2 cissp ® Official Study Guide Eighth Edition

Buffer Overflows 
Buffer overfl ow
vulnerabilities exist when a developer does not properly validate user input 
to ensure that it is of an appropriate size. Input that is too large can “overfl ow” a data 
structure to affect other data stored in the computer’s memory. For example, if a web form 
has a fi eld that ties to a backend variable that allows 10 characters, but the form processor 
does not verify the length of the input, the operating system may try to simply write data 
past the end of the memory space reserved for that variable, potentially corrupting other 
data stored in memory. In the worst case, that data can be used to overwrite system com-
mands, allowing an attacker to exploit the buffer overfl ow vulnerability to execute arbi-
trary commands on the server. 
When creating software, developers must pay special attention to variables that allow user 
input. Many programming languages do not enforce size limits on variables intrinsically—they 
rely on the programmer to perform this bounds checking in the code. This is an inherent 

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Chapter 21 
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Malicious Code and Application Attacks
vulnerability because many programmers feel parameter checking is an unnecessary burden 
that slows down the development process. As a security practitioner, it’s your responsibility 
to ensure that developers in your organization are aware of the risks posed by buffer over-
flow vulnerabilities and that they take appropriate measures to protect their code against 
this type of attack.
Anytime a program variable allows user input, the programmer should take steps to 
ensure that each of the following conditions is met:
■
The user can’t enter a value longer than the size of any buffer that will hold it (for 
example, a 10-letter word into a 5-letter string variable).
■
The user can’t enter an invalid value for the variable types that will hold it (for example, 
a letter into a numeric variable).
■
The user can’t enter a value that will cause the program to operate outside its specified 
parameters (for example, answer a “yes” or “no” question with “maybe”).
Failure to perform simple checks to make sure these conditions are met can result in a 
buffer overflow vulnerability that may cause the system to crash or even allow the user to 
execute shell commands and gain access to the system. Buffer overflow vulnerabilities are 
especially prevalent in code developed rapidly for the web using Common Gateway Interface 
(CGI) or other languages that allow unskilled programmers to quickly create interactive 
web pages. Most buffer overflow vulnerabilities are mitigated with patches provided by 
software and operating system vendors, magnifying the importance of keeping systems and 
software up to date.