Buffer Overflow

Abstract
Writing outside the bounds of a block of allocated memory can corrupt data, crash the program, or cause the execution of malicious code.

Description
Buffer overflow is probably the best known form of software security vulnerability. Most software developers know what a buffer overflow vulnerability is, but buffer overflow attacks against both legacy and newly-developed applications are still quite common. Part of the problem is due to the wide variety of ways buffer overflows can occur, and part is due to the error-prone techniques often used to prevent them.

Buffer overflows are not easy to discover and even when one is discovered, it is generally extremely difficult to exploit. Nevertheless, attackers have managed to identify buffer overflows in a staggering array of products and components.

In a classic buffer overflow exploit, the attacker sends data to a program, which it stores in an undersized stack buffer. The result is that information on the call stack is overwritten, including the function's return pointer. The data sets the value of the return pointer so that when the function returns, it transfers control to malicious code contained in the attacker's data.

Although this type of stack buffer overflow is still common on some platforms and in some development communities, there are a variety of other types of buffer overflow, including Heap buffer overflow and |Off-by-one error among others. Another very similar class of flaws is known as Format string attack. There are a number of excellent books that provide detailed information on how buffer overflow attacks work, including Building Secure Software [1], Writing Secure Code [2], and The Shellcoder's Handbook [3].

At the code level, buffer overflow vulnerabilities usually involve the violation of a programmer's assumptions. Many memory manipulation functions in C and C++ do not perform bounds checking and can easily overwrite the allocated bounds of the buffers they operate upon. Even bounded functions, such as strncpy, can cause vulnerabilities when used incorrectly. The combination of memory manipulation and mistaken assumptions about the size or makeup of a piece of data is the root cause of most buffer overflows.

Buffer overflow vulnerabilities typically occur in code that:


 * Relies on external data to control its behavior
 * Depends upon properties of the data that are enforced outside of the immediate scope of the code
 * Is so complex that a programmer cannot accurately predict its behavior

Buffer Overflow and Web Applications

Attackers use buffer overflows to corrupt the execution stack of a web application. By sending carefully crafted input to a web application, an attacker can cause the web application to execute arbitrary code – effectively taking over the machine.

Buffer overflow flaws can be present in both the web server or application server products that serve the static and dynamic aspects of the site, or the web application itself. Buffer overflows found in widely used server products are likely to become widely known and can pose a significant risk to users of these products. When web applications use libraries, such as a graphics library to generate images, they open themselves to potential buffer overflow attacks.

Buffer overflows can also be found in custom web application code, and may even be more likely given the lack of scrutiny that web applications typically go through. Buffer overflow flaws in custom web applications are less likely to be detected because there will normally be far fewer hackers trying to find and exploit such flaws in a specific application. If discovered in a custom application, the ability to exploit the flaw (other than to crash the application) is significantly reduced by the fact that the source code and detailed error messages for the application are normally not available to the hacker.

Environments Affected
Almost all known web servers, application servers, and web application environments are susceptible to buffer overflows, the notable exception being environments written in interpreted languages like Java or Python, which are immune to these attacks (except for overflows in the Interpretor itself).

Examples and References

 * OWASP Guide Project to Building Secure Web Applications and Web Services, Chapter 8: Data Validation
 * Aleph One, “Smashing the Stack for Fun and Profit”, http://www.insecure.org/stf/smashstack.txt
 * Mark Donaldson, “Inside the Buffer Overflow Attack: Mechanism, Method, & Prevention”, http://www.sans.org/reading_room/whitepapers/securecode/386.php

How to Determine If You Are Vulnerable
For server products and libraries, keep up with the latest bug reports for the products you are using. For custom application software, all code that accepts input from users via the HTTP request must be reviewed to ensure that it can properly handle arbitrarily large input.

How to Protect Yourself
Keep up with the latest bug reports for your web and application server products and other products in your Internet infrastructure. Apply the latest patches to these products. Periodically scan your web site with one or more of the commonly available scanners that look for buffer overflow flaws in your server products and your custom web applications. For your custom application code, you need to review all code that accepts input from users via the HTTP request and ensure that it provides appropriate size checking on all such inputs. This should be done even for environments that are not susceptible to such attacks as overly large inputs that are uncaught may still cause denial of service or other operational problems.

Related Attacks
Format string attack

Related Vulnerabilities
Heap buffer overflow Off-by-one error

Related Countermeasures
Input Validation

Categories
[[Category:C]]