CVE-2018-15131 in Zimbra Collaboration Suite
Summary
by MITRE
An issue was discovered in Synacor Zimbra Collaboration Suite 8.6.x before 8.6.0 Patch 11, 8.7.x before 8.7.11 Patch 6, 8.8.x before 8.8.8 Patch 9, and 8.8.9 before 8.8.9 Patch 3. Account number enumeration is possible via inconsistent responses for specific types of authentication requests.
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Analysis
by VulDB Data Team • 09/25/2023
The vulnerability identified as CVE-2018-15131 represents a significant security weakness within the Synacor Zimbra Collaboration Suite across multiple version ranges. This issue stems from inconsistent response behavior during authentication requests, which allows attackers to enumerate account numbers through carefully crafted authentication attempts. The vulnerability affects versions 8.6.x before 8.6.0 Patch 11, 8.7.x before 8.7.11 Patch 6, 8.8.x before 8.8.8 Patch 9, and 8.8.9 before 8.8.9 Patch 3, indicating a widespread problem that impacted the core authentication mechanism of the email collaboration platform. The flaw operates at the protocol level where the system's response timing and error messages differ between valid and invalid account attempts, creating a side-channel attack vector that adversaries can exploit to determine which accounts exist within the system.
The technical implementation of this vulnerability exploits the inconsistent response patterns that occur during authentication processes, particularly when handling account validation requests. When an attacker submits an authentication request, the system's response varies depending on whether the account exists or not, even when the authentication credentials are incorrect. This inconsistency manifests in different response times, error message formats, or HTTP status codes that reveal information about account existence. The vulnerability directly maps to CWE-200, which covers "Information Exposure," and specifically relates to information disclosure through inconsistent responses that leak account enumeration data. Attackers can leverage this inconsistency to systematically test account names and observe response variations, effectively mapping the user base of the email system without requiring valid credentials.
The operational impact of this vulnerability extends beyond simple account enumeration, creating a foundation for more sophisticated attacks within the Zimbra environment. Once an attacker has identified valid accounts, they can proceed with targeted credential stuffing, brute force attacks, or social engineering campaigns with significantly higher success rates. The vulnerability undermines the fundamental security principle of not revealing information about system components through response variations, which is a core requirement of secure system design. From an attacker's perspective, this vulnerability provides an initial foothold that can be combined with other techniques to escalate privileges or compromise additional system components. The impact is particularly severe in enterprise environments where Zimbra serves as a primary email platform, as the enumeration process can reveal the complete user directory structure to unauthorized parties.
Mitigation strategies for CVE-2018-15131 must focus on standardizing authentication responses to prevent information leakage through timing variations and error message differences. Organizations should immediately apply the available patches for their specific Zimbra version ranges, with the most critical updates being 8.6.0 Patch 11, 8.7.11 Patch 6, 8.8.8 Patch 9, and 8.8.9 Patch 3. The implementation of constant-time authentication responses ensures that all authentication attempts, regardless of account validity, return consistent response patterns. Additional protective measures include implementing account lockout mechanisms, rate limiting for authentication attempts, and monitoring for unusual authentication patterns that might indicate enumeration attempts. From a defensive perspective, this vulnerability aligns with ATT&CK technique T1078 which covers legitimate credentials, and T1566 which covers credential harvesting, making it a critical target for both defensive and offensive security teams to address. Network-level monitoring should be implemented to detect and alert on suspicious authentication patterns that could indicate enumeration activities.