CVE-2024-23184: Dovecot Email Server DoS Vulnerability

CVE-2024-23184 Overview

CVE-2024-23184 is a resource exhaustion vulnerability affecting Dovecot mail server that occurs when processing emails with an excessive number of address headers (From, To, Cc, Bcc, etc.). The vulnerability allows external attackers to send specially crafted messages that consume excessive CPU resources on the target system, potentially causing service outages.

Critical Impact
External attackers can cause denial of service conditions by sending emails with large numbers of address headers, consuming CPU resources for extended periods (up to 18 minutes observed for 500k header lines).

Affected Products

Dovecot Mail Server (versions prior to security patch)
Systems running Dovecot without MTA-level address header restrictions

Discovery Timeline

September 10, 2024 - CVE-2024-23184 published to NVD
November 4, 2025 - Last updated in NVD database

Technical Details for CVE-2024-23184

Vulnerability Analysis

This vulnerability is classified under CWE-770 (Allocation of Resources Without Limits or Throttling). The core issue lies in Dovecot's email header parsing mechanism, which does not impose adequate limits on the number of address headers processed within a single email message. When an email contains an abnormally large number of address headers, the parsing operation becomes excessively CPU intensive due to algorithmic inefficiency in processing these headers.

Performance testing revealed that processing 100,000 header lines requires approximately 12 seconds of CPU time. In production environments, emails with 500,000 header lines were observed taking 18 minutes to parse. This disproportionate resource consumption relative to input size indicates potential algorithmic complexity issues, where the processing time may scale non-linearly with the number of headers.

Root Cause

The root cause is the lack of resource allocation limits during the address header parsing phase. Dovecot's header parser processes each address header without enforcing maximum thresholds, allowing malicious actors to exploit this unlimited processing capability. This falls under CWE-770, where the application allocates resources (CPU cycles) based on untrusted input without implementing appropriate limits or throttling mechanisms.

Attack Vector

The attack is network-based and can be executed by external actors sending specially crafted emails to victim systems. The attacker constructs an email message containing thousands or hundreds of thousands of address headers (From, To, Cc, Bcc fields). When the Dovecot server receives and attempts to parse this email, the excessive header count triggers prolonged CPU consumption.

The attack requires low privileges (basic ability to send email) and does not require user interaction, as the parsing occurs automatically upon email receipt. The vulnerability affects system availability by exhausting CPU resources, potentially impacting other services on the same host.

Detection Methods for CVE-2024-23184

Indicators of Compromise

Abnormally high CPU utilization on mail server processes during email processing
Extended email delivery times or mail queue backlogs
Incoming emails with unusually large header sections (hundreds of thousands of address headers)
Dovecot process consuming CPU for extended periods on single message parsing

Detection Strategies

Monitor Dovecot process CPU usage for anomalous spikes during mail processing operations
Implement logging at the MTA level to track emails with excessive header counts before they reach Dovecot
Configure alerts for mail delivery latency exceeding normal thresholds
Analyze mail server logs for patterns indicating slow message processing

Monitoring Recommendations

Deploy CPU monitoring with alerting thresholds on Dovecot server processes
Implement MTA-level header counting and logging for incoming messages
Monitor mail queue depth and processing times for sudden degradation
Track system resource metrics (CPU, memory) during peak mail processing periods

How to Mitigate CVE-2024-23184

Immediate Actions Required

Apply vendor security patches for Dovecot as soon as available
Implement address header restrictions at the MTA component preceding Dovecot
Configure MTA rejection policies for emails exceeding reasonable header counts
Monitor systems for signs of active exploitation while patching

Patch Information

Security patches addressing this vulnerability are available from Dovecot/Open-Xchange. Administrators should consult the Open-Xchange Security Advisory for patch details and version-specific guidance. Additional information is available through the Debian LTS Announcement for Debian-based systems.

Workarounds

Configure the MTA (Mail Transfer Agent) preceding Dovecot to reject or quarantine emails with excessive address headers
Implement rate limiting on incoming mail connections to slow potential attack attempts
Set up resource limits (ulimit, cgroups) on Dovecot processes to contain CPU consumption
Consider temporary mail filtering rules to drop emails with abnormal header patterns until patching is complete

bash

# Example Postfix header_checks configuration to limit address headers
# Add to /etc/postfix/header_checks
/^(From|To|Cc|Bcc):/  WARN

# Main.cf configuration to enable header checks
header_checks = regexp:/etc/postfix/header_checks

# Consider implementing milter-based filtering for more granular control
# Reload Postfix after changes
postfix reload