CVE-2021-41178 in Nextcloud
Summary
by MITRE • 10/26/2021
Nextcloud is an open-source, self-hosted productivity platform. Prior to versions 20.0.13, 21.0.5, and 22.2.0, a file traversal vulnerability makes an attacker able to download arbitrary SVG images from the host system, including user provided files. This could also be leveraged into a XSS/phishing attack, an attacker could upload a malicious SVG file that mimics the Nextcloud login form and send a specially crafted link to victims. The XSS risk here is mitigated due to the fact that Nextcloud employs a strict Content-Security-Policy disallowing execution of arbitrary JavaScript. It is recommended that the Nextcloud Server be upgraded to 20.0.13, 21.0.5 or 22.2.0. There are no known workarounds aside from upgrading.
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Analysis
by VulDB Data Team • 10/30/2021
The vulnerability identified as CVE-2021-41178 represents a critical file traversal flaw within Nextcloud's file handling mechanisms that affects versions prior to 20.0.13, 21.0.5, and 22.2.0. This vulnerability stems from inadequate input validation when processing SVG image files, allowing attackers to exploit path traversal techniques to access arbitrary files on the host system. The flaw specifically targets the SVG processing functionality where Nextcloud fails to properly sanitize user-supplied file paths, enabling attackers to manipulate the file resolution process and retrieve sensitive system files that should remain inaccessible to regular users. This type of vulnerability falls under the Common Weakness Enumeration category CWE-22, which describes improper limitation of a pathname to a restricted directory, commonly known as path traversal or directory traversal attacks.
The operational impact of this vulnerability extends beyond simple unauthorized file access, creating a significant attack surface that could be leveraged for more sophisticated exploitation techniques. Attackers can not only download system files but also craft malicious SVG files that appear legitimate to users, particularly those designed to mimic the Nextcloud login interface. This creates a dangerous phishing vector where victims might be tricked into entering credentials on what appears to be a genuine login page. The vulnerability's exploitation potential is particularly concerning because it combines file system access with social engineering capabilities, making it a multi-layered threat that can bypass traditional security controls. The attacker's ability to upload malicious SVG files and then distribute them through specially crafted links transforms this vulnerability from a simple information disclosure issue into a full-fledged attack vector capable of compromising user authentication.
While Nextcloud's implementation includes a strict Content-Security-Policy that mitigates the risk of arbitrary JavaScript execution, this protection does not eliminate the overall threat posed by the vulnerability. The security model relies heavily on the principle of least privilege and proper input validation, which are fundamentally compromised in this scenario. The Content-Security-Policy, although effective against direct script execution, cannot prevent the malicious SVG from containing embedded references to external resources or from being used in combination with other vulnerabilities. This vulnerability demonstrates the importance of defense-in-depth strategies where multiple security controls work together rather than relying on a single protective mechanism. The attack chain typically involves uploading a malicious SVG file through the vulnerable file processing endpoint, then distributing the crafted link to victims who believe they are accessing legitimate Nextcloud resources, creating a deceptive environment that can successfully harvest user credentials.
The remediation approach for CVE-2021-41178 is straightforward but critical for maintaining system integrity and user security. Organizations must immediately upgrade their Nextcloud installations to versions 20.0.13, 21.0.5, or 22.2.0, depending on their current version, as these releases contain the necessary patches to address the path traversal vulnerability. The upgrade process should be carefully planned and executed to ensure no service disruption occurs during the patching procedure. While no workarounds are available for this specific vulnerability, organizations should conduct thorough security assessments of their Nextcloud deployments to identify any other potential vulnerabilities that might exist in their environment. Security teams should also implement monitoring for suspicious file upload activities and unauthorized access attempts that could indicate exploitation attempts. The vulnerability serves as a reminder of the importance of maintaining up-to-date software components and the necessity of implementing robust input validation controls to prevent similar issues from occurring in other applications. Organizations should also consider implementing additional security measures such as web application firewalls and enhanced logging to detect and prevent similar path traversal attacks across their entire infrastructure.