What is Directory Traversal?

Directory Traversal (also known as Path Traversal) is a security vulnerability in web applications that allows an attacker to access files or directories outside the intended directory. The attacker manipulates file paths to navigate through the server’s filesystem.

How Does a Directory Traversal Attack Work?

A vulnerable web application often processes file paths directly from user input, such as an URL:

https://example.com/getFile?file=report.pdf

If the server does not properly validate the input, an attacker could modify it like this:

https://example.com/getFile?file=../../../../etc/passwd

Here, the attacker uses ../ (parent directory notation) to move up the directory structure and access system files like /etc/passwd (on Linux).

Risks of a Successful Attack

• Exposure of sensitive data (configuration files, source code, user lists)
• Server compromise (stealing SSH keys or password hashes)
• Code execution, if the attacker can modify or execute files

Prevention Measures

• Input validation: Sanitize user input and allow only safe characters
• Use secure file paths: Avoid directly using user input in file operations
• Least privilege principle: Restrict the web server’s file access permissions
• Whitelist file paths: Allow access only to predefined files

CORS (Cross-Origin Resource Sharing) is a security mechanism implemented by web browsers to control which websites can access resources from other domains. By default, browsers block cross-origin requests—requests made from one website to another domain, protocol, or port—for security reasons.

Why does CORS exist?

Without CORS, malicious websites could secretly send requests to other servers (e.g., API servers or banking sites), potentially stealing or misusing sensitive data (Cross-Site Request Forgery, CSRF). CORS ensures that only explicitly allowed websites can access resources.

How does CORS work?

When a web application makes a cross-origin request (e.g., from http://example.com to https://api.example.com), the browser automatically sends a CORS request. The server must then respond with specific HTTP headers to indicate whether the request is allowed:

1. Without CORS headers:
   The browser blocks the request.

2. With CORS headers:
   The server can respond with Access-Control-Allow-Origin: * (allowing all domains) or a specific domain (Access-Control-Allow-Origin: https://example.com). This enables access.

Preflight Requests

For certain requests (e.g., PUT, DELETE, or requests with custom headers), the browser sends a preflight request using the OPTIONS method. The server must respond with the correct CORS headers to allow the main request.

Conclusion

CORS is a crucial security measure that prevents unauthorized websites from accessing foreign resources. Developers must configure the correct server-side headers to allow legitimate clients to access the data.

Remote Code Execution (RCE) is a severe security vulnerability where an attacker can execute malicious code on a remote computer or server. This can happen when a system has software vulnerabilities that allow an attacker to inject and execute arbitrary code. RCE attacks can have serious consequences because they can give the attacker control over the affected system.

How does Remote Code Execution work?

RCE occurs when an attacker exploits vulnerabilities in an application, operating system, or network component to inject and execute code on the system. These vulnerabilities can be found in various parts of an application, such as:

1. Web Applications: Insecure input validation, SQL injection, insecure deserialization, or other web application vulnerabilities can lead to RCE.
2. Server Software: Vulnerabilities in web servers, database servers, or other server applications can be exploited.
3. Network Services: Services accessible over the network with vulnerabilities can be targets for RCE attacks.

Example of an RCE Attack:

A common example is an insecure web application that does not properly validate user inputs. If an attacker inputs malicious code into a form field and the application processes this input without proper validation, the code can be executed on the server.

# A simple example in Python
import os

def execute_command(user_input):
    os.system(user_input)

# Attacker inputs: "ls; rm -rf /"
execute_command("ls; rm -rf /")

Potential Impacts of RCE:

• Complete System Takeover: The attacker can gain full control over the affected system.
• Data Loss or Theft: Sensitive data can be stolen or deleted.
• Malware Deployment: The attacker can install and spread malware.
• Pivoting and Exploiting Other Systems: The compromised server can be used as a launch point for attacks on other systems in the network.

Mitigation Measures against RCE:

1. Input Validation: Thoroughly validate and sanitize all user inputs.
2. Updates and Patches: Regularly update and patch all software components to fix known vulnerabilities.
3. Principle of Least Privilege: Applications should run with the minimum necessary permissions.
4. Secure Coding Practices: Use secure coding techniques and libraries to avoid vulnerabilities.
5. Intrusion Detection Systems (IDS): Implement IDS to detect and prevent suspicious activities.

By implementing these measures, the risk of an RCE attack can be significantly reduced.

Server Side Includes (SSI) Injection is a security vulnerability that occurs in web applications that use Server Side Includes (SSI). SSI is a technique allowing HTML files to be dynamically generated on the server by embedding special commands within HTML comments. These commands are interpreted and executed by the web server before the page is delivered to the client.

How does SSI Injection work?

In an SSI Injection attack, an attacker injects malicious SSI commands into input fields, URLs, or other mechanisms through which the application accepts user data. If the application does not properly validate and filter these inputs, the injected commands can be executed on the server.

Example of an SSI command:

This command would list the contents of the current directory on a vulnerable server.

Potential impacts of SSI Injection:

• File System Manipulation: Attackers can read, modify, or delete files.
• Remote Code Execution: Execution of arbitrary commands on the server, potentially leading to full system compromise.
• Information Theft: Access to sensitive information, such as configuration files or database contents.
• Denial of Service: Executing commands that crash or overload the server.

Mitigation measures against SSI Injection:

1. Validate and Sanitize Inputs: All user inputs should be thoroughly validated and restricted to acceptable values.
2. Use of Prepared Statements: Where possible, use prepared statements and parameterized queries to minimize the risk of injections.
3. Limit SSI Usage: Avoid using SSI if it is not necessary, to reduce exposure to such vulnerabilities.
4. Leverage Server Security Features: Configure the web server to accept only trusted SSI commands and avoid executing dangerous shell commands.

By implementing these measures, the risk of SSI Injection can be significantly reduced.

A web application firewall (WAF) is a security solution that has been specially developed to protect web applications. It monitors traffic between web browsers and web applications to detect and block potentially harmful or unwanted activity. Essentially, a WAF acts as a shield that protects web applications from a variety of attacks, including

1. SQL injection: an attack technique where attackers inject malicious SQL queries to access or manipulate the database.
2. Cross-site scripting (XSS): An attack method where attackers inject scripts into websites to compromise users, such as by stealing session cookies or performing malicious actions on the user's behalf.
3. Cross-site request forgery (CSRF): An attack in which an attacker makes a fraudulent request on behalf of an authenticated user to perform unwanted actions.
4. Brute force attacks: Repeated attempts to log into a system using stolen or guessed credentials.
5. Distributed Denial of Service (DDoS): Attacks in which a large number of requests are sent to a web application in order to overload it and make it inaccessible.
   
   A WAF analyzes HTTP and HTTPS traffic and applies specific rules and filters to identify and block suspicious activity. It can be implemented both at server level and as a cloud-based solution and is an important part of a comprehensive security strategy for web applications.