entering an IPv6 address. For example, an attacker can enter a crafted URL like [::1] that refers to localhost. Since the address is enclosed in square brackets, it can circumvent the input validation method. In addition, the IPv6 address may also be treated as valid by the application that results in loading localhost.

Local File Inclusion

When an application allows entering a file path in the URL and doesn’t sanitize it, it is likely to face server-side request forgery. In this case, an attacker can exploit this vulnerability by inserting a crafted URL. Let’s understand with an illustration:

An attacker attempts to access “some_dir/file” on the server with IP 10.10.10.10. The attacker can manipulate the URL as follows:

http://testapp.com/?file=http://10.10.10.10/some_dir/file

How to Prevent Server-Side Request Forgery?

Implement Input Validation

Validating user input is one of the most effective ways to reduce SSRF risk. It ensures the application only processes safe and expected values. Proper validation blocks attackers from injecting internal URLs, private IPs, or unexpected protocols. This creates a strong first layer of defense against malicious requests.

Here are the practices you can follow to implement input validation:

• Allow only approved URL formats and block everything else.
• Reject private IP ranges, loopback addresses, and internal hostnames.
• Normalize and re-check user input before processing.
• Block dangerous protocols like file://, gopher://, and ftp://.
• Use server-side validation and never rely solely on client-side checks.

Create Whitelists

Whitelisting restricts the server to communicate only with trusted and approved destinations. This prevents attackers from redirecting requests toward internal networks or cloud metadata endpoints. When done well, it limits your server’s reach and reduces the blast radius of any attempted SSRF attack.

Best practices you can follow when creating whitelists:

• Maintain a strict list of allowed domains or IPs.
• Resolve DNS names to IPs and verify they match the whitelist.
• Block redirects to unapproved locations.
• Use allowlists at both application and network layers.
• Log and monitor any requests blocked by the whitelist.

Secure Access Controls

Strong access control helps isolate internal services and prevents the server from reaching sensitive resources without proper authorization. Even if an attacker triggers an SSRF request, well-defined controls reduce the chances of that request exposing critical data. This approach focuses on limiting internal trust and reducing the risk of broken access control.

To ensure secure access controls:

• Apply role-based access to internal APIs and admin endpoints.
• Use firewall rules to block unnecessary outbound connections.
• Separate sensitive services into protected network segments.
• Require authentication for internal services wherever possible.
• Continuously review which systems the server is allowed to reach.

How ZeroThreat Can Help You Mitigate SSRF Attacks

ZeroThreat is an AI-powered security testing platform that helps teams find and fix vulnerabilities across web apps and APIs before attackers can exploit them. It uses advanced scanning, threat intelligence, and automated testing to strengthen your application security with minimal setup.

Here is how ZeroThreat’s help you secure web apps and APIs from SSRF attacks:

1. Comprehensive Vulnerability Detection

ZeroThreat runs dynamic scans on web applications and APIs to uncover hidden vulnerabilities, including Server-Side Request Forgery (SSRF) risks. Its AI-driven engine looks beyond simple checks, finding threats that traditional tools may miss.

• Detects OWASP Top 10 and complex logic flaws at runtime.
• Identifies SSRF patterns in API behavior and internal request handling.
• Scans authenticated and restricted areas for hidden weaknesses.

2. AI-Powered Analysis and Evidence

Once a potential SSRF vulnerabilities are spotted, ZeroThreat flags and prioritizes it. The platform correlates real exploit evidence with vulnerability signals so teams can trust their findings and act quickly.

• Provides request/response evidence for each identified flaw.
• Delivers contextual insights tailored to your codebase.
• Reduces false positives so you focus only on true risks.

3. Actionable Fix Guidance

Fixing SSRF is often harder than finding it. ZeroThreat helps bridge that gap by offering detailed, AI-generated remediation steps that are easy to follow. These suggestions help developers patch weaknesses faster and more accurately.

• Includes code-level recommendations in reports.
• Prioritizes vulnerabilities based on risk and impact.
• Integrates with development workflows so fixes fit seamlessly into your process.

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The Bottom Line

Protecting your web application and server resources can be challenging with the growing number of threat vectors. Attackers are always looking for new kinds of vulnerabilities to attempt a data breach. However, you can win this cat race by using a sophisticated web app security scanner – ZeroThreat, which can help you detect emerging security risks.

It automates vulnerability detection and enables you to identify OWASP Top 10 security risks, including SSRF and beyond. You can leverage shift left with ZeroThreat by integrating it into your SDLC to ensure secure APIs and applications from the outset.

With an AI-powered scanning engine, ZeroThreat can identify common and hard-to-detect vulnerabilities with a high rate of accuracy. It can uncover vulnerabilities with zero false positives. Get faster scanning speed and priority-based results on our DAST platform to streamline your AppSec process.

You can take a free test to find out its benefits on your own.