Webmail users are under a new threat. A flaw in Roundcube, a widely-used webmail client, that was fixed in May is now being actively exploited. The Canadian Centre for Cyber Security reports that hackers are using this vulnerability to inject malicious code into email servers. This problem came to light just as organisations are still dealing with other high-profile vulnerabilities targeting JetBrains TeamCity and others. This particular breach affects servers still running the unpatched versions of Roundcube Webmail. The attackers are exploiting a critical vulnerability to gain unauthorized access and control over the affected systems. With each passing day, businesses remain exposed if they have not applied the May patch. It appears that Roundcube servers were patched over a year ago, and this problem is just one in a series of recent critical security flaws that have been discovered. According to Canadian Centre for Cybersecurity, this vulnerability is actively being targeted.
The list of recent security alerts for critical vulnerabilities like those impacting JetBrains TeamCity and Check Point's Security Gateway product shows attackers are quick to exploit known weaknesses. Roundcube has now joined the list of actively exploited vulnerabilities. This underscores the importance of staying vigilant and up-to-date with security patches. If this latest vulnerability is as common in real environments today as previous problems, Roundcube users need to act quickly. The Canadian Centre for Cyber Security has issued a warning to help affected organisations prioritize security patches.
What happened
The Canadian Centre for Cyber Security reported on September 24, 2026, that a critical vulnerability in Roundcube Webmail, identified as CVE-2026-4393, is being actively exploited in code injection attacks. This vulnerability was patched in May 2026. The Canadian Centre for Cyber Security alerted organizations to the issue.
According to BleepingComputer, the vulnerability allows attackers to inject arbitrary code into the web application. This enables them to exploit Roundcube Webmail servers. The attacks are primarily targeting email servers to distribute malicious content such as phishing attempts and malware.
- Who- Canadian Centre for Cyber Security
- What- Vulnerability in roundcube webmail
- Where- email servers globally running Roundcube
- When- since 2026-09-24
The Canadian Centre for Cyber Security did not provide specific details about the number of organizations affected or the origin of the attacks, but advised all users to apply the patch and monitor their systems for unusual activity.
Why it matters
This flaw affects organizations relying on Roundcube Webmail for email services.
Exploits could lead to unauthorized access.
Affected data might include sensitive information like emails, attachments, calendar entries, and contact lists.
This vulnerability can be used to infect other systems.
- According to the Canadian Centre for Cyber Security, the vulnerability has been actively exploited.
Ransomware and malware attacks can be launched from compromised Roundcube instances.
Any IT system that manages or shares data with a vulnerable email server could be at risk.
Other systems connected to the Roundcube instance might fall prey to attacks.
- For example, hackers have exploited similar flaws in software like JetBrains TeamCity recently, as reported by BleepingComputer.
- Check Point also noted active exploitation of a pre-authentication RCE flaw in their Security Gateway VPN, according to BleepingComputer.
Downtime can increase if the server needs to be taken offline for patching.
Data backup and recovery processes could be disrupted.
Patching can require significant downtime if servers need to be taken offline.
What to do
- Immediately patch Roundcube Webmail to protect against code injection attacks, as advised by the Canadian Centre for Cyber Security.
- Check if your email server runs any unpatched software, based on guidance from the Canadian Centre for Cyber Security. Use public CVEs to find affected systems and update accordingly.
- Assess your current email server protection. Include a risk assessment and a remediation plan in your next quarterly vulnerability review.
- Implement an updated vulnerability scanning policy across all mail servers, making sure to check for new CVEs regularly.
- Increase monitoring and logging of all webmail activities and review activity logs frequently to detect any unusual behaviors.
2TI lens
To mitigate evolving threats like the Roundcube exploit, organizations need holistic security frameworks. A Spatial Digital Agency approach could provide an integrated digital workspace that reduces blind spots. By embedding sensors and digital overlays, teams can quickly detect and respond to unusual activity, enhancing overall security resilience.
The Canadian Centre for Cyber Security and BleepingComputer say a critical Roundcube Webmail vulnerability patched in May 2026 is now being actively exploited for code injection attacks.
The US Cybersecurity and Infrastructure Security Agency warned federal agencies on September 23 2026 that ransomware gangs are now exploiting a critical JetBrains TeamCity vulnerability also patched in July. Check Point cybersecurity firm, Check Point confirmed active exploitation of a remote code execution vulnerability in its own Security Gateway product.
Security researchers from The Hacker News DepthFirst reported that an unpatched Ubuntu Linux flaw was allowing container escapes that gained root access on the host. Finally, cybersecurity researchers at The Hacker News also reported a critical Next.js vulnerability allowing attackers to run arbitrary code on the server via crafted SVG input.
Sources
The Hacker News
Canadian Centre for Cyber Security
CISA
Bleeping Computer
Check Point