CVE-2020-8155 in Nextcloud Server
Summary
by MITRE
An outdated 3rd party library in the Files PDF viewer for Nextcloud Server 18.0.2 caused a Cross-site scripting vulnerability when opening a malicious PDF.
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Analysis
by VulDB Data Team • 05/13/2020
The vulnerability CVE-2020-8155 represents a critical security flaw in Nextcloud Server 18.0.2 that stems from the integration of an outdated third-party library within the Files PDF viewer component. This issue demonstrates how modern web applications often rely on numerous external dependencies that may contain known security weaknesses, creating attack vectors that can be exploited by malicious actors. The specific library in question was likely a PDF rendering engine or similar component that had not been updated to address previously discovered security vulnerabilities, leaving the Nextcloud instance exposed to potential exploitation.
The technical nature of this vulnerability manifests as a cross-site scripting flaw that occurs specifically when the Files PDF viewer component processes a maliciously crafted PDF file. When a user opens such a file through the Nextcloud interface, the outdated library fails to properly sanitize or validate the PDF content, allowing malicious script code embedded within the document to execute in the context of the user's browser session. This behavior aligns with the common characteristics of XSS vulnerabilities where untrusted data flows into the application's output without proper sanitization or encoding mechanisms.
The operational impact of CVE-2020-8155 extends beyond simple script execution, as it can potentially enable attackers to steal user session cookies, perform actions on behalf of authenticated users, or redirect victims to malicious websites. Given that Nextcloud serves as a collaborative file sharing platform where users frequently access sensitive business documents, this vulnerability could allow unauthorized access to confidential information. The attack vector requires user interaction through the PDF viewer, meaning that successful exploitation typically depends on social engineering tactics to convince users to open malicious files, though the actual exploitation occurs automatically upon file opening.
Organizations utilizing Nextcloud Server 18.0.2 should immediately implement mitigations including updating to a patched version of the application that addresses the outdated library dependency. The vulnerability directly relates to CWE-79 which describes cross-site scripting flaws, and may map to ATT&CK technique T1059.007 for script execution through web browsers. Additionally, administrators should consider implementing content security policies and regular security audits of third-party components to prevent similar vulnerabilities from arising in the future. The incident underscores the importance of maintaining up-to-date dependencies and implementing robust security monitoring practices to detect and respond to potential exploitation attempts.