OWASP Developer Guide

Validate All Inputs Checklist

4.2.5 Checklist: Validate All Inputs

Input validation is a collection of techniques that ensure only properly formatted data may enter a software application or system component.

It is vital that input validation is performed to provide the starting point for a secure application or system. Without input validation the software application/system will continue to be vulnerable to new and varied attacks.

Refer to proactive control C5: Validate All Inputs for more context from the OWASP Top 10 Proactive Controls project, and use the list below as suggestions for a checklist that has been tailored for the individual project.

Syntax and semantic validity

  • Identify all data sources and classify them into trusted and untrusted
  • Validate all input data from untrusted sources such as client provided data
  • Encode input to a common character set before validating
  • Specify character sets, such as UTF-8, for all input sources
  • If the system supports UTF-8 extended character sets then validate after UTF-8 decoding is completed
  • Verify that protocol header values in both requests and responses contain only ASCII characters
  • Validate data from redirects
  • Validate data range and also data length
  • Utilize canonicalization to address obfuscation attacks
  • All validation failures should result in input rejection

Libraries and frameworks

  • Conduct all input validation on a trusted system
  • Use a centralized input validation library or framework for the whole application
  • If the standard validation routine cannot address some inputs then use extra discrete checks
  • If any potentially hazardous input must be allowed then implement additional controls
  • Validate for expected data types using an allow-list rather than a deny-list

Validate serialized data

  • Implement integrity checks or encryption of the serialized objects to prevent hostile object creation or data tampering
  • Enforce strict type constraints during deserialization before object creation; typically a definable set of classes is expected
  • Isolate features that deserialize so that they run in very low privilege environments such as temporary containers
  • Log security deserialization exceptions and failures
  • Restrict or monitor incoming and outgoing network connectivity from containers or servers that deserialize
  • Monitor deserialization, for example alerting if a user agent constantly deserializes


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