[CVE-2020-5267] Possible XSS vulnerability in ActionView

[Aaron Patterson]

There is a possible XSS vulnerability in ActionView's JavaScript literal
escape helpers. Views that use the `j` or `escape_javascript` methods
may be susceptible to XSS attacks.

Versions Affected: All.
Not affected: None.
Fixed Versions: 6.0.2.2, 5.2.4.2

### Impact

There is a possible XSS vulnerability in the `j` and `escape_javascript`
methods in ActionView. These methods are used for escaping JavaScript string
literals. Impacted code will look something like this:

```erb
<script>let a = `<%= j unknown_input %>`</script>
```

or

```erb
<script>let a = `<%= escape_javascript unknown_input %>`</script>
```

### Releases

The 6.0.2.2 and 5.2.4.2 releases are available at the normal locations.

### Workarounds

For those that can't upgrade, the following monkey patch may be used:

```ruby
ActionView::Helpers::JavaScriptHelper::JS_ESCAPE_MAP.merge!(
{
"`" => "\\`",
"$" => "\\$"
}
)

module ActionView::Helpers::JavaScriptHelper
alias :old_ej :escape_javascript
alias :old_j :j

def escape_javascript(javascript)
javascript = javascript.to_s
if javascript.empty?
result = ""
else
result = javascript.gsub(/(\\|<\/|\r\n|\342\200\250|\342\200\251|[\n\r"']|[`]|[$])/u, JS_ESCAPE_MAP)
end
javascript.html_safe? ? result.html_safe : result
end

alias :j :escape_javascript
end
```

### Patches

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

* 5-2-js-helper-xss.patch - Patch for 5.2 series
* 6-0-js-helper-xss.patch - Patch for 6.0 series

Please note that only the 5.2 and 6.0 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

Thanks to Jesse Campos from Chef Secure

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