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
# Example: Configure process resource limits for Dovecot LMTP
# Add to systemd service override for dovecot.service
[Service]
CPUQuota=50%
MemoryLimit=1G
TasksMax=100
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
