CVE-2017-0893 in Server
Summary
by MITRE
Nextcloud Server before 9.0.58 and 10.0.5 and 11.0.3 are shipping a vulnerable JavaScript library for sanitizing untrusted user-input which suffered from a XSS vulnerability caused by a behaviour change in Safari 10.1 and 10.2. Note that Nextcloud employs a strict Content-Security-Policy preventing exploitation of this XSS issue on modern web browsers.
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Analysis
by VulDB Data Team • 09/24/2020
The vulnerability identified as CVE-2017-0893 affects Nextcloud Server versions prior to 9.0.58, 10.0.5, and 11.0.3, representing a critical security flaw in the web application's input sanitization mechanism. This issue stems from the use of a JavaScript library that processes untrusted user input, which had undergone behavioral modifications in Safari 10.1 and 10.2 browsers. The flaw creates a cross-site scripting vulnerability that could potentially allow attackers to execute malicious scripts in the context of a victim's browser session. The vulnerability specifically exploits how the sanitization library handles certain input patterns, creating a window of opportunity for malicious code injection when user-supplied data is processed and rendered within the web interface.
The technical implementation of this vulnerability demonstrates a classic input validation failure where the sanitization library fails to properly escape or filter user-controllable data before it is rendered in the browser environment. When users submit content containing malicious script tags or other potentially harmful input, the vulnerable library does not adequately sanitize these inputs, particularly when processed through Safari browsers with the specific version changes. This behavior change in Safari's JavaScript engine created a gap in the sanitization logic that attackers could leverage to inject persistent or reflected XSS payloads. The vulnerability operates at the client-side rendering level where user input is processed and displayed, making it particularly dangerous as it can affect any user who views the malicious content.
The operational impact of this vulnerability extends beyond simple data theft or session hijacking, as it represents a fundamental weakness in the application's security architecture that could enable attackers to execute arbitrary code within user browsers. While Nextcloud's implementation includes a strict Content-Security-Policy that provides defense-in-depth against exploitation, this protection is not foolproof and may be bypassed in certain scenarios or through sophisticated attack vectors. The vulnerability affects the core functionality of user-generated content processing, potentially allowing attackers to modify user interfaces, steal sensitive information, or redirect users to malicious sites. The impact is particularly concerning given that Nextcloud serves as a collaboration and file sharing platform where users frequently input and share various types of content, making the attack surface relatively large.
Security mitigations for this vulnerability primarily involve updating to the patched versions of Nextcloud Server that address the specific JavaScript library issue. Organizations should immediately implement the recommended version upgrades to ensure the sanitization library is properly updated and patched. Additionally, administrators should review and validate their existing Content-Security-Policy configurations to ensure they provide adequate protection against potential bypass attempts. The vulnerability aligns with CWE-79 which describes cross-site scripting flaws, and may relate to ATT&CK technique T1211 which covers exploitation of web application vulnerabilities. Organizations should also consider implementing additional monitoring for suspicious user input patterns and conduct regular security assessments of their web applications to identify similar vulnerabilities in third-party libraries and components that may be subject to similar behavioral changes in different browser environments.