CVE-2026-27857 Open Xchange CVE debrief

Who should care

Administrators and users of Dovecot email servers should be aware of this vulnerability and take steps to mitigate it. This vulnerability could be particularly problematic for organizations that rely on Dovecot for email services, as a successful exploit could lead to a denial-of-service condition, impacting email availability.

Technical summary

The vulnerability exists in the Dovecot email server, specifically in its handling of IMAP and POP3 commands. When a command with a large number of parentheses (such as 'NOOP (((...)))') is sent to the server, it can cause the server to allocate an excessive amount of memory. If the command is not properly terminated with a newline character (LF), the allocated memory can remain in use for an extended period. An attacker could exploit this vulnerability by establishing multiple connections from a single IP address, each allocating a large amount of memory, potentially causing the Dovecot process to be terminated due to reaching its virtual size (VSZ) limit. This would not only affect the exploited process but also impact other proxied connections.

Defensive priority

Medium priority should be given to patching this vulnerability, as it could be exploited to cause a denial-of-service condition. Given the medium CVSS score, it is essential to apply the fix in a timely manner, especially if the Dovecot server is exposed to untrusted networks or users.

Recommended defensive actions

• Apply the fixed version of Dovecot as recommended by the vendor.
• Review and update access controls to limit connections from untrusted sources.
• Monitor Dovecot server performance and memory usage for anomalies.
• Consider implementing rate limiting for connections from single IP addresses.
• Keep Dovecot and related software up-to-date with the latest security patches.

Evidence notes

The CVE-2026-27857 vulnerability was publicly disclosed on March 27, 2026, and has been modified on June 30, 2026. The vulnerability affects Dovecot versions prior to 2.4.3, 3.0.5, and 3.1.4, depending on the specific configuration and version in use. The Common Vulnerabilities and Exposures (CVE) score for this vulnerability is 4.3, indicating a medium level of severity. The Common Weakness Enumeration (CWE) for this vulnerability is CWE-400 (Uncontrolled Resource Consumption) and CWE-770 (Allocation of Resources Without Limits or Throttling).

Official resources