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Securing Uploads

Shrine does a lot to make your file uploads secure, but there are still a lot of security measures that could be added by the user on the application's side. This guide will try to cover some well-known security issues, ranging from the obvious ones to not-so-obvious ones, and try to provide solutions.

Validate file type​

Almost always you will be accepting certain types of files, and it's a good idea to create a whitelist (or a blacklist) of extensions and MIME types.

By default Shrine stores the MIME type derived from the extension, which means it's not guaranteed to hold the actual MIME type of the the file. However, you can load the determine_mime_type plugin to determine MIME type from magic file headers.

# Gemfile
gem "marcel", "~> 0.3"
class MyUploader < Shrine
  plugin :determine_mime_type, analyzer: :marcel
  plugin :validation_helpers

  Attacher.validate do
    validate_extension %w[jpg jpeg png webp]
    validate_mime_type %w[image/jpeg image/png image/webp]
  end
end

Limit filesize​

It's a good idea to generally limit the filesize of uploaded files, so that attackers cannot easily flood your storage. There are various layers at which you can apply filesize limits, depending on how you're accepting uploads. For starters you can add a filesize validation to prevent large files from being uploaded to :store:

class MyUploader < Shrine
  plugin :validation_helpers

  Attacher.validate do
    validate_max_size 100*1024*1024 # 100 MB
  end
end

In the following sections we talk about various strategies to prevent files from being uploaded to Shrine's temporary storage and the system's temporary directory.

Limiting filesize in direct uploads​

If you're doing direct uploads with the upload_endpoint plugin, you can pass in the :max_size option to reject files that are larger than the specified limit:

plugin :upload_endpoint, max_size: 100*1024*1024 # 100 MB

If you're doing direct uploads to Amazon S3 using the presign_endpoint plugin, you can pass in the :content_length_range presign option:

plugin :presign_endpoint, presign_options: -> (request) do
  { content_length_range: 0..100*1024*1024 }
end

Limiting filesize at application level​

If your application is accepting file uploads, it's good practice to limit the maximum allowed Content-Length before calling params for the first time, to avoid Rack parsing the multipart request parameters and creating a Tempfile for uploads that are obviously attempts of attacks.

if request.content_length >= 100*1024*1024 # 100MB
  response.status = 413 # Request Entity Too Large
  response.body = "The uploaded file was too large (maximum is 100MB)"
  request.halt
end

request.params # Rack parses the multipart request params

Alternatively you can allow uploads of any size to temporary Shrine storage, but tell Shrine to immediately delete the file if it failed validations by loading the remove_invalid plugin.

plugin :remove_invalid

Failsafe filesize limiting​

If you want to make sure that no large files ever get to your storages, and you don't really care about the error message, you can override Shrine#upload:

class MyUploader < Shrine
  def upload(io, **options)
    fail FileTooLarge if io.size >= 100*1024*1024

    super
  end
end

Limit image dimensions​

It's possible to create so-called image bombs, which are images that have a small filesize but very large dimensions. These are dangerous if you're doing image processing, since processing them can take a lot of time and memory. This makes it trivial to DoS the application which doesn't have any protection against them.

So, in addition to validating filesize, we should also validate image dimensions:

# Gemfile
gem "fastimage"
class ImageUploader < Shrine
  plugin :store_dimensions
  plugin :validation_helpers

  Attacher.validate do
    validate_max_size 100*1024*1024

    if validate_mime_type %w[image/jpeg image/png image/webp]
      validate_max_dimensions [5000, 5000]
    end
  end
end

If you want to be extra safe, you can add a failsafe before performing processing:

class ImageUploader < Shrine
  # ...
  Attacher.derivatives do |original|
    width, height = Shrine.dimensions(original)

    fail ImageBombError if width > 5000 || height > 5000

    # ...
  end
end

Prevent metadata tampering​

When cached file is retained on validation errors or it was direct uploaded, the uploaded file representation is assigned to the attacher. This also includes any file metadata. By default Shrine won't attempt to re-extract metadata, because for remote storages that requires an additional HTTP request, which might not be feasible depending on the application requirements.

However, this means that the attacker can directly upload a malicious file (because direct uploads aren't validated), and then modify the metadata hash so that it passes Shrine validations, before submitting the cached file to your app. To guard yourself from such attacks, you can load the restore_cached_data plugin, which will automatically re-extract metadata from cached files on assignment and override the received metadata.

Shrine.plugin :restore_cached_data

Limit number of files​

When doing direct uploads, it's a good idea to apply some kind of throttling to the endpoint, to ensure the attacker cannot upload an unlimited number files, because even with a filesize limit it would allow flooding the storage. A good library for throttling requests is rack-attack.

Also, it's generally a good idea to limit the minimum filesize as well as maximum, to prevent uploading large amounts of small files:

class MyUploader < Shrine
  plugin :validation_helpers

  Attacher.validate do
    validate_min_size 10*1024 # 10 KB
    # ...
  end
end

References​