CVE-2023-39955 : What You Need to Know

A vulnerability has been identified in the Notes attachment rendering feature of Nextcloud, allowing for HTML code to be executed in preview mode, potentially leading to a cross-site scripting (XSS) attack.

Understanding CVE-2023-39955

This CVE affects Nextcloud's Notes app, specifically versions equal to or greater than 4.4.0 and less than 4.8.0. HTML content within note files is improperly rendered in preview mode, exposing users to XSS vulnerabilities.

What is CVE-2023-39955?

The CVE-2023-39955 vulnerability in Nextcloud's Notes app enables attackers to inject and execute malicious HTML code within note files. This can result in a cross-site scripting (XSS) attack, compromising user data and system integrity.

The Impact of CVE-2023-39955

The impact of this vulnerability is significant as it allows threat actors to manipulate the HTML content of note files, potentially leading to the execution of unauthorized scripts within the application. Exploitation of this vulnerability can result in unauthorized access to sensitive information and compromise user privacy.

Technical Details of CVE-2023-39955

The vulnerability is classified as CWE-79, indicating improper neutralization of input during web page generation. The CVSSv3.1 base score for this CVE is 3.5, with low severity due to requirements such as low privileges and user interaction.

Vulnerability Description

The flaw allows HTML code to be executed in preview mode, enabling attackers to launch cross-site scripting attacks by injecting malicious scripts into note files.

Affected Systems and Versions

Nextcloud's Notes app versions >= 4.4.0 and < 4.8.0 are affected by this vulnerability, specifically impacting users who create note files containing HTML content.

Exploitation Mechanism

Attackers can exploit the vulnerability by crafting malicious note files with embedded HTML code. When users preview these files within the Notes app, the HTML content is executed, potentially leading to XSS attacks.

Mitigation and Prevention

To mitigate the risks associated with CVE-2023-39955, affected users and organizations should take immediate action to secure their systems and data.

Immediate Steps to Take

Update Nextcloud's Notes app to version 4.8.0 or later, which includes a patch for this vulnerability.
Educate users about the risks of opening note files containing HTML content.

Long-Term Security Practices

Regularly monitor for security advisories and updates from Nextcloud to stay informed about potential vulnerabilities.
Implement content security policies to prevent the execution of unauthorized scripts within applications.

Patching and Updates

Nextcloud has provided a patch in version 4.8.0 to address the vulnerability. It is essential for users to promptly apply this update to protect their systems from exploitation.