[CVE-2018-16471] Possible XSS vulnerability in Rack

[Aaron Patterson]

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
```

Patches
-------
To aid users who aren't able to upgrade immediately we have provided patches for
the supported release series. They are in git-am format and consist of a
single changeset.

* 2-0-scheme-xss.patch - Patch for 2.0 series
* 1-6-scheme-xss.patch - Patch for 1.6 series

Please note that only the 1.6.x and 2.0.x series are supported at present. Users
of earlier unsupported releases are advised to upgrade as soon as possible as we
cannot guarantee the continued availability of security fixes for unsupported
releases.

Credits
-------

* Patrick Tulskie <patrick...@gmail.com>

Thank you!

--
Aaron Patterson
http://tenderlovemaking.com/