CVE-2018-8927 in Calendar
Summary
by MITRE
Improper authorization vulnerability in SYNO.Cal.Event in Calendar before 2.1.2-0511 allows remote authenticated users to create arbitrary events via the (1) cal_id or (2) original_cal_id parameter.
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Analysis
by VulDB Data Team • 03/27/2023
The vulnerability identified as CVE-2018-8927 represents a critical improper authorization flaw within the SYNO.Cal.Event component of Synology Calendar applications prior to version 2.1.2-0511. This issue affects the authorization mechanisms that govern event creation operations, creating a significant security risk for organizations relying on Synology's calendar services. The vulnerability stems from inadequate validation of user permissions when processing calendar event creation requests, allowing authenticated users to bypass normal access controls and manipulate calendar data.
The technical exploitation of this vulnerability occurs through manipulation of two specific parameters within the calendar event creation API endpoint: cal_id and original_cal_id. These parameters are intended to reference specific calendar identifiers and original event references respectively, but due to insufficient authorization checks, attackers can submit crafted requests that reference calendars or events they do not have permission to access. The flaw exists because the application fails to verify whether the authenticated user possesses appropriate privileges to create events within the target calendar or to reference specific original calendar identifiers. This weakness falls under the CWE-862 category of "Missing Authorization" and specifically aligns with ATT&CK technique T1078.004 for Valid Accounts and T1566.001 for Phishing, as it enables unauthorized data manipulation through legitimate authenticated sessions.
The operational impact of this vulnerability extends beyond simple data modification, as it allows attackers to create arbitrary calendar events that may contain malicious links, phishing attempts, or misleading information that could deceive other users within the organization. An attacker could potentially create events that appear to originate from legitimate calendar owners, making the malicious activity harder to detect and trace. The vulnerability affects any authenticated user who can access the calendar application, meaning that even users with limited calendar permissions could exploit this flaw to create events in calendars they should not have access to. This creates a significant risk for enterprise environments where calendar systems are used for scheduling meetings, sharing sensitive information, and coordinating business activities.
Organizations should immediately implement the vendor-provided patch for Calendar version 2.1.2-0511 or later, which addresses the authorization validation issues by implementing proper access control checks for both cal_id and original_cal_id parameters. System administrators should also review existing calendar permissions and implement principle of least privilege controls to minimize potential damage from similar vulnerabilities. Additional mitigations include monitoring calendar creation activities for suspicious patterns, implementing network segmentation to limit access to calendar services, and conducting regular security assessments of calendar applications. The vulnerability demonstrates the importance of proper input validation and authorization checking in web applications, particularly those handling user-generated content and calendar data, aligning with security best practices outlined in NIST SP 800-53 and ISO/IEC 27001 frameworks.