Preventing Cross-Site Request Forgery (CSRF) in Google Apps Script Dialogs and Sidebars

Cross-Site Request Forgery (CSRF) is a web security vulnerability that allows a nefarious entity to take actions on a website that are unintended by the user.

Typically, this is done by tricking the user into using another website resembling the intended site and then submitting a form or clicking a button. The dodgy site then sends a request with its own payload of nasty stuff on the user’s behalf.

Google Workspace and Google Apps Script’s HMTL Service API protect the user with OAuth2 authorization standards and embed dialogues and sidebars in restrictive iframes to sandbox these environments. However, there may be a requirement to further protect your users from unintentionally sending form data using the google.script.run Client-side API that sends data back to Google Apps Script, with a server-side generated anti-CSRF token.

Indeed, when completing a CASA Tier 2 security assessment for a Google Workspace Editor add-on this was a requirement for me to not only pass the assessment but to also meet GDPR requirements.

An anti-CSRF token will allow us to create a unique ID for the current sidebar or dialogue session. We can store this token server-side in the User Properties of the Properties Service and then add the token to a hidden input element in our form client-side on our dialogues and sidebars. We can then send this token along with our form payload back to Apps Script where we can first validate the token before continuing.

The following script and tutorial provide a brief example of how to do this.

The Example Dialogue

We will first open a Google Sheet (but you can open a Google Doc or Slide and do the same thing) and create a bound Google Apps Script.

Our simple tasks will be to:

  1. Create a menu to access a modal dialogue in the Google Sheet.
  2. Create the modal dialogue with a form containing a radio selection and a submit button.
  3. On submission, the form is validated with the anti-CSRF token before

anti-CSRF token example Google Sheets DialogueCreate a Menu Item and Modal Dialogue in Google Sheets

First, let’s get our UIs out of the way.

Lines 5-12 generate the Google Apps Script simple trigger onOpen(). This will create our menu item that will access the dialogue using the Spreadsheet App Class’ getUi() method.

Next, we build the dialogue. Here, we invoke HtmlService to create a template from a file, referencing the ‘Index.html’ file as our source file. We will take this approach to use scriptlets in our HTML to define our CSRF token.

Finally, we will call the UI method again to display the HTML in a modal dialogue.

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Create the Front-end ‘Index’ HTML Page


<!DOCTYPE html>
<html>
<head>
<base target="_top">
<link rel="stylesheet" href="https://ssl.gstatic.com/docs/script/css/add-ons1.css">
</head>
<body>
<div id="container">
<h1>Choose Your Goat</h1>
<form id="goatForm" onsubmit="event.preventDefault();">
<input type="hidden" name="_csrf" value="<?= getCsrfToken() ?>" />
<label for="goat_type">Type of Goat:</label><br>
<input type="radio" name="goat_type" id="goat_type_pygmy" value="pygmy">
<label for="goat_type_pygmy">Pygmy Goat</label><br>
<input type="radio" name="goat_type" id="goat_type_nigerian_dwarf" value="nigerian_dwarf">
<label for="goat_type_nigerian_dwarf">Nigerian Dwarf Goat</label><br>
<input type="radio" name="goat_type" id="goat_type_boer" value="boer">
<label for="goat_type_boer">Boer Goat</label><br>
</form>
<input type="button" value="Submit" onclick="submit()">
<div id="resp">Response: <span id="response">…</span></div>
</div>
</body>
<script>
/**
* When the validation process is completed successfully without Apps Scripting errors.
* @param {String.<Object>} e – event parameter containing 'hasError' boolean and 'text' string.
*/
function onSuccess(e){
const message = JSON.parse(e)
let color = message.hasError? "red" : "blue"
const resp = document.getElementById("response")
resp.innerText = message.text
resp.style.color = color
}
/**
* Submits the response back to Google Apps Script.
*/
function submit(){
const form = document.getElementById('goatForm');
// Create a FormData object
const formData = new FormData(form);
let payload = {}
// Iterate over the form data
for (const [key, value] of formData.entries()) {
console.log(key, value);
Object.assign(payload, {[key]: value})
}
console.log(payload)
google.script.run.withSuccessHandler(onSuccess).formInputs(JSON.stringify(payload));
}
</script>
</html>

view raw

Index.html

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 The HTML form & CSRF token

Lines 10- 19: Here we are adding our form containing our three choices of goats for our survey.

Note the first input type in the form:

<input type="hidden" name="_csrf" value="<?= getCsrfToken() ?>" />

This is where we are inserting a custom token into the current dialogue session. we have named it “_csrf”. You can also see that we have used Google Apps Script’s HTML printing scriptlets to display the anti-CSRF token returned from the getCsrfToken() function.

