I had blogged about this in NCC Group’s website. I thought it is the best to add a link to it here as well.
It is possible to bypass the ASP.NET request validation capability when errors are ignored using request encoding techniques. This can be abused to perform stored cross-site scripting (XSS) attacks.
The full article can be read here: https://www.nccgroup.trust/uk/about-us/newsroom-and-events/blogs/2017/september/rare-aspnet-request-validation-bypass-using-request-encoding/
PDF version can be downloaded from: https://soroush.secproject.com/downloadable/Rare_ASP.NET_Request_Validation_Bypass_Using_Request_Encoding.pdf