CVE-2021-39220 : What You Need to Know

Nextcloud Mail application versions prior to 1.10.4 and 1.11.0 are vulnerable to an image blocking bypass issue.

Understanding CVE-2021-39220

This CVE involves a privacy filter failure that allows images in emails to be displayed, potentially leaking user IP and read state.

What is CVE-2021-39220?

The Nextcloud Mail application, before versions 1.10.4 and 1.11.0, has a default configuration that doesn't block images in emails, leading to privacy risks.

The Impact of CVE-2021-39220

The CVSS v3.1 base score for this vulnerability is 3.5 (Low severity) with user interaction required for exploitation. It could result in the exposure of sensitive information.

Technical Details of CVE-2021-39220

This section dives into the specific technical aspects of the CVE.

Vulnerability Description

The issue stems from the failure of the privacy filter in the Nextcloud Mail application to block images with a relative protocol, potentially exposing user data.

Affected Systems and Versions

Vendor: Nextcloud
Product: security-advisories
Vulnerable Versions: < 1.10.4, < 1.11.0

Exploitation Mechanism

The attack complexity is low, and the vector is through the network with low privileges required and user interaction. This vulnerability does not impact availability or integrity.

Mitigation and Prevention

Protecting systems from this vulnerability is crucial to maintaining security.

Immediate Steps to Take

Upgrade Nextcloud Mail application to version 1.10.4 or 1.11.0.

Long-Term Security Practices

Regularly update applications to patch known vulnerabilities.
Implement email security best practices to minimize privacy risks.
Monitor for any unusual image loading behavior in emails.

Patching and Updates

Ensure all software, including Nextcloud Mail, is kept up to date with the latest security patches to prevent exploitation.