CVE-2021-40439 : Exploit Details and Defense Strategies

Apache OpenOffice up to version 4.1.10 is vulnerable to a "Billion Laughs" entity expansion denial of service attack. Here's what you need to know about CVE-2021-40439.

Understanding CVE-2021-40439

CVE-2021-40439, also known as "Billion Laughs," affects Apache OpenOffice versions prior to 4.1.11 due to an issue with the expat software dependency. Let's delve into the details.

What is CVE-2021-40439?

Apache OpenOffice, relying on expat software, is susceptible to a "Billion Laughs" denial of service attack through specially crafted XML files. This vulnerability impacts ODF files that comprise a collection of XML files, affecting versions up to 4.1.10.

The Impact of CVE-2021-40439

The impact of CVE-2021-40439 is classified as medium severity, emphasizing the importance of addressing this security issue promptly.

Technical Details of CVE-2021-40439

Let's explore the technical aspects related to CVE-2021-40439.

Vulnerability Description

The vulnerability (CWE-611) in Apache OpenOffice arises from an improper restriction of XML external entity reference, allowing for the "Billion Laughs" entity expansion denial of service attack via crafted XML files.

Affected Systems and Versions

Product: Apache OpenOffice
Vendor: Apache Software Foundation
Versions Affected:
Apache OpenOffice up to version 4.1.10
OpenOffice.org up to version 3.4

Exploitation Mechanism

The exploitation occurs through crafted XML files that trigger the "Billion Laughs" entity expansion, leading to denial of service attacks on Apache OpenOffice installations.

Mitigation and Prevention

To address CVE-2021-40439 effectively, consider the following mitigation strategies:

Immediate Steps to Take

Update Apache OpenOffice to version 4.1.11, where the expat software vulnerability is patched.
Employ security measures to detect and block crafted XML files triggering entity expansion attacks.

Long-Term Security Practices

Regularly update software dependencies to mitigate known vulnerabilities.
Implement XML parsing best practices to restrict entity expansion abuse.

Patching and Updates

Stay informed about security advisories and promptly apply patches released by Apache OpenOffice.