Headline
CVE-2021-32679: Filenames not escaped by default in controllers using DownloadResponse
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.
Impact
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. (e.g. EXE)
Patches
It is recommended that the Nextcloud Server is upgraded to 19.0.13, 20.0.11 or 21.0.3
Workarounds
- As an administrator of a Nextcloud instance there is no known workaround.
- As an developer of a Nextcloud app, you can manually escape the filename before passing it into DownloadResponse.
References
- HackerOne
- Pull Request
For more information
If you have any questions or comments about this advisory:
- Create a post in nextcloud/security-advisories
- Customers: Open a support ticket at support.nextcloud.com
Related news
Gentoo Linux Security Advisory 202208-17 - Multiple vulnerabilities have been found in Nextcloud, the worst of which could result in denial of service. Versions less than 23.0.4 are affected.