Archive Inspector and Extractor

Table of Contents

Archive Inspector and Extractor
- Intro
- Details
- Screenshots
  - Preferences
  - Files-List
    - Mount
    - Unmount
    - Extract
    - Pass-Phrase
    - ZIP-Bomb

Intro

This is an app for the Nextcloud cloud software.

archive inspection is implemented as an external mount where archive members are streamed on the fly as they are accessed
archive extraction extracts the archive contents into the file-space of the cloud. The app tries to be quota-aware while doing this.

The app adds an item to the file-actions menu which lets you mount the archive file as "external" mount into the current directory. This mount is "movable" and can be renamed or moved into another folder.

The app further adds an entry to the details-view in the right side-bar wheren archive information is displayed, and controls for mounting and extracing the archive to any location in the cloud file-system are available.

Details

State

Works for me.

Usage

The file-actions menu contains a menu-item "Mount Archive" which lets you mount the archive file as virtual folder into the current directory.
The mount is movable, so it can be renamed later or moved somewhere else.
Unmounting an archive storage is available through the file-actions menu of the mount-point, just click on "Disconnect Storage".
Additionally, there is an "Archive"-tab in the details view of the archive-file. There you find some "meta-information" like uncompressed size, number of archive members, creator's comment, as well as the controls to mount the archive file anywhere into the cloud file-system or to extract it somewhere.

Supported Archive Formats

In principle every format supported by the underlying backend wapmorgan/UnifiedArchive should be supported. There are some caveats, however:

the way the files_app is integrated into Nextcloud means that is relies on Nextcloud emitting useful MIME-types for a file. This might need some configuration of the Nextcloud server which has to be done by an administrator. One notable example are ISO-images, which are just labelled as application/octett-stream (AKA: I DO NOT KNOW) by Nextcloud, but should be application/x-iso9660-image. Please have a look at the Nextcloud documentation. Example configuration files can be found in the config directory.
for some file-types there are different MIME-types. One example of such a case are .rar files which are labelled by Nextcloud as application/x-rar-compressed while wapmorgan/UnifiedArchive expects application/x-rar. These sort of things can be easily cured but may also be reason why particular archive formats are not supported by this app although the underlying backend can handle them.

Security

ZIP-Bombs

in order to somehow reduce the danger of zip-bombs there is a configurable upper limit of the decompressed archive size
administrators can set this limit in order to reduce resource usage on the server or if they feel that the builtin limit of 2^32 (4 GiB) bytes is too high.
users may decrease this limit further on a per-user basis
zip-bombs which manage too fool the archive extraction software about their uncompressed size can obviously not be detected by this approach.
the classical 42.zip should not be an issue as it works by nesting zip-files into each other, so it can only attack recursive archive extractors.
archive extraction respects the quota limits of the respective user
however, there might be further issues if the underlying archive extraction software creates temporary files on the server
- perhaps it would be possible to "jail" the extraction software to an OS-quota-limited directory in order to avoid resource-exhaustion by temporary files
memory consumption in principle should already be capped by the web-server configuration, so this should not be a security issue in the context of a zip-bomb attack.

Passphrases and Encrypted Archives

Decryption of encrypted archives is in principle supported as stated in the documentation of the used backend.
Unfortunately, the Nextcloud CRON-jobs need to have access to the file-space as they periodically scan the file-system. However, the cron-jobs run unauthenticated. In order to make this work, the archvie passwords for the active mounts are stored in the app's database table. The passwords are encrypted with the server password. However, that is stored in plain-text in the config.php of the Nextcloud instance running on the server.
I do not like this, but BTW this is also the way these things are handled by other apps (eg. the mail app).
One note about encrypted ZIP-archives: depends in a delicate way on the used backend whether decrypion works as some of the open-source archive backend do not support all available ZIP encryption methods (and also not all unencrypted ZIP archive formats).
At the moment we are choosing the first available archive-backend from the following list
There is no real preference for any of these backends from my side, just that I was not able to decrypt one archive with the standard PHP ZIP extension and stopped testing after I found one that worked.

Efficiency

Archive access is implemented on a single-file access basis. This is probably not the fastest way to deal with archive files. OTOH, it makes it easier to deal with quota and other limits, which then are handled easily by the Nextcloud file-system framework.

Implementation

This package relies on wapmorgan/unified-archive as archive handling backend. Please see there for a list of supported archive formats and how to support further archive formats.

All what this package does is to provide the Nextcloud integration. Possibly the archive backend could be changed or there could be more than one backend in the future, but intentionally this package just cares about the integration into the cloud-software, but does not add own archive-file handling stuff.