New to Telerik UI for ASP.NET AJAX? Download free 30-day trial


This article explains how to ensure information about the RadAsyncUpload configuration is secure and non-readable. Its transmission between the client and the server must be encrypted and impossible to decode, so the data cannot be used by a malicious entity in an attack against the server.

Configuration information includes temporary and target folder on the server, allowed file extensions and the type of the file metadata object (by default, a class from the Telerik.Web.UI.dll assembly).

This article contains the following sections:

There are three appSettings keys you should add to your web.config to ensure information security with file uploads:

  1. Set a custom Telerik.AsyncUpload.ConfigurationEncryptionKey.

  2. Set a custom Telerik.Upload.ConfigurationHashKey.

  3. Set the Telerik.Upload.AllowedCustomMetaDataTypes key. Check the Metadata Type Whitelisting section to avoid any breaking changes.

You can use the IIS MachineKey Validation Key generator to get the encryption keys (make sure to avoid the ,IsolateApps portion). You can see the steps of how to generate the security keys in this YouTube video. Do not forget to select the HMACSHA256 validation method that is the recommended one to generate the keys.

As of R1 2020, the Machine Key is used automatically for the ConfigurationEncryptionKey, ConfigurationHashKey and DialogParametersEncryptionKey keys if they are not set explicitly. You will still need to set your own custom keys if you are using older version of the controls.

        <add key="Telerik.AsyncUpload.ConfigurationEncryptionKey" value="YOUR-FIRST-UNIQUE-STRONG-RANDOM-VALUE-UNIQUE-TO-YOUR-APP&" />
        <add key="Telerik.Upload.ConfigurationHashKey" value="YOUR-SECOND-UNIQUE-STRONG-RANDOM-VALUE-UNIQUE-TO-YOUR-APP&" />
        <add key="Telerik.Upload.AllowedCustomMetaDataTypes" value="Telerik.Web.UI.AsyncUploadConfiguration" />

You can encrypt the appSettings section in the web.config.

In case you do not use RadAsyncUpload, you can disable file uploads for your application via the Telerik.Web.DisableAsyncUploadHandler key web.config switch. This feature is available as of R2 2017 SP2.

Configuration Keys Details

The information below provides more details on the available keys and their usage.

If you do not set custom encryption and hashing keys, default (hardcoded) values are used to encrypt/decrypt the information for versions prior to R2 2017 SP1. If you are using such an old version, we recommend upgrading to the latest.

As of R2 2017 SP1, hardcoded keys are not used anymore. Instead, standard .NET methods are used for encryption. Nevertheless, you should still set your own unique custom keys.

Other cryptographic operations in the UI for ASP.NET AJAX suite may also use these two keys. Telerik avoids adding more keys in order to improve backwards compatibility of your applications and to reduce the number of properties you have to set.

Available keys:


To provide secure encryption of the control configuration, we strongly advise that you set a custom encryption key for Telerik.AsyncUpload.ConfigurationEncryptionKey:

    <add key="Telerik.AsyncUpload.ConfigurationEncryptionKey" value="YOUR-FIRST-UNIQUE-STRONG-RANDOM-VALUE-UNIQUE-TO-YOUR-APP&" />

The Telerik.AsyncUpload.ConfigurationEncryptionKey is available as of Q3 2012 SP1 (version 2012.3.1205).

You can use the IIS MachineKey Validation Key generator to get the encryption keys (make sure to avoid the ,IsolateApps portion).


As of R1 2017, the Encrypt-then-MAC approach is implemented, in order to improve the integrity of the encrypted temporary and target folders.

The additional Telerik.Upload.ConfigurationHashKey key is used to hash the encrypted text. The value returned from the client is checked in the upload handler for integrity. If the hashing attempt is incorrect, a new CryptographicException("The hash is not valid!"); exception will be thrown.

    <add key="Telerik.Upload.ConfigurationHashKey" value="YOUR-SECOND-UNIQUE-STRONG-RANDOM-VALUE-UNIQUE-TO-YOUR-APP&" />

You can use the IIS MachineKey Validation Key generator to get the encryption keys (make sure to avoid the ,IsolateApps portion).


As of R3 2019 SP1, the metadata classes (upload configurations) can be whitelisted. That allows the application to use only the metadata classes from a whitelisted collection of configurations.

As of R1 2020 this feature is enabled by default to improve the application security, and allows the built-in Telerik type only. In R3 2019 SP1 the feature is opt-in. If you add any types, you must add all types that you use, otherwise those that are not whitelisted will throw a The cryptographic operation has failed! error when uploading.

There are several situations when you may be using a custom metadata class, you can read more on the most common cases in the following resources. This can help you determine whether you have such code in your application for any purpose. If you do, read the information after this list to see how to apply whitelisting for them.

To whitelist your custom types, add the Telerik.Upload.AllowedCustomMetaDataTypes key in the appSettings section of the web.config. As a value for the key, provide the metadata class full name, including the namespace, in a list delimited by a semicolon (;). The built-in type that we use out-of-the-box is always whitelisted. For an easy way to obtain the fully qualified name of the configuration class you need to add, check the example in the following KB article:

Whitelist custom metadata types

    <add key="Telerik.Upload.AllowedCustomMetaDataTypes" value="SomeNameSpace.SampleAsyncUploadConfiguration;SomeOtherNameSpace.OtherAsyncUploadConfiguration" />

This is an additional security measure and it does not replace setting the main custom encryption keys.

Failure to deserialize a custom metadata type will also throw a CryptographicException on the server and the handler request will fail, resulting in a The cryptographic operation has failed! error on the client.

Custom handlers are affected by this feature.


You can disable file uploads through RadAsyncUpload's built-in configuration altogether. This feature is available as of R2 2017 SP2 (2017.2.711).

Setting the Telerik.Web.DisableAsyncUploadHandler key to true disables the built-in RadAsyncUpload handler that is used for storing files in the temporary folder before they are moved to the target folder.

When you set this key to true, no files can be uploaded to the default handler (Telerik.Web.UI.WebResource.axd) and async upload requests to it will return a 404 error. You may want to handle the OnClientFileUploadFailed event to prevent the page from throwing JavaScript errors.

Custom handlers are not affected by this feature and you can still use them to upload and save files with the desired level of security.

How to disable (make unavailable) the default Async Upload handler so no files can be uploaded through it.

    <add key="Telerik.Web.DisableAsyncUploadHandler" value="true"/>

Even when disabling file uploads, we recommend setting the main custom encryption keys, especially for versions prior to R3 2019 SP1. The DisableAsyncUploadHandler behavior is improved in R3 2020 SP1 (2019.3.1023) and we recommend upgrading to 2019.3.1023 or a later release when using it.

Frequently Asked Questions

See Also

In this article