October 16, 2021

Input Validation

Input Validation is used to help mitigate XSSSQL Injection and other attacks.  Input validation is performed to ensure that the attackers can’t gain access to the entire backend database though a single unprotected textbox. Input validation should happen as early as possible in the data flow, preferably as soon as the data is received from the external party.

Data from the client should be subject to input validation, including not only frontend Internet-facing web pages but also the backend.  Don’t always trust input from suppliers, partners, vendors, or regulators, since anyone of which may be get compromised on their side and start sending you malicious data to your system.

  • Conduct all data validation on the server side and never trust the client side.
  • There should be a centralized input validation routine for the application.
  • Specify proper character sets, such as UTF-8, for all sources of input.
  • Encode data to a common character set before validating (Canonicalize).
  • All validation failures should result in input rejection.
  • Determine if the system supports UTF-8 extended character sets and if so, validate after UTF-8 decoding is completed.
  • Validate all client provided data before processing, including all parameters, URLs and HTTP header content (e.g. Cookie names and values). Be sure to include automated post backs from JavaScript, Flash or other embedded code.
  • Verify that header values in both requests and responses contain only ASCII characters.
  • Validate data from redirects (An attacker may submit malicious content directly to the target of the redirect, thus circumventing application logic and any validation performed before the redirect).
  • Validate for expected data types.
  • Validate data range.
  • Validate data length.
  • Validate all input against a whitelist of allowed characters, whenever possible.
  • If any potentially hazardous characters must be allowed as input, be sure that you implement additional controls like output encoding, secure task specific APIs and accounting for the utilization of that data throughout the application . Examples of common hazardous characters include: < > " ' % ( ) & + \ \' \".
  • If your standard validation routine cannot address the following inputs, then they should be checked discretely:
    • Check for null bytes (%00).
    • Check for new line characters (%0d, %0a, \r, \n).
    • Check for “dot-dot-slash” (../ or ..\) path alterations characters. In cases where UTF-8 extended character set encoding is supported, address alternate representation like: %c0%ae%c0%ae/ (Utilize canonicalization to address double encoding or other forms of obfuscation attacks).