CVE-2017-2669 in Dovecot
Summary
by MITRE
Dovecot before version 2.2.29 is vulnerable to a denial of service. When 'dict' passdb and userdb were used for user authentication, the username sent by the IMAP/POP3 client was sent through var_expand() to perform %variable expansion. Sending specially crafted %variable fields could result in excessive memory usage causing the process to crash (and restart), or excessive CPU usage causing all authentications to hang.
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Analysis
by VulDB Data Team • 03/28/2023
The vulnerability identified as CVE-2017-2669 represents a significant denial of service weakness in Dovecot email server software affecting versions prior to 22.2.29. This flaw specifically impacts systems utilizing dict passdb and userdb configurations for user authentication mechanisms. The vulnerability stems from improper handling of variable expansion within the authentication process, creating a condition where maliciously crafted input can trigger resource exhaustion behaviors that compromise system availability.
The technical implementation of this vulnerability occurs within the var_expand() function which processes username information submitted by IMAP/POP3 clients during authentication. When the system encounters specially crafted %variable fields in the authentication request, the variable expansion mechanism becomes susceptible to manipulation that causes exponential memory consumption patterns. This occurs because the dict passdb and userdb components process these variables without adequate input validation or resource limits, allowing attackers to construct payloads that trigger cascading memory allocation requests. The vulnerability is classified under CWE-400 as an Uncontrolled Resource Consumption weakness, specifically manifesting as a memory exhaustion attack vector.
The operational impact of CVE-2017-2669 extends beyond simple service disruption to create substantial availability concerns for email infrastructure. When exploited, the vulnerability can cause immediate process crashes requiring system restarts, or alternatively trigger sustained high CPU utilization that effectively hangs all authentication operations. This creates a denial of service condition where legitimate users cannot authenticate to email services while the system becomes unresponsive to new authentication requests. The attack requires minimal privileges as it operates at the protocol level through standard IMAP/POP3 client connections, making it particularly dangerous for email servers that handle high volumes of authentication requests.
Security practitioners should implement immediate mitigations including upgrading to Dovecot version 2.2.29 or later, which contains patches addressing the variable expansion vulnerability. Additionally, administrators should consider implementing rate limiting and connection throttling mechanisms to reduce the impact of potential exploitation attempts. The vulnerability aligns with ATT&CK technique T1499.004 for Network Denial of Service, where adversaries exploit weaknesses in network services to cause availability disruption. Organizations should also implement monitoring for unusual memory consumption patterns and authentication request processing times that could indicate exploitation attempts. The fix implemented in the patched versions includes enhanced input validation and resource consumption limits within the var_expand() function to prevent the recursive expansion behaviors that enabled the attack vector.