CVE-2020-5239 in Mailu
Summary
by MITRE
In Mailu before version 1.7, an authenticated user can exploit a vulnerability in Mailu fetchmail script and gain full access to a Mailu instance. Mailu servers that have open registration or untrusted users are most impacted. The master and 1.7 branches are patched on our git repository. All Docker images published on docker.io/mailu for tags 1.5, 1.6, 1.7 and master are patched. For detailed instructions about patching and securing the server afterwards, see https://github.com/Mailu/Mailu/issues/1354
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Analysis
by VulDB Data Team • 05/11/2025
The vulnerability identified as CVE-2020-5239 represents a critical authentication bypass flaw in Mailu email server software versions prior to 1.7. This vulnerability specifically targets the fetchmail script component within Mailu's architecture, allowing authenticated users to escalate their privileges and achieve full administrative access to the Mailu instance. The flaw stems from inadequate input validation and privilege management within the fetchmail processing functionality, creating a pathway for malicious actors to leverage their existing authenticated session to gain elevated system privileges. The vulnerability is particularly concerning because it can be exploited by users who already possess legitimate authentication credentials, making it difficult to detect and prevent through traditional network monitoring approaches.
The technical exploitation of this vulnerability occurs through the manipulation of the fetchmail script execution process, where authenticated users can inject malicious commands or manipulate configuration parameters that control how mail fetching operations are performed. This allows attackers to execute arbitrary code with the privileges of the Mailu service account, potentially leading to complete system compromise. The vulnerability is categorized under CWE-269 Improper Privilege Management, which specifically addresses issues where software fails to properly enforce access controls and privilege boundaries. The attack vector requires an existing authenticated user session, making it a privilege escalation vulnerability rather than a direct authentication bypass. However, the impact is severe because once exploited, the attacker gains full administrative control over the email server infrastructure.
The operational impact of CVE-2020-5239 extends beyond simple privilege escalation, as it can enable attackers to access all email accounts managed by the compromised Mailu instance, exfiltrate sensitive communications, modify email routing configurations, and potentially use the compromised server as a launching point for further attacks within the network. Mailu instances with open registration or those that trust unverified users are particularly vulnerable because they provide more opportunities for attackers to establish initial authenticated sessions. This vulnerability affects the core mail server functionality and can result in data breaches, email spoofing, and disruption of email services. Organizations using Mailu with open registration policies face the highest risk since any user can potentially exploit this vulnerability to gain unauthorized access. The attack can be classified under the MITRE ATT&CK framework as privilege escalation technique, specifically leveraging valid accounts to gain higher privileges.
Security mitigation for this vulnerability requires immediate patching of all Mailu installations to version 1.7 or later, with the patched Docker images available from the official mailu/docker.io repository. System administrators should implement comprehensive access controls and user verification processes to minimize the risk of unauthorized users gaining initial authenticated sessions. Network segmentation and monitoring should be enhanced to detect unusual patterns in fetchmail script usage or privilege escalation attempts. Organizations should conduct thorough security audits of their Mailu installations, review user access controls, and implement proper privilege separation. The patch addresses the underlying input validation issues in the fetchmail script and enforces proper privilege boundaries to prevent unauthorized escalation. Additional security measures include implementing multi-factor authentication for administrative accounts, regular security scanning of the Mailu installation, and maintaining up-to-date security patches for all components of the email infrastructure. The vulnerability demonstrates the importance of proper privilege management in mail server software and highlights the need for thorough security testing of automated script execution components within email systems.