CVE-2026-27859: LMTP RFC 2231 MIME DoS Vulnerability

CVE-2026-27859 Overview

CVE-2026-27859 is a resource exhaustion vulnerability affecting Dovecot's LMTP (Local Mail Transfer Protocol) service. A specially crafted mail message containing an excessive number of RFC 2231 MIME parameters can cause the mail delivery process to consume large amounts of CPU time, leading to a denial of service condition. This vulnerability allows remote attackers to degrade mail server performance without requiring authentication.

Critical Impact
Remote attackers can disrupt mail delivery services by sending maliciously crafted emails with excessive RFC 2231 MIME parameters, causing CPU exhaustion on affected Dovecot LMTP servers.

Affected Products

Dovecot LMTP Service (versions prior to patched release)
Open-Xchange Dovecot deployments

Discovery Timeline

2026-03-27 - CVE-2026-27859 published to NVD
2026-03-30 - Last updated in NVD database

Technical Details for CVE-2026-27859

Vulnerability Analysis

This vulnerability is classified as CWE-400 (Uncontrolled Resource Consumption). The flaw resides in how Dovecot's LMTP process handles RFC 2231 MIME parameter continuations in email messages. RFC 2231 defines a mechanism for encoding extended parameter values in MIME headers, allowing parameters to be split across multiple lines for character set encoding and language tagging purposes.

When the LMTP service receives an email containing an abnormally large number of these MIME parameters, the parsing and processing logic consumes disproportionate CPU resources. The lack of proper limits on the number of RFC 2231 parameters that can be processed allows an attacker to craft messages that trigger algorithmic complexity issues during mail delivery.

Root Cause

The root cause stems from insufficient validation and limiting of RFC 2231 MIME parameter counts during email processing. The LMTP service fails to implement adequate bounds checking on the number of MIME parameter continuations it will process per message, allowing attackers to abuse this parsing behavior for resource exhaustion attacks.

Attack Vector

The attack can be executed remotely over the network without requiring any authentication or user interaction. An attacker simply needs to send a specially formatted email to a mail server running the vulnerable Dovecot LMTP service. The malicious email would contain MIME headers with an excessive number of RFC 2231 encoded parameters, designed to maximize CPU consumption during parsing.

The vulnerability manifests during the MIME parameter parsing phase of mail delivery. When the LMTP service processes the crafted message, the excessive parameters cause the delivery process to enter a computationally expensive processing loop. See the Open-Xchange Security Advisory for additional technical details.

Detection Methods for CVE-2026-27859

Indicators of Compromise

Abnormally high CPU usage by Dovecot LMTP processes
Mail delivery delays or timeouts coinciding with receipt of specific messages
Unusual MIME parameter patterns in mail logs showing excessive RFC 2231 continuations
Process monitoring alerts for LMTP workers consuming extended CPU cycles

Detection Strategies

Monitor LMTP process CPU utilization and alert on sustained spikes above baseline thresholds
Implement mail content inspection rules to flag messages with abnormally high MIME parameter counts
Configure logging to capture detailed MIME parsing information for forensic analysis
Deploy network-based email inspection to identify malformed RFC 2231 parameter patterns before delivery

Monitoring Recommendations

Set up real-time monitoring of Dovecot LMTP process resource consumption metrics
Configure alerting thresholds for mail delivery latency increases that may indicate exploitation attempts
Implement centralized log aggregation for mail server processes to correlate potential attack patterns
Establish baseline metrics for normal MIME parameter processing to detect anomalies

How to Mitigate CVE-2026-27859

Immediate Actions Required

Upgrade to the patched version of Dovecot that includes RFC 2231 parameter processing limits
Configure MTA (Mail Transfer Agent) capabilities to limit RFC 2231 MIME parameters in incoming mail messages
Implement rate limiting on incoming mail connections to reduce the impact of exploitation attempts
Consider temporarily increasing LMTP process resources while preparing for patching

Patch Information

Open-Xchange has released a security advisory addressing this vulnerability. Administrators should consult the Open-Xchange Security Advisory for specific patch information and upgrade instructions. The fixed version implements proper limits on RFC 2231 MIME parameter processing to prevent CPU exhaustion.

Workarounds

Configure upstream MTA to filter or reject emails with excessive MIME parameters before they reach Dovecot
Implement mail relay policies that sanitize RFC 2231 encoded parameters
Deploy email security gateways with content inspection capabilities to block malicious messages
Use process isolation and resource limits (cgroups, CPU quotas) to contain LMTP process resource consumption

bash

# Example: Configure process resource limits for Dovecot LMTP
# Add to systemd service override for dovecot.service
[Service]
CPUQuota=50%
MemoryLimit=1G
TasksMax=100