Ransomware continues to evolve. While well-known groups like LockBit, Cl0p, and BlackCat dominate the headlines with high-profile attacks, a new wave of emerging ransomware groups is turning its attention to less defended systems—particularly open-source email platforms. These actors are exploiting vulnerabilities in software such as Zimbra Collaboration Suite, often with a focus on data theft, extortion, and reputation damage rather than system encryption.
This blog explores the changing nature of the ransomware threat and includes a technical bulletin focused on hardening Zimbra and similar platforms.
Beyond Encryption: A Shift in Strategy
Several of the most active emerging ransomware and extortion groups in 2024 and 2025 have moved away from traditional encryption-based attacks. Instead, they favour the exfiltration of email data, including messages, attachments, and contact information, followed by low-pressure extortion attempts.
Groups such as MalasLocker, Dunghill Leak, and KelvinSec have focused heavily on organisations running open-source or self-hosted email platforms, especially in the public sector, education, and civil society. These groups typically:
- Exploit known vulnerabilities in public-facing email interfaces
- Extract inbox data and directories
- Post samples on dark web forums or Telegram channels
- Issue demands or simply leak the data publicly
This model allows threat actors to achieve impact with minimal effort. Many targets are unaware they have been compromised until their data is published.
Why Zimbra Is at Risk
Zimbra Collaboration Suite is widely used by local government, educational institutions, and small to medium-sized businesses across the UK and Europe. Its popularity stems from its flexibility, open-source licensing, and lower cost compared to enterprise platforms. However, it also presents several security risks:
- Frequent patch cycles and short timelines between vulnerability disclosure and exploitation
- Wide internet exposure, particularly for webmail and admin panels
- Limited inbuilt logging and alerting unless properly configured
- Third-party plugin ecosystem, which can introduce additional vulnerabilities
For threat actors, Zimbra provides a high-yield, low-complexity target—especially when administrators fail to apply patches or restrict access to admin panels.
Technical Bulletin: Securing Zimbra and Open-Source Email Platforms
This section provides actionable steps to secure Zimbra and other self-hosted email systems against emerging ransomware threats and data extortion groups.
Patch Management
- Regularly monitor the Zimbra Security Centre for new CVEs
- Apply patches within 48 hours of release, particularly those affecting authentication, webmail, and admin interfaces
- Review historical CVEs, especially those affecting versions 8.8.x and 9.x, and ensure legacy servers are updated
Access Control
- Restrict access to Zimbra’s admin interface to trusted IP ranges only
- Enforce strong, unique passwords for all admin accounts
- Apply rate-limiting or fail2ban to mitigate brute-force attempts on webmail login pages
- Use HTTPS with a trusted SSL certificate and disable older TLS protocols
Authentication and Monitoring
- Enable multi-factor authentication (MFA) for admin users and high-risk staff
- Monitor login attempts for failed access or unusual geographic patterns
- Enable Zimbra’s logging features, including mailbox audit logs and authentication logs
- Use tools like fail2ban or OSSEC to generate alerts for suspicious activity
Email Content Protection
- Review attachment filtering policies to block executable files and suspicious file types
- Disable auto-execution of HTML or JavaScript within webmail clients
- Implement spam and malware filtering using tools such as Amavis, ClamAV, or SpamAssassin
- Integrate with DLP (data loss prevention) tools where possible to monitor email exfiltration
Backup and Incident Response
- Maintain offsite backups of mailboxes and configuration files
- Encrypt backups and test restoration procedures quarterly
- Develop an incident response plan specifically for email compromise or data theft scenarios
- Subscribe to threat intelligence feeds that monitor for leaked email datasets or credentials
What Makes These Threats Different?
Unlike conventional ransomware, groups targeting Zimbra often avoid deploying malware. Their behaviour may include:
- Logging into the webmail interface with stolen credentials
- Exporting entire inboxes or calendar entries via the user interface or IMAP
- Uploading a script or web shell via the Zimbra file system, if accessible
- Initiating extortion through anonymous email or by listing the organisation on a leak site
Because no files are encrypted, many organisations do not treat the intrusion as critical until reputational damage has occurred.
Recommendations for At-Risk Organisations
Organisations relying on self-hosted or open-source email systems should consider the following strategic actions:
- Conduct a full vulnerability assessment of email infrastructure
- Review all publicly exposed ports and admin interfaces
- Restrict administrative access by IP and enforce strong authentication
- Regularly back up and test email data recovery
- Train staff to recognise suspicious login alerts or signs of inbox access
If moving to a managed platform is not an option, hardening and visibility are essential. Even a basic SIEM or endpoint monitoring solution can reveal early signs of compromise.
Conclusion
Emerging ransomware and extortion actors are adapting to the changing threat landscape by targeting overlooked and undersecured platforms like Zimbra. These attacks are quiet, fast, and focused on data theft—not disruption. For UK organisations in education, local government, and small enterprise sectors, it is vital to address these gaps before attackers do.
Security should not be reserved for large cloud platforms. With practical steps and strong operational hygiene, even modest organisations can defend against low-complexity but high-impact threats.
