SaferViewer

README

SaferViewer

This utility is a Mac application which you can 'drag and drop' files onto, as a safer way to open them avoiding using applications on your host machine. It is intended for end users to preview documents, such as those attached to emails. Note, this is only of use to you if you are already hooked into using Google Docs for your organisation. Of course, you could re-implement the concept to suite your use case.

Most of the attacks we see via email are usually attachments, such as spurious CSV files or document attacks which are expect to be loaded via Microsoft Word/Pages or Excel/Numbers by the victim user.

While most office applications prompt users with something like "Are you completely, absolutely, infalliably certain you want to open this document?" people make mistakes, or are under pressure and could well click through the warnings.

The concept here is that we simply open the document via a safer mechanism, namely we'll open it in the users web browser using all the cleverness that google has implemented in their google drive/google docs, which is to say things like the local filesystem won't be available to the document, nor will macros execute, etc.

This removes a suprising amount of exploit vectors... A comedic side effect is that it's often fast to open the document via this method than for a user to cold load their document in their office suite

When a document is dragged onto the app, the very first time, the user will be directed to allow the application access to the user google drive via a standard google login dialog. Once approved, the contents of the "responsefile.html" will be displayed. Note: this file is embedded at build time.

Going forwards, when a user drags a document onto the icon the application will upload it in the background to the users google drive into a predefined folder (SaferViewer) and then, open the users web browser to view the document using the Google Docs 'preview' view. At this point, the user can review the document, and if they wish to print or do more with it, can click the icon on the top right to be presented with more options. From this latter screen, they can then choose to edit the document in normal Google Docs type interface. Alternatively they could simply 'double click' the original source to open the document in the default editor on the host machine.

Downloading

Using go get won't work - you need to just clone the repo. This is because of the absence of the client secret, discussed below.

Building

To start, you'll need to create a "client_secret.json" file, which is the output of following the process documented at https://developers.google.com/workspace/guides/get-started

In terse summary, you'll need to enable the GoogleDrive API in your organisations account, and set up OAuth 2.0 for authorization. After creating the OAuth stuff, you'll download a JSON file which is the authentication data for this application, named as above. At build time, this file content is embedded into the binary. There's an obvious residual risk as this data is sensitive, which you may wish to remedy somehow.

To actually build, the included `Makefile' has default target for building a Darwin/amd64 target, which should support both the intel and arm architectures, assuning you've already got Rosetta installed. It's rather unlikely users on M1's have got this far without requiring it, but ymmv.

The `make install' target should be used once you've created the wrapper binary, as described below. You will need to customise the Makefile to change file locations if you deviate from naming the app SaferViewer, or the app install location.

Wrapper

Once built, you need to create a 'wrapper' application for the utility, so that the Apple Events framework is supported. The utility itself just requires the name of the input file as argv[1].

The easiest way to do this is to create an app using the 'Automator' command on MacOS. Open the App, then select 'Application' as the 'type for your document'. Under the 'Library' on the left pane, select 'Utilities' and 'Run Shell Script', drag this to the right Workflow pane. You can leave the preselected 'zsh' as the interpreter, it makes no difference. Change 'pass input' to be 'as arguments', then change the contents to be something like this:

/Applications/SaferViewer.app/Contents/MacOS/SaferViewer "$@"

exit 0

(Note: we exit 0 as users will be confused by 'raw' error messages. Debugging information is written to logfile variable definition in the code)

Then, when you save the file, it will create a universal binary.

Adding the golang binary to the .app

The next thing to do is to copy the golang binary to the above referenced file, the Makefile `install' target will do this.

Optionally, change the icon and re-sign the application (left as an exercise to the reader).