Vulnerability Instance

DOS vulnerability for Quoted Quality CSV headers
### Impact
When Jetty handles a request containing request headers with a large number of “quality” (i.e. q) parameters (such as what are seen on the `Accept`, `Accept-Encoding`, and `Accept-Language` request headers), the server may enter a denial of service (DoS) state due to high CPU usage while sorting the list of values based on their quality values.  A single request can easily consume minutes of CPU time before it is even dispatched to the application.

The only features within Jetty that can trigger this behavior are:

- Default Error Handling - the `Accept` request header with the `QuotedQualityCSV` is used to determine what kind of content to send back to the client (html, text, json, xml, etc)
- `StatisticsServlet` - uses the `Accept` request header with the `QuotedQualityCSV` to determine what kind of content to send back to the client (xml, json, text, html, etc)
- `HttpServletRequest.getLocale()` - uses the `Accept-Language` request header with the `QuotedQualityCSV` to determine which “preferred” language is returned on this call.
- `HttpservletRequest.getLocales()` - is similar to the above, but returns an ordered list of locales based on the quality values on the `Accept-Language` request header.
- `DefaultServlet` - uses the `Accept-Encoding` request header with the `QuotedQualityCSV` to determine which kind of pre-compressed content should be sent back for static content (content that is not matched against a url-pattern in your web app)

### Versions
`QuotedQualityCSV` was introduced to Jetty 9.3.9.v20160517 and the bug that introduced the vulnerability was in 9.4.6.v20170531. 

Currently, known vulnerable versions include:

- 9.4.6.v20170531 thru to 9.4.36.v20210114
- 10.0.0
- 11.0.0

### Workarounds

Quality ordered values are used infrequently by jetty so they can be avoided by:

 * Do not use the default error page/handler.
 * Do not deploy the `StatisticsServlet` exposed to the network
 * Do not call `getLocale` API
 * Do not enable precompressed static content in the `DefaultServlet` 

### Patches

All patches are available for download from the Eclipse Jetty website at [https://www.eclipse.org/jetty/download.php](https://www.eclipse.org/jetty/download.php)
- 9.4.37.v20210219 and greater
- 10.0.1 and greater 
- 11.0.1 and greater