CVE-2021-43860: Permissions granted to applications can be hidden from the user at install time
Ryan Gonzalez discovered that Flatpak doesn't properly validate that the permissions displayed to the user for an app at install time match the actual permissions granted to the app at runtime, in the case that there's a null byte in the metadata file of an app. Therefore apps can grant themselves permissions without the consent of the user.
Flatpak shows permissions to the user during install by reading them from the "xa.metadata" key in the commit metadata. This cannot contain a null terminator, because it is an untrusted GVariant. Flatpak compares these permissions to the actual metadata, from the "metadata" file to ensure it wasn't lied to.
However, the actual metadata contents are loaded in several places where they are read as simple C-style strings. That means that, if the metadata file includes a null terminator, only the content of the file from before the terminator gets compared to xa.metadata. Thus, any permissions that appear in the metadata file after a null terminator are applied at runtime but not shown to the user. Maliciously crafted apps can use this to give themselves hidden permissions.
In addition, a similar weakness was discovered, where if the permissions in the summary metadata are invalid, they would not be displayed to the user, but the the actual permissions would be granted, even though it didn't match the invalid version.
Users who have Flatpaks installed from untrusted sources are at risk in case the Flatpak has a maliciously crafted metadata file, either initially or in an update.
Users can manually check the permissions of installed apps by running
flatpak info --show-permissions APP or checking the metadata file manually.
For more information
If you have any questions or comments about this advisory, please contact