How to break software pdf

One conven- ient attack along these lines is forcing output areas to be recomputed by changing the length of inputs and input strings. A good conceptual example is setting a clock to and watching it roll over to In the first case the display area is 4 characters long and the second it is 5. Going the other way, we establish 5 characters and then watch the text shrink to 4 characters.

Too often developers write code to work with the initial case of a blank display area and are often disappointed when the display area already has data in it and new data of different size is used to replace it. Suppose we enter a long string as shown be- low. Two things went on when the OK button was pressed. First, the routine computed the size of the output field needed and then populated the field with the text we entered.

Notice that the display area stays the same size despite the fact that only one character is inserted and the font size was not changed.

If we edit the string again and type a multi-line string the output is even more interesting. I think the point is made and we can move on to the next attack. Make sure you explore the edges of display areas. This is another attack based on outputs that is very similar to the previous attack.

However, instead of looking for ways to cause the area inside the dis- play to get corrupted, we are going to concentrate on out- side the display area. Considering PowerPoint again, we can draw a textbox and fill it with a superscripted string. Changing the size of the su- perscript to a large font causes the top of the exponent to be truncated. This feature is demonstrated below in conjunction with the following related problem. Try to force screen refresh problems.

This is a major problem for users of modern windows-based GUIs. It is an even bigger problem for de- velopers: refresh too often and you slow down your application, failing to refresh causes anything from minor an- noyances i. The general idea in searching for refresh problems is to add, delete and move objects around on the screen. It is a good idea to try varying the distance you move an object from its original loca- tion.

Move it a little, then move it a lot; move it a once or twice, then move it a dozen times. Continuing with the large superscript example from above, try moving it around on the screen a little at a time.

Note the nasty refresh problem shown below. Another recurring problem in Office associated with screen refresh is disappearing text. This is most annoy- ing in Word just around the page boundaries. For example, a API that can be called with two parameters requires selection of values for one parameter based on the value chosen for the other parameter.

Often it is the combination of values that was misprogrammed because of the complexity of the logic in the code. Find input value combinations that cannot co- exist. So which combinations are problematic? This is an issue still being actively researched but an approach we have found to be especially effec- tive is to determine an output you want to gener- ate and then try to find input combinations that cause the output to occur.

Try to make the target application produce an invalid output. This is a very effective attack for testers who really understand their problem do- main. For example, if you are testing a calculator and understand that some functions have a re- stricted range for their result then trying to find input value combinations that force that result is a worthwhile effort.

However, if you do not understand mathematics, then it is likely that such an endeavor will be a waste of time—you might even interpret an incorrect results as correct. Sometimes the window itself will give you clues about which inputs are interrelated. When this is the case, then testers can experiment with ranges of values and try to violate the stated relationship. Attacks by Input Order Software inputs form a formal language. Individual inputs make up the alphabet of the language and strings of inputs constitute sentences of the language.

Some such sentences should be prevented by the interface via enabling and disabling of controls and input fields and this behavior can be tested by applying numerous strings of input and varying the order of inputs as much as possible. Select input strings that force invalid output. This is a good strategy for identifying problematic input sequences just as it is a good strategy for finding problematic input combinations as described above. For example, when we noticed the disappearing text problem in Office we formulated an attack on the title text box on PowerPoint slides.

The following series of screen shots shows how a specific sequence of inputs causes the text to disappear. It is interesting to note that just rotating the text box degrees does not reveal the bug. Undo-ing the sequence of operation does not correct the problem either, each time one clicks outside the title area, it disappears. The reason that input sequencing is such a bug-rich attack strategy is that many operations complete successfully but leave side-effects that cause future operations to fail.

A thorough investigation of input sequences will expose many of these problems. Repeat the same input or input sequence over and over again. Unfortunately, most applications are unaware of their own space and time limitations and many developers like to assume that plenty of resources are always available.

Attacking Data Data is the lifeblood of software; if you manage to corrupt it the software will eventually have to use the bad data and what happens then may not be pretty. So it is worthwhile to understand how and where data values are estab- lished. Essentially, data is stored either by reading input and then storing it internally or by storing the result of some in- ternal computation. So it is through supplying input and forcing computation that we enable data to flow through the application under test.

The attacks on data follow this simple fact as outlined below. Force incorrectly typed data to be stored Attacks by variable value 2. Force data values to exceed allowable range 3. Overflow input buffers Data Attacks Attacks by data element size 4. Force too many values to be stored 5. Force too few values to be stored Attacks by data access 6.

Find alternate ways to modify the same data Attacks by Variable Value This class of attacks require investigation of the data type and allowable values associated with internally stored data objects. If one has access to the source then this information is readily available, however, significant type information can be determined through a little exploratory testing and attention to error messages.

Vary the data type used in input fields to find type mismatches. Entering characters where the program expects integers and similar attacks have long proven fruitful but we have found that such attacks are less successful than before because of the ease at which type checking and type conversion are handled by modern programming lan- guages. Try to exceed allowable ranges of data values. Variable data that is stored is subject to the same attacks as vari- able data entered as input. Attacks by Data Element Size The second class of data attacks is aimed at overflowing and underflowing data structures.

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