I found an amazing and easy solution to determine the correct version of jetty-servlets.jar for GeoServer.
The solution is to open the jetty directory
- Windows: C:\Program Files (x86)\Boundless\OpenGeo\jetty\lib
- OS X: /Users//Library/Application Support/GeoServer/jetty/lib
You will find some jar files ends with the jetty version before the extension.
(like: jetty-client-7.6.13.v20130916.jar) for Geoserver 2.8.
Remember the version number from the file (7.6.13.v20130916) and browes to this URL and select the matching version to the version we got from the last step.
Choose the file (like: jetty-servlets-7.6.13.v20130916.jar ) no other file needed.
Paste the downloaded jar into jetty directory