CVE-2021-32707 in Mail
Summary
by MITRE • 07/13/2021
Nextcloud Mail is a mail app for Nextcloud. In versions prior to 1.9.6, the Nextcloud Mail application does not, by default, render images in emails to not leak the read state. The privacy filter failed to filter images with a `background-image` CSS attribute. Note that the images were still passed through the Nextcloud image proxy, and thus there was no IP leakage. The issue was patched in version 1.9.6 and 1.10.0. No workarounds are known to exist.
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Analysis
by VulDB Data Team • 05/03/2025
The vulnerability CVE-2021-32707 affects the Nextcloud Mail application, which serves as a mail client within the Nextcloud ecosystem. This security flaw exists in versions prior to 1.9.6 and represents a privacy concern that undermines the application's intended protection mechanisms. The Nextcloud Mail application was designed with privacy as a core principle, specifically implementing a default setting that prevents automatic image rendering in emails to avoid revealing user read states. This approach aligns with standard privacy practices that prevent tracking mechanisms from being activated through simple email opening actions. The security model relies on the assumption that email clients should not automatically load external resources that could be used for user tracking or behavioral analysis.
The technical flaw in this vulnerability stems from an incomplete implementation of the privacy filter mechanism. While the application correctly blocked standard image tags and basic CSS image references, it failed to properly handle emails containing background-image CSS attributes. This oversight allowed malicious actors to craft emails that would bypass the privacy protection measures through the use of CSS background properties. The vulnerability specifically targets the CSS background-image attribute, which is a legitimate web development feature that can be used to display images as backgrounds within email content. The flaw demonstrates a classic case of incomplete input validation where the security filter only addressed certain image loading mechanisms while leaving others unaddressed. This represents a CWE-20 issue related to improper input validation, where the application fails to properly sanitize or filter all potential attack vectors.
The operational impact of this vulnerability extends beyond simple privacy concerns to potentially enable sophisticated tracking mechanisms. Although the Nextcloud image proxy did prevent direct IP address leakage, the background-image bypass allowed for indirect tracking through the image loading process. When users opened affected emails, the background images would still be loaded through the Nextcloud proxy system, potentially revealing user activity patterns and device information. This creates a scenario where threat actors could determine when users accessed specific emails through the timing and frequency of image requests. The vulnerability essentially undermines the privacy-by-design approach that Nextcloud implements, potentially exposing user behavior to surveillance. From an ATT&CK perspective, this vulnerability maps to T1566 (Phishing) and T1071.004 (Application Layer Protocol: DNS) as it enables more sophisticated tracking mechanisms that could be used to enhance phishing campaigns or gather intelligence about user behavior.
The fix implemented in versions 1.9.6 and 1.10.0 addresses the root cause by enhancing the privacy filter to properly handle background-image CSS attributes. This update represents a comprehensive approach to the vulnerability by ensuring that all potential image loading mechanisms are properly sanitized. The patch demonstrates the importance of thorough testing and validation of security filters, particularly in applications that handle user privacy as a core feature. Organizations using Nextcloud Mail should immediately upgrade to the patched versions to eliminate the risk of indirect tracking through background image loading. The lack of known workarounds makes this vulnerability particularly concerning as users cannot implement temporary mitigations while waiting for the official update. This vulnerability highlights the critical need for comprehensive security testing that covers all potential CSS and HTML attributes that could be exploited for privacy bypass mechanisms. The incident serves as a reminder that even seemingly minor implementation details in privacy protection systems can create significant security gaps that may be exploited for user tracking purposes.