Add ability to easily sanitize attributes

What does this MR do?

introduces a model concern that makes it straightforward to sanitize fields by stripping out html and also validates that these fields do not already contain escaped html entities. validation is used to prevent issues like Stored XSS in milestone tooltips which leveraged entity escaping to introduce an xss vulnerability.

the intention is to help prevent xss attacks in the event of a by-pass in the frontend sanitizer due to a configuration issue or a vulnerability in the sanitizer. this approach is commonly referred to as defense-in-depth. this approach was validated with appsec.

Notes

i would prefer not to have added validations but ruby html sanitization libraries typically use nokogiri which always does entity escaping and there's no obvious way around this. you can read more about this decision in earlier discussion on this merge request.

What does this MR not do?

this merge request is not a silver bullet. it is a redundancy measure.

When should this be used?

when you know that your upfront data should never include any html.

Example

Definition

class SomeModel
  include ActiveModel::Model
  include ActiveModel::Attributes
  include ActiveModel::Validations
  include ActiveModel::Validations::Callbacks
  include Sanitizable

  attribute :id, :string
  attribute :name, :string
  attribute :description, :string
  attribute :label, :string
  attribute :html_body, :string

  sanitizes! :name, :description, :label
end

Output

Setup

[1] pry(main)> instance = SomeModel.new(
  id: 1,
  name: 'hello<script>alert(1)</script>, world',
  description: 'hello&world',
  label: 'hello&lt;script&gt;alert(1)&lt;/script&gt;, world',
  html_body: '<div>hello, world</div>'
)

Validation

[1] pry(main)> instance.valid?
=> false
[2] pry(main)> instance.errors.full_messages
=> ["Label cannot contain escaped HTML entities"]

Sanitization

[3] pry(main)> puts instance.attributes
=> {"id"=>"1", "name"=>"hello, world", "description"=>"hello&world", "label"=>"hello&lt;script&gt;alert(1)&lt;/script&gt;, world", "html_body"=>"<div>hello, world</div>"}

please note that sanitization happens in a before_validation callback.

Relates Issue(s)

https://gitlab.com/gitlab-org/gitlab/-/issues/334653

Follow-up Merge requests

Use new Sanitizable concern in other models

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team