CVE-2023-20861: Spring SpEL Vulnerability & Fix

Hey everyone! Today, we're diving deep into a critical security vulnerability, CVE-2023-20861, affecting the Spring Expression Language (SpEL). This is a big deal, so let's break it down in a way that's easy to understand and see how it could impact your projects, especially those using WebGoat8. We'll cover everything from what SpEL is, the nitty-gritty details of the vulnerability, and most importantly, how to fix it.

What is CVE-2023-20861?

CVE-2023-20861 is a high-severity vulnerability that lurks within the Spring Expression Language (SpEL). SpEL is a powerful expression language used within the Spring Framework, allowing for dynamic manipulation and evaluation of objects at runtime. Think of it as a mini-programming language embedded within your Spring applications. But with great power comes great responsibility, and in this case, a potential security risk.

The Vulnerable Library: Spring-Expression-5.2.2.RELEASE.jar

Specifically, this vulnerability affects versions of the spring-expression library, particularly spring-expression-5.2.2.RELEASE.jar, which is the version we'll focus on here. This library is a core component of the Spring Framework and is used extensively in many Spring-based applications. The vulnerability resides in how SpEL expressions are processed, opening a door for potential attacks. It’s crucial to understand that failing to address this vulnerability could lead to significant security breaches, impacting the availability of your applications.

Where is the Vulnerable Library Located?

The vulnerable library, spring-expression-5.2.2.RELEASE.jar, can be found in various locations depending on your project's setup. For WebGoat8, it's often located within the project's Maven repository, typically under paths like /home/wss-scanner/.m2/repository/org/springframework/spring-expression/5.2.2.RELEASE/. You might see multiple instances of this path, as the library can be a dependency in various modules or sub-projects. The presence of multiple paths underscores the widespread use of this library within the application and, consequently, the breadth of the vulnerability’s potential impact.

Dependency Hierarchy

The vulnerable library is often pulled in as a transitive dependency, meaning it's not directly included in your project but is brought in by other libraries you depend on. For example, in WebGoat8, the dependency chain looks something like this:

• webgoat-container-v8.1.0.jar (Root Library)
  • spring-boot-starter-web-2.2.2.RELEASE.jar
    • spring-boot-starter-2.2.2.RELEASE.jar
      • spring-boot-2.2.2.RELEASE.jar
        • spring-context-5.2.2.RELEASE.jar
          • spring-expression-5.2.2.RELEASE.jar (Vulnerable Library)

This dependency hierarchy shows that spring-expression is a dependency of spring-context, which is part of the broader Spring Boot framework. This illustrates how deeply embedded the vulnerable library can be within your project, making it essential to identify and address the issue promptly. The vulnerability’s reach is amplified by its position in the dependency tree, affecting numerous components that rely on Spring’s expression-handling capabilities.

Impact on WebGoat8

For those using WebGoat8, this vulnerability is present in the version found in the HEAD commit 7eeb88fd39d36dd93918e5286e18c3b3a6834a91 and the main branch. This means that if you're running WebGoat8, you're potentially exposed to this vulnerability. WebGoat8, being a deliberately insecure web application, serves as an excellent platform to understand and practice exploiting vulnerabilities like CVE-2023-20861. However, it also means that you need to be aware of these vulnerabilities to prevent accidental or intentional exploitation in a real-world scenario. The presence of the vulnerability in the main branch highlights the ongoing need for vigilance and regular security updates.

Vulnerability Details: Denial-of-Service (DoS)

The crux of CVE-2023-20861 lies in the possibility of a user crafting a malicious SpEL expression that can trigger a Denial-of-Service (DoS) condition. In simpler terms, an attacker can send a specially designed input that overwhelms the application, making it unavailable to legitimate users. This is achieved by exploiting how SpEL evaluates expressions, potentially causing it to consume excessive resources or enter an infinite loop. A DoS attack can severely impact your application's uptime and reliability, leading to user frustration and potential financial losses.

How a DoS Attack Works Through SpEL

Imagine SpEL as a powerful calculator that can perform complex calculations based on user input. Now, imagine someone giving the calculator an equation so complex or convoluted that it takes forever to solve. That's essentially what's happening here. An attacker crafts a SpEL expression that is deliberately inefficient or resource-intensive, causing the application to grind to a halt while trying to evaluate it. The attacker's goal is to exhaust the server's resources, preventing it from handling legitimate requests. This can manifest as slow response times, application crashes, or complete unavailability. The insidious nature of this vulnerability lies in its ability to disrupt services without directly compromising data, making it a stealthy yet potent threat.

