CVE-2021-32733 in Text
Summary
by MITRE • 07/13/2021
Nextcloud Text is a collaborative document editing application that uses Markdown. A cross-site scripting vulnerability is present in versions prior to 19.0.13, 20.0.11, and 21.0.3. The Nextcloud Text application shipped with Nextcloud server used a `text/html` Content-Type when serving files to users. Due the strict Content-Security-Policy shipped with Nextcloud, this issue is not exploitable on modern browsers supporting Content-Security-Policy. The issue was fixed in versions 19.0.13, 20.0.11, and 21.0.3. As a workaround, use a browser that has support for Content-Security-Policy.
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Analysis
by VulDB Data Team • 07/16/2021
The vulnerability described in CVE-2021-32733 represents a cross-site scripting weakness within the Nextcloud Text collaborative document editing application that affects multiple versions of the Nextcloud server platform. This vulnerability specifically impacts deployments where the Nextcloud Text application serves files with an incorrect Content-Type header set to text/html instead of the appropriate application/octet-stream or text/plain formats. The flaw exists in the application's handling of document rendering and file serving mechanisms, creating a potential attack surface for malicious actors to inject and execute arbitrary scripts within user sessions.
The technical implementation of this vulnerability stems from the improper Content-Type specification in the HTTP response headers when serving document files through the Nextcloud Text application. When files are served with text/html Content-Type, browsers may interpret the content as HTML markup rather than plain text, particularly when the Content-Security-Policy is not properly configured to prevent script execution. This issue is categorized under CWE-79 as Cross-Site Scripting, which represents one of the most prevalent web application security vulnerabilities in the industry. The vulnerability's exploitation potential is significantly mitigated by modern browser security mechanisms that enforce Content-Security-Policy directives, making the issue less critical in contemporary browser environments.
The operational impact of this vulnerability extends beyond simple script injection, as it could potentially enable attackers to steal user session cookies, perform unauthorized actions on behalf of users, or redirect victims to malicious websites. The attack surface is particularly concerning in collaborative environments where multiple users share documents, as a single compromised document could affect all users accessing that content. Security practitioners should note that while the vulnerability exists in versions prior to 19.0.13, 20.0.11, and 21.0.3, the risk assessment should consider the specific browser compatibility and security policy enforcement mechanisms in place within the organization's infrastructure. The Nextcloud development team addressed this issue by implementing proper Content-Type headers and ensuring that document files are served with appropriate security measures that prevent unintended HTML interpretation.
The mitigation strategy for this vulnerability aligns with established security practices outlined in the ATT&CK framework under the T1211 technique for exploitation of web application vulnerabilities. Organizations should immediately upgrade to the patched versions of Nextcloud Text that properly implement Content-Type headers and Content-Security-Policy directives. The recommended workaround of using browsers with proper Content-Security-Policy support provides an additional layer of defense, though it does not eliminate the underlying issue. Security configurations should include proper Content-Security-Policy headers that include directives such as default-src 'self' and script-src 'self' to prevent unauthorized script execution. System administrators should also implement monitoring for unusual file serving patterns and ensure that all Nextcloud deployments are regularly updated to maintain security posture against similar vulnerabilities in the future.