Outline the common vulnerabilities in web servers and how those vulnerabilities are exploited in attacks.

Web servers are crucial components of the internet infrastructure, responsible for storing, processing, and delivering web content. However, they are also frequent targets of cyberattacks due to numerous vulnerabilities. These vulnerabilities can be exploited to gain unauthorized access, steal sensitive data, disrupt services, or launch further attacks. Understanding common web server vulnerabilities is essential for ensuring effective cybersecurity.

One of the most common vulnerabilities is insecure server configurations. This can involve leaving default configurations in place, which often have known weaknesses, such as default usernames and passwords or unnecessary features enabled. For example, a web server running with default credentials would allow attackers to easily gain administrative access. Also, leaving default configuration files exposed could inadvertently disclose sensitive data about the server itself which can be used by attackers. Another aspect of misconfiguration involves the use of out-of-date software, running software that has known security vulnerabilities due to missing patches. For instance, an old version of Apache or IIS might have known security holes, which can then be exploited using available exploits. These misconfigurations and security gaps can be found using automated tools and manual checks.

Another significant vulnerability is related to web application flaws. Many vulnerabilities exist within the applications running on the server and are not caused by the web server software itself, like SQL injection or Cross-Site Scripting (XSS). SQL injection vulnerabilities allow an attacker to execute malicious SQL code by injecting SQL commands into application input fields. For instance, a web application with an input form that does not properly validate user input might be susceptible to SQL injection attacks. This allows an attacker to query the database, retrieve sensitive data, or modify information in the database. XSS vulnerabilities can be exploited to inject malicious scripts into websites, usually through user input. These scripts are then executed when the page is viewed by other users, allowing the attacker to steal cookies, redirect users to malicious sites, or deface websites. For example, an attacker can inject JavaScript code that steals cookies from other users or redirects them to a phishing site.

Directory traversal vulnerabilities also pose a significant risk, they allow attackers to access files and directories outside of the web server's root directory. By manipulating file paths in the web application's URL, attackers can potentially view sensitive files, access system data, or execute commands on the server. For example, an attacker might use "../" sequences in a URL to navigate outside the web server's web root to access other important files or even system files if the webserver does not properly handle the directory paths provided by the user.

Insufficient access control vulnerabilities occur when web servers do not properly enforce access controls. This may allow unauthorized users to access restricted areas or administrative functions. For instance, improperly set file permissions on the server might enable attackers to upload malicious files or modify existing ones. Another form of access control issues includes inadequate authentication or authorization mechanisms, which might allow attackers to bypass login pages or gain administrative privileges by exploiting software vulnerabilities.

Denial-of-Service (DoS) vulnerabilities, which can be exploited to overload a web server with excessive traffic, making the server unresponsive to legitimate users are also a significant threat. DoS or DDoS attacks can cause service disruptions and affect the availability of websites or web applications. For example, an attacker may launch an HTTP flood attack or a TCP SYN flood attack to cause resource exhaustion on the targeted server and cause disruptions.

In summary, common vulnerabilities in web servers include misconfigurations, out-of-date software, web application flaws like SQL injection and XSS, directory traversal issues, insufficient access control, and vulnerabilities that can be exploited for denial-of-service attacks. These vulnerabilities are exploited by attackers to gain unauthorized access, steal sensitive data, disrupt services, and launch further attacks. Proper server configuration, regular patching, secure coding practices, and strong security controls are vital for mitigating these risks.