Possible XSS vulnerability in Rack

There is a possible vulnerability in Rack. This vulnerability has been
assigned the CVE identifier CVE-2018-16471.

Versions Affected: All.
Not affected: None.
Fixed Versions: 2.0.6, 1.6.11

Impact

There is a possible XSS vulnerability in Rack. Carefully crafted requests can
impact the data returned by the scheme method on Rack::Request.
Applications that expect the scheme to be limited to “http” or “https” and do
not escape the return value could be vulnerable to an XSS attack.

Vulnerable code looks something like this:

<%= request.scheme.html_safe %>

Note that applications using the normal escaping mechanisms provided by Rails
may not impacted, but applications that bypass the escaping mechanisms, or do
not use them may be vulnerable.

All users running an affected release should either upgrade or use one of the
workarounds immediately.

Releases

The 2.0.6 and 1.6.11 releases are available at the normal locations.

Workarounds

The following monkey patch can be applied to work around this issue:

require "rack"
require "rack/request"

class Rack::Request
SCHEME_WHITELIST = %w(https http).freeze

def scheme
  if get_header(Rack::HTTPS) == 'on'
    'https'
  elsif get_header(HTTP_X_FORWARDED_SSL) == 'on'
    'https'
  elsif forwarded_scheme
    forwarded_scheme
  else
    get_header(Rack::RACK_URL_SCHEME)
  end
end

def forwarded_scheme
  scheme_headers = [
    get_header(HTTP_X_FORWARDED_SCHEME),
    get_header(HTTP_X_FORWARDED_PROTO).to_s.split(',')[0]
  ]

  scheme_headers.each do |header|
    return header if SCHEME_WHITELIST.include?(header)
  end

  nil
end
end