Security Policies

Overview

By default, requests that modify existing files or account settings are authenticated, but requests to upload or deliver files are not. Requests are authenticated by checking a policy string that is signed by a shared secret. Applications can be configured on an individual basis so that every request is authenticated. All security settings are configurable in the Developer Portal.

Request	Default Authentication
Modifies existing file	yes
Modifies account settings	yes
Uploads new file	no
Delivers file	no

Policies and Signatures

Authentication and authorization against our APIs relies on Base64URL-encoded JSON “policies” and HMAC-SHA256 “signatures”. The policy determines which actions are authorized and the signature authenticates the policy. Depending on the API, these values may be required as part of the path, query parameters, or body of a request.

The secret used for signing is automatically generated for each application. The secret should be carefully protected and never exposed client-side. A secure application stack requires backend code that generates and signs short-lived and limited-scope policies for clients. The secret can be regenerated as needed, but this will invalidate existing signatures.

Policy Structure

A policy must contain an “expiry” but can exclude all other values. A minimal policy, containing only an expiry, permits nearly all requests. The “call” value limits the types of requests allowed, and additional values limit request parameters. Getting EXIF data from image files requires explicit permission.

This policy allows any request except one to get EXIF data:

{
  "expiry": 501379200
}

This policy only allows uploading and saving to custom storage:

{
  "expiry": 501379200,
  "call": ["pick", "store"]
}

Policy Keys

A valid policy must contain an “expiry” value set in the future, but can exclude any other values.

Key	Value	Purpose
expiry	Unix timestamp	Sets policy expiration
call	Array of call names	Limits allowed requests, defaults to all
handle	ID string	Limits file modifications to a single file
container	Regex	Limits storage containers allowed for uploads
path	Regex	Limits storage paths allowed for uploads
url	Regex	Limits source URL’s allowed for transformations
minSize	Size in bytes	Sets a min size for uploads
maxSize	Size in bytes	Sets a max size for uploads

Please notice that while using the container, path, or URL policy keys, all special characters should be escaped with a backslash in order to use them in regular expressions.

Example

The example of the url:

https://sample-domain.com/default/file_sample(1).docx

Regular expression with the properly escaped special characters:

https\:\/\/sample\-domain\.com\/default\/file_sample\(1\)\.docx

Calls

Policies without a “call” array allow every call by default except “exif”. Requests to get EXIF data require the “exif” call explicitly.

Name	Allows
pick	Uploading
read	Reading
remove	Deleting
store	Saving to custom storage (also need “pick”)
write	Overwriting (not new uploads)
convert	Converting (and using the document viewer)
exif	Getting exif metadata for image files
stat	Getting file metadata
runWorkflow	Running workflow jobs

Example

The Developer Portal contains a widget to create and sign policies and each of the SDK’s contain helper functionality for the same. The below example shows how to create and use a policy for file delivery.

Create a JSON policy string:

{
  "expiry": 1523595600,
  "call": ["read", "convert"],
  "handle": "bfTNCigRLq0QMOrsFKzb"
}

Base64URL encode the policy string:

ewogICJleHBpcnkiOiAxNTIzNTk1NjAwLAogICJjYWxsIjogWyJyZWFkIiwgImNvbnZlcnQiXSwKICAiaGFuZGxlIjogImJmVE5DaWdSTHEwUU1PcnNGS3piIgp9

Create a HMAC-SHA256 signature of the encoded policy (using ‘mysecret’ as key):

5191e4c6c304c08296eab217ee05236a5bacaab9b581b535d5922a41079b77e0

Using the policy and signature with a download URL:

https://cdn.filestackcontent.com/bfTNCigRLq0QMOrsFKzb?policy=ewogICJleHBpcnkiOiAxNTIzNTk1NjAwLAogICJjYWxsIjogWyJyZWFkIiwgImNvbnZlcnQiXSwKICAiaGFuZGxlIjogImJmVE5DaWdSTHEwUU1PcnNGS3piIgp9&signature=5191e4c6c304c08296eab217ee05236a5bacaab9b581b535d5922a41079b77e0

Using a policy and a signature when transforming your files (resize transformation URL):

https://cdn.filestackcontent.com/resize=width:300/security=policy:ewogICJleHBpcnkiOiAxNTIzNTk1NjAwLAogICJjYWxsIjogWyJyZWFkIiwgImNvbnZlcnQiXSwKICAiaGFuZGxlIjogImJmVE5DaWdSTHEwUU1PcnNGS3piIgp9,signature:5191e4c6c304c08296eab217ee05236a5bacaab9b581b535d5922a41079b77e0/bfTNCigRLq0QMOrsFKzb