CVSS 4 Score: 7.1 (High Severity)

The Common Vulnerability Scoring System (CVSS) version 4 assigns this vulnerability a score of 7.1, classifying it as high severity. This score reflects the potential impact and ease of exploitation. Let's break down the key metrics:

Base Score Metrics:

• Exploitability Metrics:
  • Attack Vector: Network: This means the attack can be launched remotely over a network, making it highly accessible.
  • Attack Complexity: Low: The vulnerability is relatively easy to exploit, requiring minimal technical skill from the attacker.
  • Privileges Required: Low: An attacker with low-level privileges can exploit this vulnerability, reducing the barrier to entry.
  • User Interaction: None: No user interaction is required to trigger the vulnerability, allowing for automated attacks.
  • Scope: N/A: The vulnerability's impact is limited to the affected component.
• Impact Metrics:
  • Confidentiality Impact: N/A: The vulnerability does not directly compromise the confidentiality of data.
  • Integrity Impact: N/A: The vulnerability does not directly compromise the integrity of data.
  • Availability Impact: N/A: The primary impact is on the availability of the application, leading to a DoS condition.

This CVSS score underscores the urgency of addressing this vulnerability. The low attack complexity and lack of required user interaction make it a prime target for attackers. While the immediate impact is on availability, the disruption caused by a DoS attack can have cascading effects, leading to reputational damage and financial losses.

Suggested Fix: Upgrade Spring Expression Language

The recommended solution to address CVE-2023-20861 is to upgrade your Spring Framework dependencies to versions that include a fix for this vulnerability. Specifically, you should upgrade to one of the following versions:

• org.springframework:spring-expression:5.2.23.RELEASE
• org.springframework:spring-expression:5.3.26
• org.springframework:spring-expression:6.0.7

These versions contain the necessary patches to mitigate the risk of DoS attacks via maliciously crafted SpEL expressions. The Spring team has diligently addressed this issue, providing clear upgrade paths for affected users. Delaying the upgrade leaves your applications exposed, making it imperative to prioritize this fix.

How to Upgrade

The process of upgrading your Spring dependencies will vary depending on your project's build system. Here are some general guidelines:

Maven

If you're using Maven, you'll need to update the version numbers in your pom.xml file. Locate the dependency for spring-expression (or a parent dependency like spring-context or spring-boot-starter-web) and change the version to one of the fixed versions mentioned above. For example:

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-expression</artifactId>
    <version>5.2.23.RELEASE</version>
</dependency>

After updating the pom.xml, run mvn clean install to rebuild your project with the updated dependencies. This will ensure that the new version of the library is downloaded and incorporated into your application. Maven's dependency management capabilities make this process relatively straightforward, but it's crucial to verify the changes and test thoroughly after the upgrade.

Gradle

For Gradle projects, you'll need to update the build.gradle file. Find the dependency declaration for spring-expression and update the version number. For example:

dependencies {
    implementation 'org.springframework:spring-expression:5.2.23.RELEASE'
}

Run gradle clean build to rebuild your project with the updated dependencies. Gradle's flexible dependency management system simplifies the upgrade process, but it's essential to conduct comprehensive testing to ensure the application functions as expected.

Verification

After upgrading, it's crucial to verify that the fix has been applied correctly. You can do this by checking the version of the spring-expression library included in your application's deployed artifacts. Additionally, you should run thorough testing, including penetration testing and vulnerability scanning, to ensure that the vulnerability has been effectively mitigated. Proactive verification is a cornerstone of a robust security posture, ensuring that upgrades have the intended effect.

Conclusion: Prioritize Security Updates

CVE-2023-20861 serves as a stark reminder of the importance of staying vigilant about security vulnerabilities and promptly applying necessary updates. The high severity of this vulnerability, coupled with the ease of exploitation, makes it a significant threat to Spring-based applications. By upgrading to the patched versions of spring-expression, you can effectively mitigate the risk of DoS attacks and ensure the continued availability of your applications.

Remember, security is an ongoing process, not a one-time fix. Regularly monitoring for vulnerabilities, applying updates, and conducting security testing are essential practices for maintaining a secure application environment. Don't wait until an attack occurs; proactive security measures are your best defense. Stay safe out there, guys! This proactive approach not only addresses immediate threats but also fosters a culture of security awareness within your development teams.