CVE-2026-27859 Open-Xchange GmbH CVE debrief

Who should care

Defenders responsible for mail server configurations, especially those handling LMTP, should assess exposure and prioritize verification to limit the impact of large CPU consumption.

Why it matters

Defenders should care about CVE-2026-27859 because it can lead to large CPU consumption by the mail delivery process, potentially impacting mail server performance. Verification of mail server configurations and possible upgrades to fixed versions are recommended.

• Potential large CPU consumption by the mail delivery process
• Need to verify mail server configurations for LMTP handling
• Possible upgrade to a fixed version of Dovecot

Technical summary

The vulnerability occurs when a mail message contains an excessive amount of RFC 2231 MIME parameters, causing the LMTP to consume large amounts of CPU time. This can be mitigated by limiting RFC 2231 MIME parameters in mail messages or upgrading to a fixed version. The issue affects mail servers handling LMTP and can lead to large CPU consumption by the mail delivery process. Defenders should assess exposure and prioritize verification of mail server configurations to limit the impact of large CPU consumption. The CVE record and NVD entry provide details on the vulnerability, its impact, and potential mitigations.

Defensive priority

Defenders should assess exposure and prioritize verification of mail server configurations, especially those handling LMTP, to limit the impact of large CPU consumption.

Recommended defensive actions

• Assess mail server configurations for LMTP handling and limit RFC 2231 MIME parameters
• Upgrade to a fixed version of Dovecot where processing is limited
• Monitor mail delivery processes for large CPU consumption
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE record and NVD entry provide details on the vulnerability, its impact, and potential mitigations. However, specific details about affected versions and exploitation are limited. Defenders should verify mail server configurations for LMTP handling and review vendor advisories for fixed versions. The CVE Program record and NVD detail page offer source-provided metadata and vulnerability assessments.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.