CVE-2018-3763 in Calendar
Summary
by MITRE
In Nextcloud Calendar before 1.5.8 and 1.6.1, a missing sanitization of search results for an autocomplete field could lead to a stored XSS requiring user-interaction. The missing sanitization only affected group names, hence malicious search results could only be crafted by privileged users like admins or group admins.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 02/25/2020
The vulnerability identified as CVE-2018-3763 represents a critical stored cross-site scripting flaw within the Nextcloud Calendar application affecting versions prior to 1.5.8 and 1.6.1. This security weakness resides in the autocomplete functionality's handling of search results, specifically where user-provided input lacks proper sanitization before being stored and subsequently rendered. The flaw demonstrates characteristics consistent with CWE-79 - Improper Neutralization of Input During Web Page Generation, which classifies this issue as a classic XSS vulnerability where malicious code can be injected and executed in the context of a victim's browser.
The technical implementation of this vulnerability exploits the autocomplete field's search functionality where group names are processed without adequate input validation or sanitization. When privileged users such as administrators or group administrators create or modify group names containing malicious script code, this content becomes stored within the application's database. The vulnerability requires user interaction to be exploited effectively, as the malicious payload must be triggered when a victim interacts with the affected autocomplete field, typically through a search operation. This interaction-based requirement aligns with ATT&CK technique T1203 - Exploitation for Client Execution, where adversaries leverage web-based applications to execute malicious code through user engagement.
The operational impact of this vulnerability extends beyond simple code injection, as it provides attackers with a pathway to compromise user sessions and potentially escalate privileges within the Nextcloud environment. The restriction that only privileged users can craft malicious search results does not mitigate the overall risk, as administrators often have elevated permissions and access to sensitive data. Attackers could leverage this vulnerability to steal session cookies, redirect users to malicious sites, or perform actions on behalf of authenticated users, potentially leading to complete account compromise or data exfiltration. The stored nature of the XSS payload means that the malicious code persists in the system and can affect multiple users over time, making the vulnerability particularly dangerous for collaborative environments where group management is frequent.
Organizations should implement immediate mitigations including updating to Nextcloud Calendar versions 1.5.8 or 1.6.1, which contain the necessary sanitization patches. Additionally, administrators should conduct thorough review of existing group names and user accounts for potential malicious content, implement proper input validation at all application entry points, and consider network-based protections such as web application firewalls to detect and block suspicious payloads. The vulnerability underscores the importance of comprehensive input sanitization practices and demonstrates how seemingly minor functionality flaws can create significant security risks in collaborative web applications. Security teams should also monitor for potential exploitation attempts and maintain updated threat intelligence regarding similar vulnerabilities in web-based calendar and collaboration platforms.