CVE-2026-27857: IMAP Server DOS Vulnerability

CVE-2026-27857 Overview

CVE-2026-27857 is a memory exhaustion vulnerability in Dovecot's IMAP proxy that allows authenticated attackers to trigger excessive memory allocation through specially crafted NOOP commands. By sending a NOOP command with approximately 4000 nested parentheses, an attacker can cause approximately 1MB of extra memory allocation per connection. This memory can be held for extended periods by not sending the command terminating line feed (LF) character.

Critical Impact

An attacker operating from a single IP address can establish 1000 connections to allocate approximately 1GB of memory, potentially reaching the Virtual Size (VSZ) limit and causing process termination, disrupting all proxied IMAP connections.

Affected Products

• Dovecot IMAP Server (specific affected versions documented in vendor advisory)

Discovery Timeline

• 2026-03-27 - CVE-2026-27857 published to NVD
• 2026-03-30 - Last updated in NVD database

Technical Details for CVE-2026-27857

Vulnerability Analysis

This vulnerability is classified as CWE-400 (Uncontrolled Resource Consumption). The flaw exists in how Dovecot's IMAP proxy handles the parsing of deeply nested parentheses within NOOP commands. While the NOOP command is typically a simple no-operation command used to keep connections alive and reset inactivity timers, the parser allocates memory proportional to the nesting depth of parentheses in the command argument.

The attack requires network access and low-privilege authentication to establish IMAP connections. The impact is limited to availability, with no effect on confidentiality or integrity of data. However, the attack can be amplified by holding connections open without completing the command, allowing memory to remain allocated indefinitely.

Root Cause

The root cause is improper resource management in the IMAP command parser. When processing commands with deeply nested parentheses structures like NOOP (((...))), the parser allocates memory for each level of nesting without adequate limits or cleanup. The implementation fails to bound the memory consumption based on the complexity of the input, allowing an attacker to craft commands that consume disproportionate amounts of memory relative to their legitimate utility.

Attack Vector

The attack is conducted over the network against the IMAP service. An attacker must first authenticate to the IMAP server (requiring low privileges). The exploitation sequence involves:

1. Establishing multiple authenticated IMAP connections to the target server
2. Sending malformed NOOP commands containing approximately 4000 open and close parentheses (e.g., NOOP ((((...)))))
3. Deliberately withholding the terminating line feed character to prevent the command from completing
4. Repeating this process across hundreds or thousands of connections
5. The accumulated memory consumption eventually causes the Dovecot process to hit its VSZ limit and terminate

The attack does not require sophisticated tooling, as standard IMAP clients or scripts can be used to construct and send the malformed commands. No publicly available exploits are currently known.

Detection Methods for CVE-2026-27857

Indicators of Compromise

• Unusual IMAP NOOP commands containing excessive parentheses characters in server logs
• Abnormally high memory consumption by Dovecot processes without corresponding legitimate user activity
• Multiple simultaneous IMAP connections from single IP addresses that remain idle or incomplete
• Dovecot process crashes with out-of-memory or VSZ limit exceeded errors

Detection Strategies

• Implement network intrusion detection rules to identify IMAP commands with abnormally long or deeply nested parentheses structures
• Monitor Dovecot process memory usage and alert when consumption patterns deviate from baseline
• Configure logging to capture full IMAP command text for forensic analysis of potential exploitation attempts
• Deploy rate limiting on IMAP connections per source IP address

Monitoring Recommendations

• Set up alerts for Dovecot process memory consumption exceeding normal operational thresholds
• Monitor connection counts per IP address and implement anomaly detection for sudden spikes
• Track IMAP command parsing errors and incomplete command timeouts as potential attack indicators
• Enable verbose IMAP protocol logging temporarily if an attack is suspected to capture malicious commands

How to Mitigate CVE-2026-27857

Immediate Actions Required

• Update Dovecot to the latest patched version as indicated in the Open-Xchange Security Advisory
• Implement connection rate limiting at the firewall or load balancer level to reduce attack surface
• Configure Dovecot process resource limits (memory cgroups, ulimits) to prevent single-process memory exhaustion from affecting the entire system
• Consider implementing IP-based connection limits for IMAP services

Patch Information

The vendor has released a fixed version addressing this vulnerability. According to the security advisory, installing the fixed version is the only recommended remediation path. Organizations should consult the Open-Xchange Security Advisory for specific version information and patch details.

Workarounds

• No effective workarounds are available according to the vendor advisory - upgrading to the fixed version is required
• Temporary mitigations include implementing aggressive connection rate limiting and per-IP connection caps at the network layer
• Configure monitoring and automatic service restart to minimize downtime impact if exploitation occurs
• Consider temporarily disabling IMAP proxy functionality if not critical to operations while awaiting patch deployment

# Example: Configure connection limits in iptables to reduce attack surface
# Limit new IMAP connections to 10 per minute per IP
iptables -A INPUT -p tcp --dport 143 -m state --state NEW -m recent --set
iptables -A INPUT -p tcp --dport 143 -m state --state NEW -m recent --update --seconds 60 --hitcount 10 -j DROP

# Example: Set memory limits for Dovecot process via systemd override
# Create /etc/systemd/system/dovecot.service.d/limits.conf
# [Service]
# MemoryMax=2G
# MemoryHigh=1.5G