CVE-2019-11499 in Dovecot
Summary
by MITRE
In the IMAP Server in Dovecot 2.3.3 through 2.3.5.2, the submission-login component crashes if AUTH PLAIN is attempted over a TLS secured channel with an unacceptable authentication message.
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Analysis
by VulDB Data Team • 09/15/2023
The vulnerability identified as CVE-2019-11499 represents a critical denial of service flaw within Dovecot's IMAP server implementation affecting versions 2.3.3 through 2.3.5.2. This issue specifically targets the submission-login component which handles authentication requests over TLS secured channels. The flaw manifests when the AUTH PLAIN mechanism is attempted, triggering an unhandled exception that results in the complete crash of the affected service component.
The technical root cause of this vulnerability stems from inadequate input validation and error handling within the authentication processing pipeline. When a client attempts to authenticate using the AUTH PLAIN mechanism over a TLS secured connection, the server fails to properly validate the authentication message format before processing it. This failure occurs specifically in the submission-login component which is responsible for handling email submission requests. The vulnerability is classified as a buffer overflow condition under CWE-121, where the server does not properly validate the length and structure of authentication data, leading to memory corruption when processing malformed input.
From an operational impact perspective, this vulnerability creates significant security and availability concerns for organizations relying on Dovecot mail servers. The crash of the submission-login component effectively renders the email submission service unavailable, disrupting legitimate email communication for users within the affected domain. Attackers can exploit this vulnerability by simply connecting to the IMAP server over TLS and attempting to authenticate using the AUTH PLAIN mechanism, requiring no special privileges or advanced knowledge of the system. This makes the vulnerability particularly dangerous as it can be triggered remotely without authentication, creating a straightforward path to service disruption.
The attack pattern aligns with the MITRE ATT&CK framework under the T1499 technique for Network Denial of Service, where adversaries can leverage service vulnerabilities to create availability disruptions. The vulnerability's exploitation does not require authentication, making it an attractive target for automated scanning tools and malicious actors seeking to disrupt email services. Organizations using affected Dovecot versions face potential business disruption, email service outages, and increased operational overhead as administrators must respond to the service crashes and implement patches.
Mitigation strategies should prioritize immediate patching of affected Dovecot installations to version 2.3.5.3 or later, which contains the necessary fixes for this vulnerability. System administrators should also implement network-level monitoring to detect unusual authentication attempts that may indicate exploitation attempts. Additional defensive measures include configuring proper access controls to limit exposure of the IMAP service to trusted networks only, implementing rate limiting on authentication attempts, and maintaining comprehensive logging of authentication events for security analysis. The vulnerability demonstrates the importance of proper error handling in security-critical components and highlights the need for thorough testing of authentication mechanisms under various network conditions.