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The interface between the web server and the rest,

CGI (i.e. the Common Gateway Interface) sends requests from clients (e.g. browsers) to web server. CGI enables web servers to execute programs (e.g. in Python, Perl, C or shell scripts) and send their output back to the browser as HTML pages. It is one of the oldest methods for generating dynamic content on web pages.

How CGI works

  1. Request from browser: A user sends a request to the web server, typically by calling a URL that points to a CGI script.

  2. Server executes program: The web server starts the corresponding CGI program by passing the request parameters (e.g. form values) to the program.

  3. Program processes the data: The CGI program processes the data, generates dynamic content and sends the output (e.g. an HTML page) back to the web server.

  4. Response to the browser: The web server sends the response generated by the CGI program back to the user’s browser.

CGI was one of the most important technologies for generating dynamic web content in the 1990s, but has now been largely replaced by more efficient methods such as PHP, Python-based frameworks (e.g. Flask, Django) or server-side JavaScript (Node.js).

CGI scripts, especially poorly implemented ones, can contain numerous security vulnerabilities that can be exploited by attackers. Here are some of the most common security issues and potential attacks:

1. Command Injection

  • Description: A common problem with CGI scripts is that user input is injected unfiltered into system commands. For example, if a CGI script injects user input into a shell command chain, an attacker can execute arbitrary commands on the server.
  • Example:
  • Attacker enters a malicious value into a form field: 
    ; rm -rf / # (Linux command to delete all files on the server)
  • If the CGI script injects this value into a shell command without validation, that command will be executed and can cause severe damage to the server.
  • Safeguards: All user input should be filtered and securely validated. User input should never be injected directly into system commands.

2. Path Traversal

  • Description: In a path traversal attack, an attacker uses relative paths (such as ../../etc/passwd) to access files or directories that are outside the webroot directory. Poorly secured CGI scripts can allow attackers to read sensitive files.
  • Example:
  • A URL might look like http://example.com/cgi-bin/script.cgi?file=../../../../etc/passwd, which leads to access to a Linux server’s password file.
  • Protection measures: CGI scripts should restrict access to file paths and ensure that relative paths are not accepted in user input.

3. Insecure file access

  • Description: Many CGI scripts provide functions for reading and writing files based on user input. An attacker can access unauthorized files or upload files with malicious content by manipulating the input.
  • Example:
  • An upload script allows an attacker to upload a file containing a dangerous script or malware that can then be executed on the server.
  • Protection measures: Strict file type validations should be implemented to allow only trusted file types. Access rights to files should be limited to the necessary minimum.

4. Cross-Site Scripting (XSS)

  • Description: XSS attacks occur when CGI scripts output a user’s input directly to a web page without filtering or escaping it. This can inject malicious JavaScript code into the page that can access other users’ data.
  • Example:
  • A CGI script returns the user name without escaping it. An attacker could send the following input: 
    <script>alert('Hacked!')</script>
  • This code would then be executed in other users’ browsers.
  • Protections: User input should always be properly filtered or escaped before being output to HTML pages.

5. Denial of Service (DoS)

  • Description: Poorly optimized CGI scripts can also be vulnerable to denial-of-service attacks. If a CGI script performs very computationally intensive tasks or requires a lot of system resources, an attacker can overload the server by making excessive requests.
  • Example:
  • An attacker sends a large number of requests to a CGI script, which performs expensive computational operations or creates files on each request. This can lead to server overload.
  • Protection measures: CGI scripts should be optimized and implement resource constraints, e.g. by limiting execution time and input size.

6. Directory listing and misconfigurations

  • Description: Sometimes access to the directory where CGI scripts are located is not properly configured, allowing users to call the scripts directly or even see their source code. If directories like /cgi-bin/ are publicly accessible, attackers can obtain information about the structure of the system or execute old, unsafe scripts.

  • Example:

  • An attacker can simply visit the URL http://example.com/cgi-bin/ and see a list of all scripts located there, including potentially unsafe or outdated versions.

  • Protection measures: Directories where CGI scripts are stored should be protected from direct access. Directory listing must be disabled and old scripts removed.

Conclusion

While the Common Gateway Interface (CGI) provides an easy way to create dynamic web content, it also brings with it a variety of security risks, especially when scripts are poorly programmed or configured. Insecure programming practices allow attackers to use various techniques to compromise servers, steal sensitive information, or perform malicious actions.

Modern alternatives to CGI such as PHP, Node.js or frameworks such as Flask or Django offer more robust security mechanisms while also providing better performance and scalability. If CGI is still used, it is crucial to implement security measures such as input validation, permission management, and output escaping to prevent possible attacks.

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