When it comes to attacking web servers without any initial access, there are more ways than one may think of. Lets start from the most common and go from there:

1. Web Application Vulnerabilities

Issues within the application itself can be exploited remotely to possibly gain control or access backend systems. Vulnerabilities such as SQL injections, XXE, serialization attacks – any security flaws in web applications – can lead the attacker to take control of the web server. These vulnerabilities are often the primary targets due to their visibility and accessibility.

Realistic Attack Scenario: An attacker exploits an SQL injection flaw in a web application’s news page, allowing the execution of arbitrary SQL commands on the backend database server. This breach is used to extract administrative credentials, write files and even run remote commands on the server.

2. Web Server Vulnerabilities

Security weaknesses in the server software itself, on which the web server operates, can result in gaining control of the web server. Examples include vulnerabilities in widely used server software like Apache and Nginx.

Realistic Attack Scenario: A well-known RCE vulnerability in Apache Struts is exploited by an attacker. By sending a specially crafted request, the attacker executes arbitrary code on the server, enabling them to deploy a reverse shell for persistent access.

3. Server Vulnerabilities

Operating systems and services that are part of the server’s operation and exposed to the internet can also be targets. Vulnerable components might include FTP, SSH, or any custom services running on the operating system.

Realistic Attack Scenario: An attacker finds an unpatched vulnerability in an SSH daemon running on an old Linux distribution. By exploiting this flaw, the attacker gains initial foothold and root access, allowing them to control the server, and by extension – our web-server.

4. Network Vulnerabilities

These involve weaknesses in the network where the server is located. If the server itself is too secure to crack directly, an attacker might target other less secure systems on the same network, and even eventually gaining domain admin privileges which provide access to all servers within the network.

Realistic Attack Scenario: An attacker scans for and identifies vulnerable IoT devices on the same network as the target web server. After compromising one of these devices, the attacker uses it as a pivot point to move laterally within the network, increasing his privileges with attacks like Pass-the-Hash and ultimately gaining domain admin privileges – which grants the attacker access to all the servers, including the one on which our web-server is sitting.

5. Human Vulnerabilities

These involve techniques designed to trick users into sharing or exposing their credentials. Social engineering is a prevalent method for exploiting human vulnerabilities.

Realistic Attack Scenario: A phishing campaign targets employees of a company with emails that mimic the IT department’s communication style. The emails direct the recipients to a fake login page designed to capture their credentials, which are then used to access the company’s web server.

6. Physical Vulnerabilities

This category focuses on exploiting physical access to the server or associated hardware.

Realistic Attack Scenario: An insider threat involves a disgruntled employee using a USB keylogger to capture the web server administrator’s login credentials. These are then used for unauthorized access and data theft.

7. Eavesdropping

This includes positioning oneself in-between the web server and the clients – man-in-the-middle (MitM) attacks – or eavesdropping on credentials sent over the network.

Realistic Attack Scenario: An attacker sets up a rogue Wi-Fi access point in a coffee shop frequented by a web developer who works remotely on a popular blog’s server. The developer connects to the rogue Wi-Fi, through which the attacker conducts a Man-in-the-Middle attack, intercepting the developer’s FTP credentials used to upload files to the server. The attacker uses these credentials to gain initial foothold on the network and for further access.

8. Bruteforcing Attacks

Bruteforcing attacks involve repeatedly trying different combinations of usernames and passwords until the attacker finds the right one that grants access. It’s like trying every key on a keyring until you find the one that unlocks a door.

Realistic Attack Scenario: An attacker targets a web server’s login interface that hasn’t implemented proper account lockout policies or CAPTCHA verification. Using automated tools, they attempt many thousands of username and password combinations, leveraging common credentials and previously breached data. Eventually, they succeed in logging in and gaining an initial foothold, perhaps even full control over the web server.

Conclusion

Hacking a web-server can include any of the above described methods and their various combinations.

Keeping a web server safe from remote attackers really boils down to staying alert and keeping everything up to date. It’s like making sure your home’s doors and windows are not only locked but also strong enough to resist break-ins. Here’s what you need to do:

• Check Inputs Rigorously: Make sure your server only accepts the kind of data it should.
• Stay Updated: Keep your software patched up and current.
• Code Smart: Write your server’s code like you’re writing instructions that leave no room for misinterpretation. Be clear and secure, minimizing the chances of someone finding a loophole.
• Tighten Network Security: Ensure your network services are set up correctly – think about it as setting up a good security system that keeps an eye on every corner of your house.

Understanding and blocking these common attack paths are key to increasing chances of keeping your web server secure.