What is Server-Side Request Forgery (SSRF)?

Server-Side Request Forgery (SSRF) is a critical web application vulnerability where attackers manipulate a server to make unauthorized requests. SSRF is particularly dangerous and is therefore recognized as one of the top 10 cybersecurity vulnerabilities by the Open Worldwide Application Security Project (OWASP).

For website owners, knowing what SSRF is and how to protect against it is a major key to preventing data breaches. To help you reduce your attack surface and shore up any SSRF vulnerabilities in your web applications, here’s an in-depth guide on what Server-Side Request Forgery is and the web application security measures that help prevent it.

How SSRF works

SSRF allows hackers to manipulate server-side components, such as APIs or HTTP clients, to send malicious requests. It’s a vulnerability that arises when servers process user input without adequate input validation, which lets hackers dictate the server’s actions.

The problem lies in the fact that many web applications are designed to accept URLs or other data from users in order to fetch resources. But without proper validation, these applications can also be made to perform harmful, unauthorized actions that put your internal systems and sensitive information at risk.

Its role in web applications

There are several common web application features that are vulnerable to SSRF. Features such as image uploaders, URL preview generators, and API integrations all rely on user-supplied input, which can inadvertently give hackers access to the application server.

Hackers can then use this access to breach internal networks by bypassing firewalls and exposing sensitive internal services such as databases, configuration management tools, or cloud metadata endpoints.

Types of SSRF attacks

Hackers can use SSRF to perform several different types of attacks that put your sensitive data at risk. Common types of SSRF attacks include:

Basic SSRF attacks

In a basic SSRF attack, the attacker receives immediate server responses, allowing direct data retrieval. For example, a vulnerable server can be made to retrieve sensitive files or expose internal APIs.

Blind SSRF attacks

In blind SSRF attacks, attackers do not receive direct feedback but infer information indirectly. For instance, timing differences or side-channel signals might reveal whether a target resource is accessible.

SSRF leading to RCE

SSRF vulnerabilities can escalate to remote code execution (RCE), enabling attackers to run malicious commands on the server. This escalation dramatically increases the severity of the attack and its potential impact on your internal systems.

Examples of SSRF attacks

Along with there being different types of SSRF attacks, there are also different types of malicious actions that hackers can use SSRF to perform. Here are a few examples of how SSRF attacks are commonly used to access a website’s sensitive data:

Accessing internal metadata services

Hackers often target cloud metadata services like AWS's metadata URL (http://169.254.169.254/). Exploiting SSRF to access these URLs can expose temporary access keys, instance configurations, and other sensitive data.

Port scanning internal networks

Using SSRF, hackers can scan internal networks to discover open ports or services. This allows them to map the network topology and use it as a blueprint for subsequent attacks.

Exploiting AWS services

Attackers may use SSRF to interact with unauthorized AWS endpoints, manipulating APIs to perform unauthorized actions like launching instances or accessing data stored in S3 buckets.

How SSRF impacts your website

SSRF attacks can have a devastating impact on a website in more ways than one. From bypassing access controls to exposing sensitive data, here are the various ways SSRF can potentially impact web security:

Unauthorized access to internal systems

Exploiting SSRF lets hackers bypass security measures and access controls such as firewalls and authentication mechanisms. This gives them easy access to sensitive or critical resources and exposes those resources to attack.

Exposure of sensitive data

SSRF vulnerabilities are commonly used to access sensitive information on a website. This not only puts your data at risk, but it can also impact your compliance with data protection regulations.

Potential for data breaches

SSRF can serve as an entry point for larger network compromises, resulting in a much more widespread data breach. The financial and reputational damage these large-scale breaches can cause is immense.

Chaining SSRF with other vulnerabilities

SSRF is often combined with other vulnerabilities like Cross-Site Scripting (XSS). This allows hackers to amplify the attack's scope and severity by exploiting multiple vectors.

How to prevent SSRF

Given the many ways that SSRF can be used to attack a website and expose its sensitive data, website owners need to take steps to shore up this vulnerability. If you want to prevent hackers from using SSRF to access your web servers, here are some effective mitigation strategies to employ:

Input validation and sanitization

Ensuring that all user input is sanitized and validated is the biggest key to preventing SSRF. Any inputs that contain malformed or suspicious data should be automatically rejected. You can achieve this easily with tools like SiteLock’s Malware Scanning and Malware Removal, which detect and remove threats automatically.

Implementing proper whitelists

Blacklists alone are often insufficient against creative hackers who know how to bypass them. Unlike blacklists, which block destinations that are known to be malicious, whitelists define a strict set of trusted domains and IPs. By implementing proper whitelists, you can ensure that only predefined and verified destinations can be accessed by server-side requests.

Access control and authentication

Any resources that are critical or sensitive should be protected with strong access control and authentication features. This will help reduce the impact of an SSRF attack and ensure that your most important resources remain protected even if a hacker can gain access to other areas of your site.

Use of WAFs

A web application firewall (WAF) is designed to filter incoming traffic and automatically block any requests that are deemed suspicious. Implementing a high-quality WAF such as the one offered by SiteLock can drastically improve your website’s security posture, helping prevent SSRF exploits and a range of other attacks.

Other best practices to consider

In addition to the security measures we’ve already covered, here are some other best practices that can help prevent and mitigate SSRF attacks:

Monitor website logs

Keeping a close eye on your website logs will help you detect anomalies and suspicious activity early. Learn how to analyze website logs.

Regular security testing

Regular security testing helps you identify any vulnerabilities that hackers could use to access your website, including SSRF vulnerabilities. There are a lot of different security testing tools you can use to identify a wide range of vulnerabilities, but be sure to look for those that can detect SSRF vulnerabilities within your web applications.

Keep software up-to-date

Software providers will regularly patch their applications to eliminate known vulnerabilities. However, if your software isn’t up-to-date, you may still be using an unpatched version. That’s why it’s always important to promptly apply updates for all of the software applications on your website. With a tool like SiteLock’s Vulnerability Patching tool, you can ensure that all updates and patches are applied automatically.

Implement network segmentation

Network segmentation is the process of separating critical systems from less sensitive ones. This allows you to protect your critical systems using more robust access controls without having to worry about hackers using other systems as a backdoor to access them.

Educate development teams

If your company develops its own web applications, it is essential to educate your development teams on secure coding practices and raise awareness regarding SSRF vulnerabilities.

Mitigate SSRF attacks with SiteLock

SSRF vulnerabilities within web applications can leave your website open to numerous types of devastating and costly attacks. Thankfully, there are effective ways to prevent and mitigate these tasks.

By following the security practices we’ve covered and using tools such as those offered by SiteLock, you can both prevent SSRF attacks from happening and reduce their potential impact in the event that an attack is successful.

At SiteLock, preventing and mitigating SSRF attacks is just one of the benefits offered by our advanced cybersecurity tools and services. If you’d like to ensure that your site doesn’t fall victim to an SSRF attack, be sure to learn more about how SiteLock works!

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