CVE-2021-32679 in Nextcloud Server
Summary
by MITRE • 07/12/2021
Nextcloud Server is a Nextcloud package that handles data storage. In versions prior to 19.0.13, 20.0.11, and 21.0.3, filenames where not escaped by default in controllers using `DownloadResponse`. When a user-supplied filename was passed unsanitized into a `DownloadResponse`, this could be used to trick users into downloading malicious files with a benign file extension. This would show in UI behaviours where Nextcloud applications would display a benign file extension (e.g. JPEG), but the file will actually be downloaded with an executable file extension. The vulnerability is patched in versions 19.0.13, 20.0.11, and 21.0.3. Administrators of Nextcloud instances do not have a workaround available, but developers of Nextcloud apps may manually escape the file name before passing it into `DownloadResponse`.
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Analysis
by VulDB Data Team • 07/15/2021
The vulnerability identified as CVE-2021-32679 affects Nextcloud Server versions prior to 19.0.13, 20.0.11, and 21.0.3, representing a critical security flaw in the file handling mechanism of the Nextcloud platform. This issue stems from inadequate input sanitization within the DownloadResponse controller implementation, where user-supplied filenames are not properly escaped or validated before being processed for file downloads. The vulnerability specifically targets applications that utilize the DownloadResponse class to serve files to end users, creating a scenario where malicious actors can manipulate file extensions to deceive users into downloading harmful content while the interface displays misleading file type information. The flaw manifests in the way Nextcloud applications present file information to users through their graphical interfaces, where the displayed file extension may appear legitimate such as .jpeg or .png, but the actual downloaded file possesses a different extension that could execute malicious code. This represents a classic case of insecure input handling that violates fundamental security principles of data validation and output encoding, allowing for potential code execution and system compromise through social engineering tactics that exploit user trust in file type indicators.
The technical implementation of this vulnerability resides in the DownloadResponse controller's failure to sanitize user-provided filenames before incorporating them into HTTP response headers, particularly the Content-Disposition header that dictates how browsers should handle file downloads. According to CWE-116, this vulnerability maps directly to improper encoding or escaping of output, where the application fails to properly escape special characters or control sequences that could alter the intended behavior of the download mechanism. The flaw enables attackers to inject malicious file extensions or manipulate the filename parameter in ways that bypass standard security checks, allowing them to present executable files with deceptive extensions that appear safe to users. When a user clicks to download what appears to be a benign image file, the actual download may contain a .exe, .bat, or other executable file type that executes automatically upon user interaction, creating a potential attack vector for malware distribution and system compromise. This vulnerability operates at the intersection of web application security and user interface deception, where the frontend presentation conflicts with backend file handling behavior, creating confusion and potential exploitation opportunities.
The operational impact of this vulnerability extends beyond simple file download manipulation, as it creates a significant risk for organizations relying on Nextcloud for file storage and collaboration. Attackers could exploit this flaw to distribute malware through seemingly legitimate file sharing activities, potentially compromising entire network infrastructures through targeted attacks on individual users. The vulnerability affects all Nextcloud installations running affected versions, with no available administrative workaround, making it particularly dangerous for organizations that cannot immediately upgrade their systems. Users who access shared files through Nextcloud applications become potential victims of phishing attacks that leverage the platform's own trust mechanisms, as the system's interface design makes it appear safe to download files that are actually malicious. This vulnerability also aligns with ATT&CK technique T1193, which involves the use of malicious file downloads to establish initial access or persistence within target environments, representing a critical threat vector for organizations that rely heavily on cloud-based file sharing solutions.
Organizations affected by this vulnerability should prioritize immediate upgrade to patched versions 19.0.13, 20.0.11, or 21.0.3, as no workaround exists for administrators to mitigate the risk without upgrading the core platform. Developers of Nextcloud applications can implement manual escaping of filenames before passing them to DownloadResponse as a temporary mitigation strategy, though this requires careful code review and implementation across all affected applications. The vulnerability highlights the importance of proper input validation and output encoding practices in web applications, particularly those handling user-generated content and file operations. Security teams should conduct thorough audits of their Nextcloud installations to identify any potentially affected versions and implement monitoring for suspicious download activities that might indicate exploitation attempts. Additionally, user education regarding the risks of downloading files from untrusted sources remains critical, as the vulnerability exploits human trust in familiar file type indicators rather than purely technical weaknesses. The incident underscores the necessity of maintaining up-to-date security patches and implementing robust security testing procedures for web applications that handle user content, particularly those with file download capabilities that could be exploited for malicious purposes.