CVE-1999-0144 in Qmail
Summary
by MITRE
denial of service in qmail by specifying a large number of recipients with the rcpt command.
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Analysis
by VulDB Data Team • 09/07/2024
The vulnerability described in CVE-1999-0144 represents a classic denial of service weakness in the qmail email server software that was widely deployed during the late 1990s and early 2000s. This issue specifically targets the mail transfer agent's handling of recipient addresses during the SMTP transaction process, where the rcpt command is used to specify intended recipients for an email message. The flaw occurs when an attacker sends a malformed SMTP command sequence that includes an excessive number of recipient addresses in a single rcpt command, causing the qmail server to consume excessive system resources or enter an unstable state.
The technical implementation of this vulnerability stems from inadequate input validation and resource management within the qmail SMTP daemon's processing of recipient lists. When the server receives a command containing a large number of recipients, it fails to properly limit or validate the quantity of addresses processed in a single transaction. This behavior aligns with CWE-122, which describes improper restriction of operations within a limited error handling context, and CWE-400, which covers resource exhaustion vulnerabilities. The qmail implementation does not enforce reasonable limits on the number of recipients that can be specified in a single rcpt command, allowing an attacker to exploit this weakness through a carefully crafted SMTP transaction sequence.
The operational impact of this vulnerability extends beyond simple service disruption, as it can lead to complete system unavailability and resource exhaustion that affects legitimate email services. Attackers can leverage this weakness to consume significant memory and processing power on the target mail server, potentially causing the system to become unresponsive to legitimate email traffic or even crash entirely. The vulnerability affects any system running qmail versions that do not implement proper bounds checking on recipient list sizes, making it particularly dangerous in environments where email services are critical for business operations. This weakness can be exploited through automated tools that send malformed SMTP commands, making it a particularly effective vector for distributed denial of service attacks against email infrastructure.
Mitigation strategies for CVE-1999-0144 should focus on implementing proper input validation and resource limits within the mail server configuration. System administrators should configure qmail to enforce maximum recipient limits in SMTP transactions, typically through configuration parameters that restrict the number of addresses processed in a single rcpt command. The implementation should align with ATT&CK technique T1499.004, which covers network denial of service attacks, by establishing proper rate limiting and resource allocation controls. Additionally, implementing proper monitoring and alerting mechanisms can help detect unusual SMTP transaction patterns that may indicate exploitation attempts. Organizations should also consider upgrading to more modern mail server implementations that have better resource management and input validation capabilities, as qmail has been largely superseded by more secure and robust email server solutions. The vulnerability demonstrates the importance of defensive programming practices and proper resource management in network services, particularly those handling user-provided data through protocol interactions.