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We spend only a few words justifying the importance of preventing buffer overflow Consider the following facts Since the highly publicized Morris Worm first used a buffer overflow exploit against fingerd to aid its spread across the fledgling Internet in 1988, buffer overflow has become the single best-known software security vulnerability With almost 20 years of high-profile exposure, you might expect that buffer overflow would no longer pose a significant threat You would be wrong In 2000, David Wagner found that nearly 50% of CERT warnings for that year were caused by buffer overflow vulnerabilities [Wagner et al, 2000] What about today Buffer overflow contributed to 14 of the top 20 vulnerabilities in 2006 [SANS 20, 2006], and data collected by MITRE as part of the Common Vulnerabilities and Exposures (CVE) project show that the overall number of buffer overflow vulnerabilities being reported has not decreased meaningfully in this decade [Christy, 2006] If that isn t enough evidence of their ongoing impact, buffer overflow vulnerabilities were behind some of the most devastating worms and viruses in recent memory, including Zotob, Sasser, Blaster, Slammer, Nimda, and Code Red
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The best way to prevent buffer over ow vulnerabilities is to use a programming language that enforces memory safety and type safety In unsafe languages, of which C and C++ are the most widely used, the programmer is responsible for preventing operations from making undesirable changes to memory Any operation that manipulates memory can result in a buffer over ow, but in practice, the mistakes that most often lead to buffer over ow are clustered around a limited set of operations Before going into the variety of ways buffer over ows can occur, we look at a classic buffer over ow exploit Exploiting Buffer Over ow Vulnerabilities To understand the risk that buffer over ow vulnerabilities introduce, you need to understand how buffer over ow vulnerabilities are exploited Here we outline a canonical buffer over ow exploit We refer readers interested in more in-depth coverage of buffer over ow exploits to Exploiting Software [Hogland and McGraw, 2004] and The Shellcoder s Handbook [Koziol et al, 2004] In a classic stack smashing attack, the attacker sends data that contain a segment of malicious code to a program that is vulnerable to a stack-based
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buffer over ow In addition to the malicious code, the attacker includes the memory address of the beginning of the code When the buffer over ow occurs, the program writes the attacker s data into the buffer and continues beyond the buffer s bounds until it eventually overwrites the function s return address with the address of the beginning of the malicious code When the function returns, it jumps to the value stored in its return address Normally, this would return it to the context of the calling function, but because the return address has been overwritten, control jumps to the buffer instead and begins executing the attacker s malicious code To increase the likelihood of guessing the correct address of the malicious code, attackers typically pad the beginning of their input with a sled of NOP (no operation) instructions The code in Example 61 de nes the simple function trouble(), which allocates a char buffer and an int on the stack and reads a line of text into the buffer from stdin with gets() Because gets() continues to read input until it nds an end-of-line character, an attacker can over ow the line buffer with malicious data
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Example 61 This simple function declares two local variables and uses gets() to read a line of text into the 128-byte stack buffer line
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void trouble() { int a = 32; /*integer*/ char line[128]; /*character array*/ gets(line); /*read a line from stdin*/ }
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In today s security climate, the code in Example 61 would be quickly labeled unsafe because gets() is almost universally understood to be dangerous This basic variety of exploit still works on older platforms, but because buffer over ows offer attackers the ability to write arbitrary data to memory, the range of possible attacks is not limited to targeting the return address of a function To better understand what happens in a classic buffer over ow exploit, consider Figure 61, which shows three different versions of a simpli ed stack frame for trouble() The rst stack frame depicts the contents of memory after trouble() is called but before it is executed The local variable line is allocated on the stack beginning at address 0xNN The local variable a is just above it in memory; the return address (0x<return>) is just above that
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