Issue description: Asp.net Server error [Unable to redirect to custom error page] when three dots (…) placed after directory name in url, Custom error page will not work if below attribute is set to false. www.yourwebsite.com\somefolder\ ... By default in asp.net application will be configured relaxedUrlToFileSystemMapping = " false, which means each http request will be validated by ‘Server OS file path validation component’. So when we pass (.) or (..) in URL first it will be validated by this component then the valid request will be passed to IIS. By setting relaxedUrlToFileSystemMapping = " true " it will bypass the ‘Server OS file path validation’, so all the http request will directly reach to IIS. Same scenario has been explained in below figure. Mitigation: In web config: <system.web> <httpRuntime targetFramework="4.5" relaxedUrlToFileSystemM