File Browser vulnerable to command execution allowlist bypass

Summary

The Command Execution feature of Filebrowser only allows the execution of shell command which have been predefined on a user-specific allowlist. The implementation of this allowlist is erroneous, allowing a user to execute additional commands not permitted.

Impact

A user can execute more shell commands than they are authorized for. The concrete impact of this vulnerability depends on the commands configured, and the binaries installed on the server or in the container image. Due to the missing separation of scopes on the OS-level, this could give an attacker access to all files managed the application, including the File Browser database.

Vulnerability Description

For a user to make use of the command execution feature, two things need to happen in advance:

1. An administrator needs to grant that account the Execute commands permission
2. The command to be executed needs to be listed in the Commands input field (also done by an administrator)

If a user tries to execute a different command, it gets rejected by the application.

The allowlist verification of a command happens in the function CanExecute in the file users/users.go:

// CanExecute checks if an user can execute a specific command.
func (u *User) CanExecute(command string) bool {
	if !u.Perm.Execute {
		return false
	}

	for _, cmd := range u.Commands {
		if regexp.MustCompile(cmd).MatchString(command) {
			return true
		}
	}

	return false
}

This check employs a regular expression which does not test if the command issued (command) is identical to a configured one (cmd, part of the array u.Commands) but rather only if the issued command contains an allowed one. This has the consequence, that, e.g., if you are only granted access to the ls command, you will also be allowed to execute lsof and lsusb.

As a prerequisite, an attacker needs an account with the Execute Commands permission and some permitted commands.

Proof of Concept

Grant a user the Execute commands permission and allow them to use only ls in the Commands field.

Afterwards, login as that user, open a command execution window and execute lsof and lsusb.

Recommended Countermeasures

The CanExecute function in the source code should be fixed to only allow exact matches of the command specified instead of doing partial matching. The correctness of this fix should be extensively tested in the application's automated test suite.