Understanding Spring Framework DoS Vulnerability CVE-2023-20861 and How to Fix It

1. What is a DoS Vulnerability

A DoS (Denial of Service) vulnerability occurs when an attacker sends a massive number of requests or malicious code, overwhelming the target system's resources and preventing it from providing normal services. This can cause crashes, data loss, or service degradation.

2. Impact and Mitigation

CVE-2023-20861: In Spring Framework versions 6.0.0‑6.0.6, 5.3.0‑5.3.25, 5.2.0.RELEASE‑5.2.22.RELEASE and earlier unsupported releases, a specially crafted SpEL expression can trigger a DoS attack.

2.1 Affected Spring Products and Versions

Spring Framework:

6.0.0‑6.0.6

5.3.0‑5.3.25

5.2.0.RELEASE‑5.2.22.RELEASE

Earlier unmaintained versions

2.2 Mitigation Methods

Users of affected versions should apply the following mitigations:

6.0.x users should upgrade to 6.0.7 or later.

5.3.x users should upgrade to 5.3.26 or later.

5.2.x users should upgrade to 5.2.23.RELEASE or later.

Users of older versions should upgrade to 6.0.7+ or 5.3.26+.

Versions that already contain the fix include:

6.0.7+

5.3.26+

5.2.23.RELEASE+

3.3 Vulnerability Root Cause

Providing a large regular expression to the

matches

operator in a SpEL expression can cause excessive processing, leading to denial of service. The fix limits the maximum allowed characters in regular expressions and improves diagnostics by throwing a

SpelEvaluationException

with a meaningful error message.

3. Credit

The vulnerability was initially discovered and reported by the Google OSS‑Fuzz team from Code Intelligence.

4. References

CVSS Calculator

CWE‑770: Allocation of Resources Without Limits or Throttling

5. History

2023‑03‑20: Initial vulnerability report published.

6. Spring Boot

Almost all Spring Boot versions are affected. Spring Boot 3.0.5 and 2.7.10 are scheduled for release this Thursday. You can also manually specify the Spring Framework version.

In a Gradle

build.gradle

file:

<code>ext['spring-framework.version'] = '6.0.7'</code>

Or in a Maven

pom.xml

file:

<code><properties>
  <spring-framework.version>6.0.7</spring-framework.version>
</properties>
</code>

Note: End‑of‑life Spring Boot versions require manual rebuilding of Spring Boot Maven or Gradle projects to upgrade to a secure Spring Framework version. It is strongly recommended to upgrade to a supported Spring Boot release.

7. Summary

Spring Framework offers robust security features, including Spring Security for authentication and authorization, SSL encryption, CSRF and XSS protection, role‑based access control, and OAuth 2.0 support. The Spring Boot Actuator provides monitoring and management capabilities. The CVE‑2023‑20861 vulnerability affects all Spring Framework and Spring Boot versions, highlighting the importance of careful code practices and timely updates.