CSS Sanitization Flaw in Roundcube Webmail

Executive Summary

CVE-2026-62643 is a vulnerability in Roundcube Webmail versions before 1.6.17 and 1.7.x before 1.7.2. It involves insufficient sanitization of Cascading Style Sheets (CSS) in HTML email messages. This flaw allows attackers to craft malicious stylesheet links that point to local network hosts, leading to Server-Side Request Forgery (SSRF) or Information Disclosure.

The vulnerability exists because earlier fixes for CVE-2026-35540 and CVE-2026-48843 were insufficient. Specifically, certain local address URLs could bypass security checks, enabling attackers to exploit SSRF vulnerabilities. The issue was addressed by improving the detection of local addresses in the `is_local_url` function, including handling IPv6-mapped IPv4 addresses and domain patterns like `.nip.io` and `.sslip.io`.

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Detection Guidance

Detecting this vulnerability on your network or system involves checking if your Roundcube Webmail instance is running a vulnerable version (before 1.6.17 or 1.7.x before 1.7.2). You can verify the installed version by accessing the Roundcube Webmail interface or checking the version file directly.

• Check the Roundcube version via the command line: cat /path/to/roundcube/program/include/iniset.php | grep RCMAIL_VERSION. Replace /path/to/roundcube with your actual Roundcube installation directory.
• Inspect web server logs for suspicious requests to local or internal network addresses (e.g., 127.0.0.1, ::1, or domains like .nip.io and .sslip.io) originating from Roundcube Webmail. Look for patterns such as HTTP requests to these addresses in the logs.
• Use network monitoring tools to detect outbound requests from the Roundcube server to unexpected local or internal hosts. Tools like tcpdump or Wireshark can help capture and analyze such traffic.

If you are unable to determine the version or detect suspicious activity, assume the system may be vulnerable if it has not been updated to the fixed versions (1.6.17, 1.7.2, or later).

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Impact Analysis

If you are using an affected version of Roundcube Webmail, this vulnerability could impact you in several ways:

• Attackers could exploit the SSRF vulnerability to send requests from your server to internal systems, potentially accessing sensitive data or services that are not publicly exposed.
• Information Disclosure risks arise if attackers can trick the server into fetching and returning data from internal resources, such as configuration files or internal network details.
• The vulnerability could be used as a stepping stone for further attacks, such as pivoting to other internal systems or escalating privileges within your network.

To mitigate these risks, it is recommended to update Roundcube Webmail to version 1.6.17 or 1.7.2 or later, where the issue has been fixed.

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Compliance Impact

This vulnerability could impact compliance with several common standards and regulations in the following ways:

• GDPR (General Data Protection Regulation): If the vulnerability leads to unauthorized access or disclosure of personal data, it could result in a violation of GDPR. Organizations are required to implement appropriate security measures to protect personal data, and failing to patch known vulnerabilities could be seen as a failure to meet these obligations.
• HIPAA (Health Insurance Portability and Accountability Act): For organizations handling protected health information (PHI), this vulnerability could lead to unauthorized access to sensitive health data. HIPAA requires covered entities to protect PHI from security threats, and exploiting this vulnerability could result in non-compliance.
• Other standards like ISO 27001 or NIST frameworks: These standards emphasize the importance of vulnerability management and protecting systems from known threats. Failure to address this vulnerability could result in non-compliance with these frameworks, potentially leading to audit failures or loss of certifications.

To maintain compliance, organizations should promptly apply the available patches and ensure their systems are protected against this vulnerability.

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Mitigation Strategies

To mitigate this vulnerability, follow these immediate steps:

• Upgrade Roundcube Webmail to the latest patched version (1.6.17 or 1.7.2 or later) as soon as possible. The fixes address the insufficient CSS sanitization and SSRF bypass issues described in the vulnerability.
• If upgrading is not immediately possible, apply the patches referenced in the resources to the `is_local_url` function in `rcube_utils.php`. The patches improve detection of local addresses, including IPv6-mapped IPv4 addresses and domains like .nip.io and .sslip.io.
• Restrict outbound network traffic from the Roundcube server to prevent potential SSRF attacks. Use firewall rules to block requests to local or internal network addresses unless explicitly required.
• Monitor Roundcube Webmail logs for any suspicious activity, such as requests to unexpected local or internal hosts. Investigate and block any malicious attempts.
• Disable HTML email rendering if it is not required for your use case. This reduces the attack surface by preventing malicious CSS or links in emails from being processed.

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