Jenkins, a widely used open-source automation server that helps developers automate various parts of the software development process, has recently been discovered to be vulnerable to a security issue. In this long-read post, we will deep dive into the details of CVE-2022-0538, a newly disclosed vulnerability affecting Jenkins versions 2.333 and earlier, as well as the Long-Term Support (LTS) versions 2.319.2 and earlier. This vulnerability allows for unconstrained resource usage by exploiting custom XStream converters, which have not been updated to apply the protections needed to address the CVE-2021-43859 vulnerability.

We will explore this vulnerability by providing code snippets, highlighting the original references, and explaining the exploit's details.

CVE-2022-0538 Description and Impact

CVE-2022-0538 is a security vulnerability in Jenkins that allows an attacker to consume excessive resources on the affected system by exploiting custom XStream converters. This vulnerability occurs because these converters have not been updated to include the necessary protections against CVE-2021-43859, another closely related security issue.

The impact of this vulnerability is significant, as it can lead to a denial of service (DoS) – effectively taking your Jenkins system offline.

Code Snippet: Vulnerable XStream Converter

Here is a code snippet demonstrating the vulnerability in a custom XStream converter that does not apply the appropriate protections:

import com.thoughtworks.xstream.XStream;
import com.thoughtworks.xstream.converters.Converter;
// more imports...

public class VulnerableConverter implements Converter {
  // ... converter methods ...

  public void marshal(Object source, HierarchicalStreamWriter writer, MarshallingContext context) {
    // vulnerable code: missing proper resource usage constraints
    SomeResource resource = (SomeResource) source;
    xstream.marshal(resource, writer);
  }

  // ... rest of the converter methods ...
}

// in another part of the code, the custom converter is used
XStream xstream = new XStream();
xstream.registerConverter(new VulnerableConverter ());

In the code snippet above, the VulnerableConverter class implements a custom XStream converter without applying proper resource usage constraints, making it prone to exploitation.

Exploit Details

To exploit this vulnerability, an attacker would send a specially crafted XML payload to Jenkins, targeting an exposed custom XStream converter not updated with the necessary protections. The payload would cause an unconstrained resource usage, consuming excessive memory, CPU, or other system resources on the affected Jenkins instance, leading to a DoS.

Mitigation and Resolution

To address this vulnerability, it is strongly recommended to upgrade your Jenkins instances to version 2.334 or later and LTS to version 2.319.3 or later. These releases include the necessary updates to custom XStream converters, applying the protections against CVE-2021-43859, and consequently, CVE-2022-0538.

Original References

For more information about CVE-2022-0538 and the associated security advisory, please refer to the following resources:

1. Jenkins Security Advisory 2022-01-26
2. CVE-2022-0538 Details on NIST NVD

Conclusion

In this post, we have explored CVE-2022-0538, a security vulnerability affecting Jenkins versions 2.333 and earlier, and LTS versions 2.319.2 and earlier. By exploiting custom XStream converters in Jenkins, this vulnerability allows for unconstrained resource usage, which can cause a denial of service on the affected system. It is crucial to upgrade your Jenkins instances to the latest versions to mitigate this vulnerability and ensure your system's continued security and functionality.

Timeline

Published on: 02/09/2022 14:15:00 UTC
Last modified on: 02/11/2022 20:44:00 UTC