CVE-2018-21252 in Mattermost Server
Summary
by MITRE
An issue was discovered in Mattermost Server before 5.2, 5.1.1, 5.0.3, and 4.10.3. Attackers could use multiple e-mail addresses to bypass a domain-based policy for signups.
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Analysis
by VulDB Data Team • 10/26/2020
The vulnerability identified as CVE-2018-21252 represents a critical access control flaw in Mattermost Server versions prior to 5.2, 5.1.1, 5.0.3, and 4.10.3. This issue stems from insufficient validation of email address domains during user registration processes, allowing malicious actors to circumvent organizational security policies that typically restrict account creation to specific email domains. The flaw particularly affects systems that implement domain-based restrictions to control who can join their Mattermost instances, which is a common practice for enterprise security management.
The technical implementation of this vulnerability occurs at the user registration validation layer where the system fails to properly enforce domain restrictions when multiple email addresses are associated with a single registration attempt. Attackers can exploit this by submitting multiple email addresses during the signup process, potentially including addresses from allowed domains alongside those from restricted ones. This allows them to bypass the intended domain filtering mechanism that should prevent unauthorized users from creating accounts using email addresses from prohibited domains. The vulnerability essentially undermines the fundamental principle of domain-based access control that organizations rely upon to maintain security boundaries within their communication platforms.
From an operational perspective, this vulnerability significantly weakens the security posture of Mattermost deployments, particularly in enterprise environments where domain restrictions are critical for maintaining secure communication channels. Organizations that depend on Mattermost for internal collaboration may find their security policies effectively nullified, allowing unauthorized individuals to gain access to sensitive discussions and data. The impact extends beyond simple unauthorized access as it can enable social engineering attacks, data exfiltration attempts, and other malicious activities that exploit the trust established within legitimate domain-based user communities. This vulnerability directly contradicts the security principles outlined in the NIST Cybersecurity Framework and can be classified under CWE-639 as an authorization bypass through multiple inputs.
The exploitation of this vulnerability aligns with several tactics described in the MITRE ATT&CK framework, specifically targeting the Initial Access phase through credential reuse and account creation techniques. Attackers can leverage this flaw to establish persistent access points within organizations, potentially using compromised accounts to gain further privileges or to conduct reconnaissance activities. The vulnerability also represents a significant concern for compliance requirements in regulated environments where domain-based access controls are mandated by standards such as SOC 2, HIPAA, or PCI DSS. Organizations implementing Mattermost for collaboration must consider this vulnerability as a potential entry point for threat actors seeking to compromise their secure communication infrastructure.
Organizations should implement immediate mitigations including updating to the patched versions of Mattermost Server as specified in the CVE references, and conducting comprehensive audits of existing user accounts to identify potential unauthorized access. Additional security measures such as implementing multi-factor authentication, regular account reviews, and enhanced monitoring of user registration activities can help detect and prevent exploitation attempts. The vulnerability demonstrates the critical importance of proper input validation and access control implementation, emphasizing that even seemingly simple security features like domain restrictions require robust technical implementation to be effective against determined attackers.