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Using information gained from the reconnaissance and scanning phases, attackers attempt to gain access to systems. The techniques employed during Phase 3, gaining access, depend heavily on the skill level of the attacker. Less experienced attackers use exploit tools developed by others, available at a variety of Web sites. More sophisticated attackers write their own customized attack tools and employ a good deal of pragmatism to gain access. This chapter explores techniques for gaining access by manipulating applications and operating systems.

Buffer overflows are among the most common and damaging attacks today. They exploit software that is poorly written, allowing an attacker to enter input into programs to execute arbitrary commands on a target machine. When a program does not check the length of input supplied by a user before entering the input into memory space on the stack or heap, a buffer overflow could result. Without this proper bounds checking, an attacker can send input that consists of executable code for the target system to run, along with a new return pointer for the stack. By rewriting the return pointer on the stack, the attacker can make the target system run the executable code. For heap-based buffer overflows, an attacker can manipulate other variables in the heap, and possibly execute malicious code.

Exploitation frameworks like Metasploit help automate the production and use of exploits, such as stack-based and heap-based buffer overflows. These tools let attackers write modular exploits and payloads, tying the two together in an easy-to-use interface.

Defenses against buffer overflow attacks include applying security patches in a timely manner, filtering incoming and outgoing traffic, and configuring systems so that their stacks cannot be used to store executable code. Software developers can also help stop buffer overflows by utilizing automated code-checking and compile-time stack protection tools.

Password attacks are also very common. Attackers often try to guess default passwords for systems to gain access, by hand or through using automated scripts. Password cracking involves taking the encrypted or hashed passwords from a system and using an automated tool to determine the original passwords. Password-cracking tools create password guesses, encrypt or hash the guesses, and compare the result with the encrypted or hashed password. The password guesses can come from a dictionary, brute-force routine, or a hybrid technique. Cain is one of the best tools for cracking passwords on Windows machines. On UNIX systems (as well as Windows), John the Ripper is excellent.

To defend against password attacks, you must have a strong password policy that requires users to have nontrivial passwords. You must make users aware of the policy, employ password filtering software, and periodically crack your own users' passwords (with appropriate permission from management) to enforce the policy. You might also want to consider authentication tools stronger than passwords, such as hardware tokens.

Attackers employ a variety of techniques to undermine Web-based applications. Some of the most popular techniques are account harvesting, undermining Web application session tracking and variables, and SQL injection. Account harvesting allows an attacker to determine account numbers based on different error messages returned by an application. To defend against this technique, you must make sure your error messages regarding incorrect user IDs and passwords are consistent.

Attackers can undermine Web application session tracking by manipulating URL parameters, hidden form elements, and cookies to try to clone another user's session. To defend against this technique, make sure your applications use strong session tracking information that cannot easily be determined by an attacker and protect all variables passed to a browser.

SQL injection allows attackers to extend SQL statements in an application by appending SQL elements to user input. The technique allows attackers to extract or update additional information in a back-end database behind a Web server. To protect your applications from this technique, you must carefully screen special characters from user input and make sure your Web application logs in to a database with minimal privileges.

Numerous browser-based vulnerabilities let an attacker take over a browsing machine that surfs to an infected Web server. By compromising trusted Web servers, attackers can spread their browser exploits to a large population. To defend against such attacks, keep your browsers patched, and utilize up-to-date antivirus tools.

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