If you're using SheetJS Community Edition before version .19.3, your application could be at risk of a dangerous vulnerability called Prototype Pollution. This vulnerability (CVE-2023-30533) makes it possible for an attacker to manipulate the behavior of objects in your application, potentially causing security issues, crashes, or incorrect data processing. In this post, we'll go through the details of this vulnerability, explain how it works, show a code snippet, and provide guidance on how to mitigate this issue and secure your applications.


SheetJS, a popular JavaScript library for parsing, writing, and manipulating spreadsheet data, has a vulnerability (CVE-2023-30533) in its Community Edition versions before .19.3 that allows an attacker to perform Prototype Pollution. The vulnerability exists due to the library's improper handling of crafted files, which could allow an attacker to modify an object's prototype—and consequently change the behavior of all objects that inherit from that prototype.

CVE-2023-30533 Exploit Details

By creating and uploading a carefully crafted file to a vulnerable application using SheetJS, an attacker could effectively inject malicious code or manipulate existing code. The vulnerability can enable an attacker to cause a denial of service, unauthorized access, or other unexpected behavior.

Code Snippet:

The following is a simple example of how an attacker might manipulate the prototype of an object

// Attacker's crafted JSON file content
  "__proto__": {
    "isAdmin": true

When an application parses such a crafted JSON file using SheetJS, it would unintentionally change the prototype of all objects to include "isAdmin" property with the value of "true." This could lead to unauthorized access or broken functionality in the application.


To protect your applications from this vulnerability, it is recommended that you update SheetJS Community Edition to version .19.3 (or higher) immediately. This update resolves the issue by properly handling files and avoiding the prototype pollution exploit.

npm install xlsx@.19.3

However, updating the library might not be enough in some cases. You should also take the following precautions:

1. Verify all user-provided files before processing, ensuring they meet your expected format and structure. You can use file type validation and content sanitization measures.
2. Employ the principle of least privilege, ensuring only authorized users can upload and process files in your application.
3. Monitor application and system logs, especially focusing on any unauthorized access or unexpected behavior that might stem from Prototype Pollution.


This post provided an overview of the CVE-2023-30533 vulnerability in SheetJS Community Edition before version .19.3, which allows for Prototype Pollution via a crafted file. To secure your application, make sure to update SheetJS to the latest version and follow best practices to protect your applications against this type of vulnerability.

Original References

1. SheetJS Github Repository: https://github.com/SheetJS/sheetjs
2. NVD - CVE-2023-30533: https://nvd.nist.gov/vuln/detail/CVE-2023-30533


Published on: 04/24/2023 08:15:00 UTC
Last modified on: 05/02/2023 18:40:00 UTC