The open-source Spring Framework, a popular Java-based application framework for creating enterprise and web applications, has been found to contain a denial-of-service (DoS) vulnerability. This vulnerability affects Spring Framework versions 6.. - 6..6, 5.3. - 5.3.25, 5.2..RELEASE - 5.2.22.RELEASE, and older unsupported versions.

This blog post will provide an overview of the Spring Framework vulnerability, designated as CVE-2023-20861, and will cover exploit details, affected code snippets, and links to the original reference materials.

Vulnerability Summary

In Spring Framework, developers use the Spring Expression Language (SpEL) for runtime expressions and data binding. This vulnerability occurs when a user submits a specially crafted SpEL expression that causes a denial-of-service (DoS) by consuming a significant amount of system resources.

Exploit Details

The vulnerability is due to a lack of validation and improper handling of the user-provided SpEL expressions, which can be exploited by an attacker to craft a malicious SpEL expression that results in an infinite loop, causing high CPU and memory usage, eventually leading to a DoS condition.

An example of a vulnerable SpEL expression in the Spring Framework is as follows

// Spring Boot application using SpEL
@RestController
public class VulnerableController {

  @RequestMapping(value = "/calc", method = RequestMethod.POST)
  public ResponseEntity<String> calculate(@RequestParam("expr") String expression) {
    try {
      ExpressionParser parser = new SpelExpressionParser();
      StandardEvaluationContext context = new StandardEvaluationContext();
      Expression exp = parser.parseExpression(expression);
      Integer result = exp.getValue(context, Integer.class);
      return ResponseEntity.ok(String.valueOf(result));
    } catch (Exception e) {
      return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR)
          .body("Error processing expression: " + e.getMessage());
    }
  }
}

In the above code example, the SpEL expression is parsed and evaluated without proper validation and handling, possibly resulting in an infinite loop when given a maliciously crafted expression.

Original References

For a comprehensive understanding of this vulnerability and its implications, it is advisable to review the original references provided by the Spring Framework team:

1. Spring Security Project: CVE-2023-20861
2. Spring Framework GitHub Issue: Improper handling of SpEL expressions

Mitigation Measures

To mitigate this vulnerability, developers should promptly update their Spring Framework version to the latest release, which contains fixes for this issue:

5.2.23.RELEASE for Spring 5.2.x users

Additionally, developers should ensure to validate and sanitize user input properly to avoid accepting malicious SpEL expressions.

Conclusion

CVE-2023-20861, a critical vulnerability in the Spring Framework, can lead to a denial-of-service condition if not addressed promptly. Developers must keep their Spring Framework version up to date and implement proper input validation to protect their applications from this vulnerability.

Timeline

Published on: 03/23/2023 21:15:00 UTC
Last modified on: 04/20/2023 09:15:00 UTC