CVE-2023-20873 - Spring Boot Security Bypass in Cloud Foundry Environments Explored

In the fast-evolving world of Java development, Spring Boot stands out as one of the most popular frameworks. But with great popularity comes a bigger target for security threats. In March 2023, a critical vulnerability tracked as CVE-2023-20873 was found in certain Spring Boot versions, potentially allowing unauthorized users to bypass established security constraints—particularly when running applications on Cloud Foundry.

This post will break down what CVE-2023-20873 is, how it can be exploited, and—most importantly—how you can secure your application. We’ll keep our language simple and direct, include direct references, and provide hands-on code snippets.

Older, unsupported versions

If your application, using one of these versions, is deployed to Cloud Foundry, it could allow unauthenticated bypass of security filters meant to protect sensitive endpoints. In other words, attackers might reach internal endpoints without proper authentication, putting your application’s data and logic at risk.

Official reference:  
- Spring Boot CVE-2023-20873 Advisory
- NVD – CVE-2023-20873

You run a vulnerable Spring Boot version on Cloud Foundry.

- You use security features such as Servlet Filters to protect API endpoints.

Important: Deployments outside of Cloud Foundry may not be impacted, but it is recommended to upgrade anyway; see “Mitigation” section below.

Exploit Details: How Can Attackers Bypass Security?

Cloud Foundry’s routing mechanism can set X-Forwarded-Host and other headers. Vulnerable Spring Boot versions could mishandle these headers, leading to a mismatch in the logic that enforces security layers, especially in URL-based authentication filters.

Suppose you have the following filter to block access to /admin except for authenticated users

@Configuration
public class SecurityConfig extends WebSecurityConfigurerAdapter {
    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http
            .authorizeRequests()
            .antMatchers("/admin/**").authenticated()
            .anyRequest().permitAll();
    }
}

In theory: Only authenticated users can reach /admin.  
With CVE-2023-20873: An attacker could craft a request with spoofed headers, confusing the interceptor logic and bypassing authentication checks.

Here’s a conceptual curl command an attacker might use, exploiting trusted headers

curl -H "X-Forwarded-Host: victim.app.com" -H "X-Forwarded-Proto: https" \
     https://your-app.cloudfoundry.net/admin

In older (vulnerable) Spring Boot versions, this could result in the filter not being applied properly, and the attacker may access /admin without authentication!

Note: Exploitation requires access to the application, and results depend on custom filters and configuration.

How To Fix It? (Mitigation)

The fix is simple: upgrade your Spring Boot version.

If on older versions: Migrate to a supported version above

Not sure which version you’re running?

Check your pom.xml or build.gradle for the Spring Boot plugin line

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>3..4</version> <!-- Vulnerable! -->
</dependency>

Change to

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>3..6</version> <!-- Safe! -->
</dependency>

Or for 2.7.x lines

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.7.11</version> <!-- Safe! -->
</dependency>

After updating, always rebuild and redeploy your application.

Upgrade dependencies promptly following vendor security advisories.

- Set up automated dependency monitoring using tools like Dependabot, Snyk, or OWASP Dependency-Check.

Quick FAQ

Q: Is only Cloud Foundry affected?  
A: This specific issue’s exploit path impacts Cloud Foundry deployments because of how it handles proxy headers.

Q: I use an older, unsupported version. Now what?  
A: You should upgrade—older unsupported versions are not patched and remain vulnerable.

Q: Where can I get more info?  
A:  
- Spring Boot GitHub Release Notes
- Spring Boot Security Docs

Conclusion

CVE-2023-20873 is a great reminder that all code—especially widely-used frameworks—can have security holes. Keeping up with versions and understanding deployment-specific threats is critical. Don’t risk your app: Upgrade today!

Further Reading:  
- Spring Blog: Security Releases
- Tanzu Security Advisory
- Cloud Foundry Documentation

Timeline

Published on: 04/20/2023 21:15:00 UTC
Last modified on: 06/01/2023 14:15:00 UTC