TNEF Decoder DoS in Roundcube Webmail

Executive Summary

CVE-2026-62641 is a denial-of-service (DoS) vulnerability in Roundcube Webmail versions before 1.6.17 and 1.7.x before 1.7.2.

The vulnerability occurs in the TNEF (Transport Neutral Encapsulation Format) decoder, which processes winmail.dat attachments. Specifically, it involves crafted compressed-RTF (Rich Text Format) sizes within these attachments.

An attacker can exploit this flaw by sending a specially crafted TNEF file with an abnormally large compressed-RTF size. When Roundcube processes this file, it may consume excessive memory or computational resources, leading to a denial-of-service condition.

The issue was fixed by adding a check in the rcube_tnef_decoder.php file to prevent the decompression process from exceeding expected size limits, thereby mitigating the risk of resource exhaustion.

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Detection Guidance

Detecting CVE-2026-62641 on your network or system involves checking for the presence of vulnerable Roundcube Webmail versions and monitoring for suspicious TNEF (winmail.dat) attachments that may exploit the compressed-RTF size issue.

• Check the installed Roundcube Webmail version. If it is before 1.6.17 or 1.7.2, it is vulnerable. You can verify the version by accessing the Roundcube installation directory and checking the version in the `program/include/iniset.php` file or via the Roundcube web interface under 'About'.
• Monitor email attachments for TNEF (winmail.dat) files, especially those with compressed-RTF content. These files can be identified by their MIME type (application/ms-tnef) or file extension (.dat).
• Use network monitoring tools to detect unusual traffic patterns or resource consumption spikes when processing TNEF attachments. Tools like Wireshark or Suricata can help identify suspicious email traffic.
• Review Roundcube logs for errors or crashes related to the TNEF decoder. Look for entries in the Roundcube error logs (e.g., `logs/errors`) that mention `rcube_tnef_decoder.php` or decompression failures.

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Impact Analysis

If you are using an affected version of Roundcube Webmail (before 1.6.17 or 1.7.x before 1.7.2), this vulnerability could impact you in the following ways:

• Denial-of-Service (DoS): An attacker could send a malicious winmail.dat attachment to your Roundcube email server. Processing this attachment could cause excessive memory or CPU usage, leading to slowdowns or crashes of the Roundcube service.
• Service Disruption: If the Roundcube service becomes unresponsive or crashes, users may be unable to access their emails, disrupting communication and workflow.
• Resource Exhaustion: The vulnerability could lead to high resource consumption on the server, potentially affecting other services running on the same machine.

To mitigate this risk, it is recommended to update Roundcube Webmail to version 1.6.17 or 1.7.2 or later.

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Compliance Impact

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

• GDPR (General Data Protection Regulation): Under GDPR, organizations must ensure the availability and resilience of processing systems and services. A denial-of-service vulnerability that disrupts email services could violate these requirements, potentially leading to non-compliance if not addressed promptly.
• HIPAA (Health Insurance Portability and Accountability Act): For organizations handling protected health information (PHI), HIPAA requires ensuring the confidentiality, integrity, and availability of electronic PHI. A DoS vulnerability that disrupts access to email systems containing PHI could be seen as a failure to maintain availability, potentially resulting in non-compliance.
• Other Standards: Many compliance frameworks, such as ISO 27001, require organizations to manage information security risks, including those related to system availability. Failure to patch known vulnerabilities like this one could be viewed as a lapse in risk management, potentially affecting certification or compliance status.

To maintain compliance, organizations should apply the available security updates for Roundcube Webmail and ensure their systems are protected against known vulnerabilities.

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Mitigation Strategies

To mitigate CVE-2026-62641, follow these immediate steps:

• Upgrade Roundcube Webmail to the latest patched version (1.6.17 or 1.7.2) as soon as possible. These versions include the fix for the compressed-RTF size issue in the TNEF decoder.
• If upgrading is not immediately possible, consider temporarily disabling the TNEF decoder by removing or renaming the `rcube_tnef_decoder.php` file in the Roundcube installation directory. Note that this may break functionality for users who rely on winmail.dat attachments.
• Implement email filtering rules to block or quarantine TNEF (winmail.dat) attachments, especially those with compressed-RTF content. This can be done at the email server level (e.g., using Postfix, Exim, or a dedicated email security gateway).
• Monitor Roundcube logs and system resources for signs of exploitation, such as unusual memory consumption or crashes related to TNEF processing.
• Educate users about the risks of opening winmail.dat attachments from untrusted sources until the system is patched.

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