CVE-2021-40347 in Mailman Postorius
Summary
by MITRE • 09/10/2021
An issue was discovered in views/list.py in GNU Mailman Postorius before 1.3.5. An attacker (logged into any account) can send a crafted POST request to unsubscribe any user from a mailing list, also revealing whether that address was subscribed in the first place.
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Analysis
by VulDB Data Team • 09/16/2021
The vulnerability CVE-2021-40347 represents a significant authorization flaw in the GNU Mailman Postorius administrative interface that affects versions prior to 1.3.5. This issue resides within the views/list.py module and demonstrates a critical weakness in access control validation mechanisms. The vulnerability allows any authenticated user to manipulate the unsubscribe functionality of mailing lists, effectively bypassing proper authorization checks that should normally restrict such actions to authorized administrators or list owners. The flaw manifests when an attacker sends a crafted POST request to the unsubscribe endpoint, enabling them to target any user address regardless of their subscription status or the attacker's own privileges within the system.
The technical implementation of this vulnerability stems from inadequate input validation and insufficient authorization verification within the unsubscribe process. When a POST request is made to the unsubscribe endpoint, the system fails to properly authenticate the requesting user against the permissions required to perform the unsubscribe action. This weakness creates a path where any logged-in user can construct a malicious request that targets specific email addresses, potentially leading to unauthorized removal of subscribers from mailing lists. The vulnerability additionally exposes information disclosure capabilities, as the system reveals whether a target address was previously subscribed to the mailing list, providing attackers with valuable reconnaissance data about list membership and user behavior patterns.
The operational impact of this vulnerability extends beyond simple unauthorized unsubscribes, creating potential risks for privacy, data integrity, and system availability. Attackers could systematically target specific users within mailing lists, potentially disrupting communication channels or creating social engineering opportunities by removing key participants from important discussions. The information disclosure aspect amplifies the threat, as attackers can map out list membership structures and identify high-value targets within organizations. This vulnerability particularly affects organizations that rely on Mailman for critical communications, as it undermines the trust model and could lead to targeted harassment or disruption of legitimate mailing list operations. The flaw also creates opportunities for attackers to enumerate users and gather intelligence about organizational communication patterns, representing a significant risk to enterprise security infrastructure.
Mitigation strategies for CVE-2021-40347 should prioritize immediate patching to version 1.3.5 or later, which implements proper authorization checks and input validation for unsubscribe operations. Organizations should also implement network-level restrictions to limit access to administrative endpoints and consider deploying web application firewalls to detect and block suspicious POST requests targeting unsubscribe functionality. The vulnerability aligns with CWE-285, which addresses insufficient authorization issues, and maps to ATT&CK technique T1566 for social engineering and T1071 for application layer protocols. Additional defensive measures include implementing multi-factor authentication for administrative accounts, conducting regular security audits of administrative endpoints, and establishing monitoring for unusual unsubscribe patterns that could indicate exploitation attempts. Organizations should also review their access control policies to ensure that only authorized personnel have the ability to perform administrative functions within their mailing list systems.