CVE-2020-18985 in Zimbra Collaboration
Summary
by MITRE • 12/16/2021
An issue in /domain/service/.ewell-known/caldav of Zimbra Collaboration 8.8.12 allows attackers to redirect users to any arbitrary website of their choosing.
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Analysis
by VulDB Data Team • 12/22/2021
The vulnerability identified as CVE-2020-18985 represents a critical security flaw within the Zimbra Collaboration platform version 8.8.12, specifically affecting the calendar and contacts service implementation. This issue resides within the well-known endpoint structure at /domain/service/.ewell-known/caldav, which serves as a standard discovery mechanism for calendar access protocols. The flaw enables malicious actors to manipulate the service's redirection behavior, allowing them to craft malicious links that will automatically redirect unsuspecting users to attacker-controlled websites without their knowledge or consent. This type of vulnerability falls under the category of open redirect vulnerabilities, which are classified as CWE-601 in the Common Weakness Enumeration catalog and are commonly associated with web application security risks.
The technical implementation of this vulnerability stems from inadequate input validation and sanitization within the caldav service endpoint. When users access the calendar service through the well-known URL structure, the application fails to properly validate the redirect parameters, allowing attackers to inject malicious URLs that will be processed and executed as legitimate redirects. This flaw essentially bypasses the normal security mechanisms that should prevent unauthorized redirection, creating an attack surface where users can be unknowingly directed to phishing sites, malicious downloads, or other harmful web destinations. The vulnerability is particularly dangerous because it leverages the standard caldav discovery protocol, making it appear legitimate to users who are simply trying to access their calendar services. From an operational perspective, this vulnerability creates a significant risk for organizations using Zimbra Collaboration, as it enables social engineering attacks that can compromise user credentials, deliver malware, or harvest sensitive information through sophisticated phishing campaigns.
The impact of this vulnerability extends beyond simple redirection, as it creates a foundation for more sophisticated attacks within the broader ATT&CK framework. Attackers can leverage this flaw to establish initial access through spear-phishing campaigns where users are directed to malicious sites through seemingly legitimate calendar invitations or notifications. The vulnerability can be exploited to create convincing phishing attacks that appear to come from trusted calendar services, potentially bypassing security awareness training and email filtering systems. Organizations may experience increased risk of credential theft, data exfiltration, and insider threats as users are unknowingly redirected to malicious sites. The attack vector is particularly concerning because it operates at the service level rather than the application level, making it harder to detect through traditional application security measures. Security teams must consider this vulnerability as part of their broader threat modeling exercises, particularly when evaluating the security posture of collaborative platforms and calendar services. The presence of such a vulnerability in a widely used collaboration platform like Zimbra increases the potential attack surface for organizations, as attackers can exploit this weakness to target multiple users simultaneously through mass email campaigns or calendar invitations, creating a scalable attack mechanism that can affect entire organizations.
Organizations should implement immediate mitigations including patching to the latest available version of Zimbra Collaboration, which addresses this specific redirection vulnerability. Network-level controls such as URL filtering and web application firewalls can provide additional protection by monitoring and blocking suspicious redirect patterns. Security awareness training should emphasize the importance of verifying URLs before clicking on calendar invitations, particularly when the redirection occurs through standard service endpoints. Regular security audits of well-known service endpoints should be conducted to identify similar vulnerabilities in other protocols. The vulnerability demonstrates the importance of validating all user inputs and implementing proper access controls for service discovery endpoints, as these mechanisms are often overlooked in security assessments. Organizations should also consider implementing monitoring solutions that can detect anomalous redirection patterns in their network traffic, as this type of attack may not be immediately apparent through standard security tools.