Handling Input in Java

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correct You can, however, be responsible for ensuring that the input you accept is not obviously wrong Don t expect input to be formatted properly, make sense, be self-consistent, follow normal encoding conventions, or adhere to any sort of standard Don t expect that you can trust input just because it comes from a source that seems like it should be wholesome and reliable Don t trust input just because you wrote the program that is supposed to generate that input; your program might nd itself receiving input from a less trustworthy source or the trusted source itself might be compromised When your input validation code identi es a problem, gracefully decline to accept the request Don t patch it up and try to soldier on In short, be suspicious about the input you handle, and ensure that when input does not match your expectations, you chart a secure course nonetheless You have to accept input, but you can t trust it so what do you do You sanity-check it You corroborate it You take control and limit it to only the values that you know for certain are acceptable We refer to these activities collectively as input validation This chapter looks at what needs to be validated, how to perform validation and how to respond when input fails a validation check, and how to structure your software to make good input validation easier We discuss the various ways that program input should be validated, strategies for performing validation, and ways to verify that your strategy has been implemented correctly Along the way, we look at a multitude of security problems that resulted from inadequate input validation In subsequent chapters, input validation problems come up repeatedly in the context of various program activities In those later chapters, we look at individual input validation requirements and speci c vulnerabilities related to mishandled input The primary message in this chapter is that no form or aspect of program input should be trusted by default The chapter unfolds as follows: What to validate Validate all input Validate every piece of input the program uses Make it easy to verify that all input is validated before it is used Validate input from all sources Validate input from all sources, including command-line parameters, con guration les, database queries, environment variables, network services, registry values, system properties, temporary les, and any other source outside your program
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Establish trust boundaries Store trusted and untrusted data separately to ensure that input validation is always performed How to validate Use strong input validation Use the strongest form of input validation applicable in a given context Prefer indirect selection or whitelisting Avoid blacklisting Do not fall back on blacklisting just because stronger input validation is dif cult to put in place Don t mistake usability for security Do not confuse validation that an application performs for usability purposes with input validation for security Reject bad data Reject data that fail validation checks Do not repair it or sanitize it for further use Make good input validation the default Use a layer of abstraction around important or dangerous operations to ensure that security checks are always performed and that dangerous conditions cannot occur Always check input length Validate input against a minimum expected length and a maximum expected length Bound numeric input Check numeric input against both a maximum value and a minimum value as part of input validation Watch out for operations that might be able to carry a number beyond their maximum or minimum value The chapter wraps up with a look at metacharacter vulnerabilities, including SQL injection, command injection, and log forging
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