This function is called from the Google Apps Script side and generated as a part of the HTML template-building process. More on this function later.

 

Line 20-21: The submit button is added outside the form so as not to generate an error when clicked.

A response line is also added to display if the token is correct or not.

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Submitting the Form

submit()

Lines 43-64

When the ‘Submit’ button is clicked, the submit() function is invoked.

Here we retrieve the form element and use the new FormData() constructor to gather all the form responses including our hidden CSRF token.

Next, we iterate over the form data entries sorting the keys and values in the payload object.

Finally, we use the google.script.run API to send a stringified version of the payload back serverside.

We also invoke the withSuccessHandler method to return a message once validation of the token has been carried out.

onSuccess(e)

Once the CSRF token has been validated against the stored token value serverside (Apps Script-side) a stringified object will be returned back to the HTML file containing a hasError boolean property and a text string property.

If there was no match between the sent CSRF token and the stored token, we change our message colour to red.

Then we report the message in the ‘response’ span under the submit button.

Generate the CSRF Token in Google Apps Script

This function is called from the Index.html file template when it is being generated.

The function uses the Apps Script Utilities Service to generate a Unique User ID using the getUuid() method.

Next, the token is stored in the user’s property service. This way the token is only accessible for that use for this script. We will generate and store this token each time the user opens dialogue to make it even more challenging for an attacker to breach.

Finally, the token is returned so it can be stored in the HTML file.

If you have found the tutorial helpful, why not shout me a coffee ☕? I'd really appreciate it.

Validating the CSRF token in Apps Script

Lines 6-10: When the user submits the form, it is sent to the formInputs() function. Here we first parse the stringified form data back to an object and then set up a message object that we will return when our script is complete.

On line 14, we grab our stored CSRF token value so that we can compare it against the one coming in.

Then on lines 17- 27, we check if the form CSRF value matches the store CSRF token value. If it doesn’t, then we return the message variable with our error text.

If the tokens match, then you can carry on a validate your other form inputs before continuing with your data.

That’s it. That’s the whole script. Google Apps Script makes it really easy to implement this security token.

Is It Really Necessary to Add CSRF Tokens to your Google Workspace Dialogues and Sidebars?

Well…probably not. Particularly if you do not intend to publish your Add-on to the public. However, if you do have some restricted scopes that need to be authorised by your users then part of the CASA Tier 2 Assessment then it probably isn’t a huge deal.

The likelihood of someone finding or caring about your dialogues and then trying to exploit them along with Google OAuth and Iframe restrictions would make it pretty hard for a baddie to do any damage to your Google Workspace environment. But, you never know.

Create and Publish a Google Workspace Add-on with Apps Script Course

Need help with Google Workspace development?

My team of experts can help you with all of your needs, from custom app development to integrations and security. We have a proven track record of success in helping businesses of all sizes get the most out of Google Workspace.

Schedule a free consultation today to discuss your needs and get started or learn more about our services here.

~Yagi.

 

Using Hyperlinks in Dialogs and Sidebars to open a URL in a new Tab with Google Apps Script

If you’ve landed on this page you’re probably wondering why your hyperlinks are not working in your Google Workspace dialogue (dialog for my U.S. friends) box or sidebar.

This affects all locations where you can build a sidebar or dialogue with Google Apps Scripts, Sheets, Docs, Slides and Forms.

You might even hit F12 in your browser to inspect the code and found this dreaded error:

Unsafe attempt to initiate navigation for frame with origin ‘https://docs.google.com’ from frame with URL ‘https://n-yyi3lctp…<<fileID>>…-0lu-script.googleusercontent.com/userCodeAppPanel’. The frame attempting navigation of the top-level window is sandboxed, but the flag of ‘allow-top-navigation’ or ‘allow-top-navigation-by-user-activation’ is not set.

linking from a Google Workspace sidebar or dialog error Unsafe attempt to initiate navigation for frame

Why your Links aren’t working in your Google Workspace Dialogs and Sidebars

So what’s going on?

Dialogues and sidebars in Google Workspace are set in iframes. Essentially, this is a nested webpage on your main page. Take a look at the examples, below. I’m in Chrome here and I have selected the Developer Tools Element Selector (Ctrl + Shift + C for PC) and clicked on the Sidebar and Dialogue box respectively.

For the Sidebar.

Google Workspace Sidebars are in iframes

And for the dialogue box.

Google Workspace Dialogs are in iframes

When you create a simple HTML hyperlink in your anchor tag like this:

<a href="https://yagisanatode.com">Website</a>
You are asking the iframe to open a URL in its parent window, your Sheet, Doc, Slide or Form, and browsers generally don’t like to let you do this.

