Handling Input in Java

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5 Handling Input
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Example 521 Wrapper methods around SessionsetAttribute() make it hard to forget about input validation
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public void setAttribute(HttpSession session, String attrib, String value, Pattern p) { if ((attrib == null) || (value == null) || (p == null) || !pmatcher(value)matches()) { throw new ValidationException(attrib, p); } sessionsetAttribute(attrib, value); } public void unsetAttribute(HttpSession session, String attrib) { sessionsetAttribute(attrib, null); }
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Static Analysis: Flag Functions Replaced by Your Security-Enhanced API
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Programmers who are unaware of your security-enhanced API might bypass it inadvertently This is disastrous from a security point of view The power of the API comes from its consistent use Don t let programmers get away with bypassing the security mechanisms you ve put in place If the names of your API functions are similar to the names of the functions they wrap, it can be hard to tell good calls from bad calls during a manual code review, but it s easy for a static analysis tool to pick out Consider the following two calls:
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setAttribute(session, "xfactor", flavor, rgx); sessionsetAttribute("size", ounces);
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The first call uses the wrapper method from Example 521, but the second call bypasses the wrapper and invokes the base method directly Because both are named setAttribute(), it s not easy to tell them apart with a quick visual inspection A static analysis tool knows the difference, though You can spot the call to
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sessionsetAttribute() with this rule:
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Structural rule:
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FunctionCall fc: (fcfunction is [name == "setAttribute" and enclosingClasssupers contains [Class: name == "javaxservlethttpHttpSession"]])
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How to Validate
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Example 522 shows what can happen when a security-enhanced API isn t adopted The code reads three Boolean values in three different ways If the input specification changes, there are three separate input validation schemes to think through When it s time to audit the code, auditors have to think through not only whether each approach is legitimate, but also whether the three are consistent Because we re talking about input validation, we critique the three methods used in this example The first is overly permissive, in that it only requires the string to contain "true" at some point The string "falsefalsefalsetrue" will yield true The second is better, but BooleanvalueOf() takes any string that doesn t match the string literal "true" (non-case-sensitive) to be false, so "T" is false The third method is good It accepts only two strings, "true" and "false", and throws an exception if it does not receive the input it expects
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Example 522 Code that checks three different pieces of input in three different ways This is trouble
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boolean argZero, argOne, argTwo; // Method 1: // search for the string "true" in the arg argZero = args[0]indexOf("true") != -1; // Method 2: // use built-in String to Boolean conversion argOne = BooleanvalueOf(args[1]); // Method 3: // throw an exception if the string is neither "true" or "false" if ("true"equals(args[2])) { argTwo = true; } else if ("false"equals(args[2])) { argTwo = false; } else { throw new IllegalArgumentException("bad Boolean " + args[2]); }
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Check Input Length Front-end validation logic should always check input against a minimum and maximum expected length Length checks are usually easy to add because they don t require much knowledge about the meaning of the input
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5 Handling Input
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Watch out, though if the program transforms its input before processing it, the input could become longer in the process Good input validation consists of much more than just evaluating the length of the input, but a length check is an absolute minimum amount of validation The more program context that can be brought to bear during input validation, the better If the program needs to validate an input eld, the more the validation logic knows about the legal values for the input eld, the more rigorous a job it can do For example, if an input eld is meant to hold the state abbreviation portion of a postal address, the validation logic can use indirect selection to check the input value against a list of valid postal abbreviations for states A more sophisticated input validation scheme might cross check the area code portion of a phone number eld against the state abbreviation Good design practices dictate that front-end validation code and business logic should not be intimately intermixed The result is that validation code rarely has the ideal context to do the best possible job of validating the input The perfect split between front-end validation and validation checks that are intermingled with application logic depends on the context of the program; at a minimum, however, it should always be possible to check input length as part of the front-end validation Checks for reasonable maximum input length can make it harder for an attacker to exploit other vulnerabilities in the system For example, if an input eld can be used as part of a cross-site scripting attack, an attacker who can write a script of any length has much more exibility than an attacker who is limited to a small number of characters By checking against a minimum input length, the attacker loses both the capability to omit input elds that are meant to be mandatory and the capability to supply data that are too small to be valid Example 523 demonstrates a basic length check performed to ensure that the variable path is nonempty and, at most, MAXPATH in length Example 524 goes one step further and uses a whitelist to verify that path consists of only valid characters and falls within the same length requirements
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