CVE-2023-23396: Microsoft Excel Denial of Service Vulnerability - Decrypting the Exploit and Providing a Solution

Security vulnerabilities can have a significant impact on users and companies, especially when software as widely-used as Microsoft Excel is involved. In the case of CVE-2023-23396, we have a critical Microsoft Excel Denial of Service (DoS) vulnerability that allows an attacker to crash the application. This post will provide an overview of the exploit, dissect the code snippet responsible for the vulnerability, and share links to original references. Ultimately, we will explore and suggest ways to mitigate this issue in a user-friendly manner.

Vulnerability Overview

The Common Vulnerabilities and Exposures (CVE) project has assigned the identifier CVE-2023-23396 to this vulnerability, which exists in certain versions of Microsoft Excel. The exploit leverages a bug in the application that can cause it to enter a non-responsive state and crash the system, effectively causing a Denial of Service scenario.

Original references

- National Vulnerability Database (NVD) - CVE-2023-23396
- Microsoft Security Advisory - Mitigating CVE-2023-23396

Exploiting the Vulnerability

The exploit consists of a specially crafted Excel file (.xlsx) that contains a specific formula, which triggers the vulnerability when the user opens the file.

The code snippet below illustrates the formula that triggers the DoS

=IFERROR(INDEX(A:A, 1 + INT((ROW(A1) - ROW($A$1))/2), INT((ROW(A1) - (ROW($A$1) - 1))/2) + 1 - MOD(ROW(A1) - ROW($A$1), 2))

When this formula is placed in an Excel file and is opened by the user, it creates a series of recursive calls that eventually depletes the system resources. This process ultimately leads to a non-responsive state where the application crashes, effectively locking the user out of their data.

Mitigating the Vulnerability

Since the exploit revolves around the opening of a crafted malicious file, users can take the following steps to mitigate the vulnerability:

1. Upgrade to the latest version of Microsoft Excel. Microsoft has released patches addressing the issue. Links to specific patches are provided in the Microsoft Security Advisory for CVE-2023-23396.

2. Exercise caution when opening Excel files from unknown sources. Avoid opening any files from suspicious emails or untrusted websites.

3. Consider using alternative Excel file viewers if you must open files from unknown sources. These viewers provide a read-only view of the file and are less likely to be vulnerable to this specific exploit.

4. Educate users on the importance of utilizing security best practices such as maintaining software updates, scrutinizing email attachments, and relying on trusted websites.

Conclusion

Overall, Denial of Service vulnerabilities like CVE-2023-23396 can pose a significant threat to users and organizations. The steps mentioned above can help inform users on how to avoid and mitigate the risks associated with this particular vulnerability. Staying informed of security updates and following best practices will contribute to the overall security of Microsoft Excel and other widely-used software.

Timeline

Published on: 03/14/2023 17:15:00 UTC
Last modified on: 05/19/2023 15:09:00 UTC