CVE-2020-7046 in Dovecot
Summary
by MITRE
lib-smtp in submission-login and lmtp in Dovecot 2.3.9 before 2.3.9.3 mishandles truncated UTF-8 data in command parameters, as demonstrated by the unauthenticated triggering of a submission-login infinite loop.
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Analysis
by VulDB Data Team • 03/30/2024
The vulnerability identified as CVE-2020-7046 affects Dovecot's lib-smtp and lmtp components in versions prior to 2.3.9.3, specifically targeting the handling of truncated UTF-8 data within command parameters. This issue manifests in the submission-login functionality and demonstrates a critical flaw in how the mail server processes malformed input sequences. The vulnerability operates through a specific edge case where improperly terminated UTF-8 byte sequences in SMTP command parameters cause the system to enter an infinite loop during processing. This behavior represents a denial of service condition that can be triggered without authentication, making it particularly dangerous in production environments where mail servers handle high volumes of traffic.
The technical root cause of this vulnerability lies in the improper validation and handling of UTF-8 encoding within the SMTP command parameter parsing logic. When Dovecot receives a malformed UTF-8 sequence that appears to be truncated or incomplete, the parsing routine fails to properly terminate or reject the invalid input. Instead, the system enters a processing loop where it continuously attempts to interpret the malformed data, leading to resource exhaustion and system unresponsiveness. This flaw is categorized under CWE-129 as an improper validation of array index, though the specific manifestation involves input validation rather than array bounds checking. The vulnerability is particularly insidious because it can be exploited through standard SMTP client interactions without requiring any authentication credentials.
The operational impact of this vulnerability extends beyond simple denial of service, as it can effectively shut down mail submission services and disrupt email communication for affected organizations. Attackers can exploit this vulnerability by sending specially crafted SMTP commands containing truncated UTF-8 sequences to the submission-login service, causing the Dovecot process to consume excessive CPU resources and potentially leading to system crashes or restarts. This vulnerability directly maps to ATT&CK technique T1499.004 for network denial of service attacks, where adversaries leverage application-level flaws to exhaust system resources. The infinite loop condition can persist for extended periods, making it difficult to identify and mitigate once the attack begins, particularly in environments where monitoring systems may not immediately detect the resource consumption patterns associated with this specific flaw.
Mitigation strategies for CVE-2020-7046 require immediate deployment of Dovecot version 2.3.9.3 or later, which includes proper UTF-8 validation and handling routines that prevent the infinite loop condition. Organizations should also implement network-level filtering to restrict SMTP traffic from suspicious sources and deploy monitoring solutions that can detect unusual CPU utilization patterns in mail server processes. Additionally, administrators should consider implementing rate limiting and input validation rules at the network perimeter to prevent exploitation attempts. The fix addresses the core issue by implementing proper UTF-8 sequence validation that terminates processing of malformed input rather than allowing it to loop indefinitely, aligning with security best practices for input sanitization as recommended in OWASP Top Ten categories. Regular vulnerability assessments and patch management procedures should be strengthened to prevent similar issues in other components of the mail infrastructure.