CVE-2024-42010 Overview
CVE-2024-42010 is an information disclosure vulnerability in Roundcube Webmail's mod_css_styles module. The vulnerability exists due to insufficient filtering of Cascading Style Sheets (CSS) token sequences in rendered e-mail messages. A remote attacker can exploit this flaw to obtain sensitive information from targeted users by sending specially crafted emails containing malicious CSS.
Critical Impact
Remote attackers can exfiltrate sensitive data from Roundcube users without any user interaction beyond viewing a malicious email. This vulnerability poses significant risk to organizations using Roundcube for email services, including government entities.
Affected Products
- Roundcube Webmail through version 1.5.7
- Roundcube Webmail versions 1.6.x through 1.6.7
Discovery Timeline
- 2024-08-04 - Roundcube releases security patches 1.5.8 and 1.6.8
- 2024-08-05 - CVE-2024-42010 published to NVD
- 2024-08-12 - Last updated in NVD database
Technical Details for CVE-2024-42010
Vulnerability Analysis
The vulnerability resides in the mod_css_styles module within Roundcube Webmail, which is responsible for sanitizing and filtering CSS content in rendered email messages. The module fails to properly filter certain CSS token sequences, creating an opportunity for attackers to inject malicious stylesheets that can exfiltrate data.
When Roundcube renders an email containing HTML content, the CSS sanitizer attempts to strip potentially dangerous style directives. However, the filtering logic contains gaps that allow carefully constructed CSS token sequences to bypass the sanitization process. This enables attackers to craft emails with CSS that can leak sensitive information back to attacker-controlled servers.
The vulnerability is classified under CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor), as it allows unauthorized access to information that should remain protected within the user's email session.
Root Cause
The root cause lies in the incomplete CSS token sequence filtering within mod_css_styles. The sanitization logic does not account for all possible CSS constructs that could be used for data exfiltration. Attackers can leverage CSS features such as attribute selectors, URL-based properties, and other CSS mechanisms to extract information from the DOM and transmit it externally.
Attack Vector
The attack is network-based and requires no privileges or user interaction beyond the victim viewing a malicious email. An attacker sends a specially crafted email containing malicious CSS to the target. When the victim opens the email in Roundcube Webmail, the improperly filtered CSS executes within the browser context, potentially leaking sensitive information such as CSRF tokens, email content, or other DOM-accessible data to an attacker-controlled endpoint.
The attack flow involves:
- Attacker crafts an email with malicious CSS token sequences designed to bypass mod_css_styles filtering
- Email is delivered to the victim's Roundcube mailbox
- Victim opens the email in their browser
- Malicious CSS renders and executes data exfiltration techniques
- Sensitive information is transmitted to attacker infrastructure
Detection Methods for CVE-2024-42010
Indicators of Compromise
- Unusual outbound network connections originating from user browsers when viewing emails
- Emails containing complex or obfuscated CSS constructs with external URL references
- HTTP requests to unknown external domains triggered during email viewing sessions
- Anomalous CSS patterns in email content logs that include attribute selectors or URL functions
Detection Strategies
- Monitor web application firewall (WAF) logs for suspicious CSS patterns in inbound email content
- Implement content security policies (CSP) to restrict external resource loading from email rendering contexts
- Analyze email gateway logs for messages containing unusual CSS constructs
- Deploy network monitoring to detect data exfiltration attempts via CSS-based techniques
Monitoring Recommendations
- Enable verbose logging for Roundcube email rendering processes
- Monitor DNS queries for unexpected external domain lookups during email sessions
- Implement browser-side monitoring for unauthorized network requests
- Review Roundcube access logs for patterns indicating exploitation attempts
How to Mitigate CVE-2024-42010
Immediate Actions Required
- Upgrade Roundcube Webmail to version 1.5.8 or 1.6.8 immediately
- Review email logs for any signs of exploitation attempts
- Consider temporarily disabling HTML email rendering if immediate patching is not possible
- Notify users about the vulnerability and advise caution with unexpected emails
Patch Information
Roundcube has released security updates addressing this vulnerability. Organizations should upgrade to the patched versions:
- For the 1.5.x branch: Upgrade to Roundcube 1.5.8
- For the 1.6.x branch: Upgrade to Roundcube 1.6.8
Full release notes and download links are available at the Roundcube Security Updates Announcement and GitHub Releases page.
Workarounds
- Disable HTML message display and force plain-text rendering until patches can be applied
- Implement strict Content Security Policy headers to block external resource loading
- Configure email gateways to strip or sanitize CSS content from incoming messages
- Use network-level controls to restrict outbound connections from webmail servers
# Roundcube configuration to disable HTML messages (config/config.inc.php)
$config['prefer_html'] = false;
$config['show_images'] = 0;
$config['htmleditor'] = 0;
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.


