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Muhammad Ahmad
Muhammad Ahmad

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Handling Client-Side Data Securely

Transmitting Data Via the Client

Many applications leave themselves exposed because they transmit critical data such as product prices and discount rates via the client in an unsafe manner.

If possible, applications should avoid transmitting this kind of data via the client. In virtually any conceivable scenario, it is possible to hold such data on the server and reference it directly from server-side logic when needed.

For example, an application that receives users’ orders for various products should allow users to submit a product code and quantity and look up the price of each requested product in a server-side database. There is no need for users to submit the prices of items back to the server. Even where an application offers different prices or discounts to different users, there is no need to depart from this model.

Prices can be held within the database on a per-user basis, and discount rates can be stored in user profiles or even session objects. The application already possesses, server-side, all the information it needs to calculate the price of a specific product for a specific user. It must. Otherwise, it would be unable, on the insecure model, to store this price in a hidden form field.

If developers decide they have no alternative but to transmit critical data via the client, the data should be signed and/or encrypted to prevent user tampering. If this course of action is taken, there are two important pitfalls to avoid:

  • Some ways of using signed or encrypted data may be vulnerable to replay attacks. For example, if the product price is encrypted before being stored in a hidden field, it may be possible to copy the encrypted price of a cheaper product and submit it in place of the original price.
    • To prevent this attack, the application needs to include sufficient context within the encrypted data to prevent it from being replayed in a different context. For example, the application could concatenate the product code and price, encrypt the result as a single item, and then validate that the encrypted string submitted with an order actually matches the product being ordered.
  • If users know and/or control the plaintext value of encrypted strings that are sent to them, they may be able to mount various cryptographic attacks to discover the encryption key the server is using. Having done this, they can encrypt arbitrary values and fully circumvent the protection offered by the solution.

In applications running on the ASP.NET platform, it is advisable never to store any customized data within the ViewState - especially anything sensitive that you would not want to be displayed on-screen to users. The option to enable the ViewState MAC should always be activated.

Validating Client-Generated Data

Data generated on the client and transmitted to the server cannot in principle be validated securely on the client:

  • Lightweight client-side controls such as HTML form fields and JavaScript can be circumvented easily and provide no assurance about the input that the server receives.
  • Controls implemented in browser extension components are sometimes more difficult to circumvent, but this may merely slow down an attacker for a short period.
  • Using heavily obfuscated or packed client-side code provides additional obstacles; however, a determined attacker can always overcome these. (A point of comparison in other areas is the use of DRM technologies to prevent users from copying digital media files. Many companies have invested heavily in these client-side controls, and each new solution usually is broken within a short time.)

The only secure way to validate client-generated data is on the server-side of the application. Every item of data received from the client should be regarded as tainted and potentially malicious.

COMMON MYTH

It is sometimes believed that any use of client-side controls is bad. In particular, some professional penetration testers report the presence of client-side controls as a “finding” without verifying whether they are replicated on the server or whether there is any non-security explanation for their existence.

In fact, despite the significant caveats arising from the various attacks that happens, there are nevertheless ways to use client-side controls that do not give rise to any security vulnerabilities:

  • Client-side scripts can be used to validate input as a means of enhancing usability, avoiding the need for round-trip communication with the server. For example, if the user enters her date of birth in an incorrect format, alerting her to the problem via a client-side script provides a much more seamless experience. Of course, the application must revalidate the item submitted when it arrives at the server.

  • Sometimes client-side data validation can be effective as a security measure — for example, as a defense against DOM-based cross-site scripting attacks. However, these are cases where the focus of the attack is another application user, rather than the server-side application, and exploiting a potential vulnerability does not necessarily depend on transmitting any malicious data to the server.

  • As described previously, there are ways of transmitting encrypted data via the client that are not vulnerable to tampering or replay attacks.

Logging and Alerting

When an application employs mechanisms such as length limits and JavaScript based validation to enhance performance and usability, these should be integrated with server-side intrusion detection defenses. The server-side logic that performs validation of client-submitted data should be aware of the validation that has already occurred on the client-side. If data that would have been blocked by client-side validation is received, the application may infer that a user is actively circumventing this validation and therefore is likely to be malicious.

Anomalies should be logged and, if appropriate, application administrators should be alerted in real-time so that they can monitor any attempted attack and take suitable action as required. The application may also actively defend itself by terminating the user’s session or even suspending his account.

NOTE

In some cases where JavaScript is employed, the application still can be used by users who have disabled JavaScript within their browsers. In this situation, the browser simply skips JavaScript-based form validation code, and the raw input entered by the user is submitted. To avoid false positives, the logging and alerting mechanism should be aware of where and how this can arise.

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