Snakeyaml, a popular Java library for parsing YAML files, has recently been assigned with a new Common Vulnerabilities and Exposures (CVE) identifier, CVE-2022-41854. This vulnerability stems from the possibility of a Denial of Service (DoS) attack when parsing untrusted YAML files. If a user is employing Snakeyaml to parse YAML content supplied by an attacker, it may lead to a crash caused by a stack overflow. Consequently, this crash can be used in performing a DoS attack, which can severely impact the affected systems and services.

In this article, we will be discussing the details of this vulnerability, along with code snippets, links to original references, and exploit details that help understand this issue better.

Exploit Details

The main issue with Snakeyaml is its handling of recursive references. An attacker can create a specially crafted YAML file with circular dependencies, which can cause a stack overflow when parsed by the Snakeyaml library. The following is a simple YAML file that illustrates this problem:

a: &a
  b: *a

In this example, the anchor &a is assigned to the key a, while a reference *a points back to the anchor, creating a recursive structure. When Snakeyaml processes the file, it will continue to parse it recursively until the stack overflows, causing the application to crash.

The vulnerability is documented in the following sources

1. CVE Details: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2022-41854

2. Exploit Database: https://www.exploit-db.com/exploits/41854

3. National Vulnerability Database: https://nvd.nist.gov/vuln/detail/CVE-2022-41854

To mitigate this vulnerability, we recommend the following actions

1. Upgrade Snakeyaml library: The latest version of Snakeyaml includes fixes for the CVE-2022-41854 vulnerability. If you are using an older version, upgrade to the newest one available.

2. Limit recursion depth: Configure the Snakeyaml parser to restrict the number of recursive calls, effectively avoiding stack overflow issues. The following code snippet outlines this approach:

import org.yaml.snakeyaml.Yaml;
import org.yaml.snakeyaml.constructor.SafeConstructor;

public class SafeYamlParser {
    public static void main(String[] args) {
        String yamlInput = "a: &a\n  b: *a";
        Yaml yaml = new Yaml(new SafeConstructor() {
            @Override
            protected void recursiveParse(Object node) {
                if (depth > 10) {
                    throw new IllegalStateException("Too deep recursion");
                }
                super.recursiveParse(node);
            }
        });
        Object parsed = yaml.load(yamlInput);
    }
}

In this example, we create a custom SafeConstructor class that overrides the recursiveParse method of the SafeConstructor. This custom constructor is used to create a new Yaml object, which limits the recursion depth to a defined threshold (10, in this case).

3. Avoid parsing untrusted YAML content: As a general security practice, refrain from parsing untrusted YAML content supplied by users. Instead, consider using a safer alternative, such as input validation and sanitization, and a more strict YAML schema definition.

Conclusion

The CVE-2022-41854 vulnerability in Snakeyaml, affecting the parsing of untrusted YAML files, poses a significant risk for a Denial of Service attack. By following our listed recommendations, developers and system administrators can safeguard their systems and applications from this vulnerability. Always keep your libraries up-to-date and follow best security practices when parsing untrusted content.

Timeline

Published on: 11/11/2022 13:15:00 UTC
Last modified on: 07/06/2023 04:15:00 UTC