The Solution

The solution is really easy. Simply add target="_blank" to your anchor element:

<a href="https://yagisanatode.com" target="_blank">Website</a>
The target ‘_blank’  attribute will generally open the link in a new tab for most browsers. Note that a user may change their settings to open in a new window instead of a new tab.
Target ‘_blank’ implicitly adds the rel="noopener" behaviour on most browsers to prevent the destination link from tampering with the original source.

The video

Example Implementation

If you want to see an example implementation you can get your own copy of the sample Sheet with bound Apps Script from the link below:
Easter eggs, a bonus Google Sheet formula for linking images in a cell and a fun way to call object methods from a Ui method are all in the example sheet.
Once you made a copy of the Sheet, click on the ‘More’ custom menu item and select from the Sidebar or Dialogue box implementation to see it in action. You will have to Authorise the scopes the first time though.
To look at the code, go to Extensions > Apps Script.
Have fun!

If you have found the tutorial helpful, why not shout me a coffee ☕? I'd really appreciate it.

Create and Publish a Google Workspace Add-on with Apps Script Course

Need help with Google Workspace development?

My team of experts can help you with all of your needs, from custom app development to integrations and security. We have a proven track record of success in helping businesses of all sizes get the most out of Google Workspace.

Schedule a free consultation today to discuss your needs and get started or learn more about our services here.

~Yagi

Google Apps Script: Google Standard Color Palette Picker for Sidebar and Dialog Boxes in Google Sheets, Google Docs and Google Slides (Updated Feb 2022)

Google Apps Script: Dev Tools, Color Picker, Side Bar, Custom Prompt, HtmlService, onOpen, Sidebar, Dialog Box

I wanted to update one of my free Google Add-on apps that works with colour. What I had is just the standard HTML color input element where the user selects from the palette and that hexadecimal colour code is returned to Google Apps Script to be used in the App. The problem is that it is really hard to get a good colour match between the palette and Google’s own colour range that is accessible from the fill or text colour buttons.

Take a look at the comparison between the HTML color input element and the Google Sheet background colour palette in the image below.

Color input vs google sheet palette colours

That’s not a user-friendly tool to match colours with the standard Google palette.

So in the back of my mind, I had always wanted to create a tool for a sidebar or dialogue box that would allow the user to easily access the standard colours or use the custom palette provided by the HTML color input.

After finding a bit of time in my recent summer break I came up with this.

Google Color array picker

And this is how it looks in action:

(Note: add-ons has been updated to Extensions)

Continue reading “Google Apps Script: Google Standard Color Palette Picker for Sidebar and Dialog Boxes in Google Sheets, Google Docs and Google Slides (Updated Feb 2022)”

Google Apps Script – Disable Enable Submit Button in Sidebar

Google Apps Script, Javascript, Jquery, HTML

I was working on a Google Apps Script project lately in Google Sheets that set up parameters in a sidebar and then ran the process once the user clicked the “Submit” button. The problem was that the process was taking a while and that “Submit” Button was ripe to be clicked multiple times by the impatient user before the server-side code could even finish its operation.

To fix this I needed to disable the submit button once it had been clicked and then enable it again once the server-side process was complete. Here, I need to:

  1. Disable the button and get the data from the client-side Javascript inside my sidebar’s HTML file.
  2. Do something awesome with it server-side.
  3. Upon the completion of the server-side awesome, call back to the HTML file and enable the button again.

The Example

I have a very simple Google Sheets Side Bar with a “Submit” and “Cancel” button. When the “Submit” button is clicked it calls the function submittington (can he get any more creative? No. No he can’t).

This function then disables the “Submit” button sends a variable to the client-side code.gs and displays it stylishly in the sheet multiple times for the users viewing pleasure. After the code.gs function is executed, it calls back to the client-side submittington function and enables the button.

Behold!!! The example:

Disable Enable Button in Sidebar Google Apps Script

Continue reading “Google Apps Script – Disable Enable Submit Button in Sidebar”

How to Get Something from Google Sheets and Display it in the Sidebar in Google Apps Script (Updated January 2022)

Google Apps Script: SpreadsheetApp, getUI, HTMLservice, 

What if you want to get a value or a range from Google Sheets and show it in your sidebar using Google Apps Script?

First, you will need to get the value or range by using Google’s server-side script. Then you will have to display it client-side in your HTML document.

Documentation on getting the server-side and client-side talking nicely to each other is a little vague. Hopefully, this very basic tutorial will help clear things up.

In this tutorial, I will also be using Jquery.

Let’s get started.

Continue reading “How to Get Something from Google Sheets and Display it in the Sidebar in Google Apps Script (Updated January 2022)”