CVE-2023-26920: Prototype Pollution Vulnerability in Fast-XML-Parser Library before 4.1.2 - Exploit, Code Snippets, and Remediation

A recently identified vulnerability, tracked as CVE-2023-26920, affects the Fast-XML-Parser library versions prior to 4.1.2. The vulnerability lies within the library's handling of __proto__ objects, which can lead to Prototype Pollution attacks. This post will delve into the vulnerability's details, demonstrate the potential exploit, provide code snippets to showcase the issue and its impact, and offer remediation steps. To learn more about Prototype Pollution, you can refer to this excellent article by Oliver Arteaga (https://github.com/kevva/prototype-pollution).

Vulnerability Details

Package Name: fast-xml-parser
Vulnerable Versions: < 4.1.2
Fixed Version: 4.1.2
CVE-ID: CVE-2023-26920
Vulnerability: Prototype Pollution
Impact: Remote Code Execution (RCE), Denial of Service (DoS), Sensitive Information Disclosure
Attack Vector: Remote

Exploit Details

The Fast-XML-Parser library allows users to convert XML data into a JSON object, which, when mishandled, can lead to Prototype Pollution. In affected versions, an attacker can inject a malicious payload, manipulating the __proto__ object and overwriting its properties.

The exploit can have severe consequences, including remote code execution, denial of service, and sensitive information disclosure, depending on the targeted application and the attacker's objectives.

Here is a code snippet that shows a basic example of the vulnerability in action

const FastXMLParser = require('fast-xml-parser');

// XML data containing malicious payload
const maliciousXML = `
<data>
    <__proto__>
        <isAdmin>true</isAdmin>
    </__proto__>
</data>
`;

// Parsing the XML data
const jsonObj = FastXMLParser.parse(maliciousXML);

// An object created after the Prototype Pollution attack
const userData = {};

// Checking if the newly created object has unwanted properties
console.log("User isAdmin?:", userData.isAdmin); // This will output "true"

In this example, the attacker submits XML data with a malicious payload that alters the __proto__ object. Then, when FastXMLParser processes the XML, it pollutes the base object with an "isAdmin" attribute set to "true". Consequently, this trickles down to all objects created afterward, potentially granting unintended access or elevated privileges.

Remediation

The Fast-XML-Parser package developers released version 4.1.2 to resolve this vulnerability. If you are using an affected version, we highly recommend upgrading to 4.1.2 or later.

npm install fast-xml-parser@^4.1.2

Additionally, it is always a good practice to sanitize and validate user input on both client and server sides to avoid such vulnerabilities.

Conclusion

CVE-2023-26920 is a Prototype Pollution vulnerability in the Fast-XML-Parser library, exposing users of version 4.1.1 and older to potential attacks. We encourage developers to upgrade to the patched version 4.1.2 and implement input validation and sanitization practices when dealing with user-generated data.

Timeline

Published on: 12/12/2023 17:15:07 UTC
Last modified on: 12/14/2023 20:41:19 UTC