What is Common Gateway Interface : Working and Its Applications

Would you like to know how browsers get us information so quickly and instantly? Here is the article that explains the working of CGI, its advantages, and disadvantages. Common Gateway Interface (CGI) is a set of standards used for running scripts and programs on a web server. As CGI is a standard interface, the communication between the web server and the client’s web browser follows specified rules. In other words, the Common Gateway Interface acts as a middleware between WWW servers and external databases and information sources. CGI was described by the WWW consortium, which also described the way the program interacts with a HyperText Transfer Protocol (HTTP) server. The scripts are written in PHP and ASP and processed on the web server before the page gets loaded and the result is sent to the client’s browser.

What is Common Gateway Interface?

The Common Gateway Interface (CGI) is described as a set of rules for exchanging information between a web server and a custom script. CGI is one of the most common ways for web servers to interact with users by sending the data. The programming languages used can be C, C++, Java, Perl, Python, or VB (Visual Basic).

Several HTML pages consist of forms, which use CGI programs to process the data available in forms. Scripts or programs are considered to be an alternative method used to provide feedback to the web users, while these operate from the user’s machine instead of the Web server and make use of programs like Java Scripts, Java applets, or ActiveX controls. These various technologies as a whole are known as client-side solutions and the use of CGI is known as server-side solutions as the processing is from the webserver.

How Common Gateway Interface Works

The following common gateway diagram helps to understand how CGI works when a user clicks a hyperlink to search and browse any web page. A web browser operating on a client machine uses HyperText Transfer Protocol (HTTP) to exchange information with a web server. The CGI program resides on the same system where the webserver is present and they operate from the same system.


Based on the request type received from the browser, the web server attempts to either provide the document from its document file system or run a CGI program. Following are a series of events to create an HTML document using CGI scripts:

  • A user requests the HTTP web server and demands the URL. The user can also type the URL in a browser’s location window, which can be a hyperlink or specified as HTML <form> tag.
  • The web server will analyze the URL and looks for the filename. Otherwise, it activates the gateway program mentioned in the URL and sends parameters to the program via the URL
  • The Common Gateway Interface gateway processes the required information and sends file/HTML text to the webserver. Additionally, the server appends the MIME header and sends the HTML text to the browser.
  • Taking the result from a web server, the web browser displays either the received document or an error message.
    Features of CGI
  • CGI is utilized to create simple shell scripts and interactive application
  • They are well defined with a set of rules
  • CGI is written using simple programming languages like Perl, C
  • CGI is a technology that easily interfaces with HTML.
  • CGI is highly compatible with existing browsers.


  • Common Gateway Interface language should conform to the specifications and hence can be written in any programing language.
  • CGI enables us to use the already created code and users can avoid writing their own code again.
  • It is simple and secure as the applications operate on the server.
  • The advanced functions are a lot easier if performed in CGI than in Java.
  • It is quick to respond.


  • Common Gateway Interface consumes lots of processing time
  • CGI incurs overhead when HTTP remains as a stateless protocol
  • It consists of a massively existing code base, mainly in Perl
  • Every time a CGI script is executed, the process initiates again
  • Reduces the functioning of the server

Applications of Common Gateway Interface

CGI transforms the Web from collecting static data into a new interactive structure, wherein users can interact with the number of questions to run applications. Some of the applications that are designed using CGI are:



Forms are one of the most significant users of CGI. Forms allow the user to share information and is a subset of HTML. A CGI program makes these forms very interactive for both user and provider by processing and selecting the appropriate forms that match the selection criteria.


Web gateways are alike programs or scripts. It is used to access the necessary information that is not directly readable by the client from the database. The CGI program is employed to serve as a gateway and use appropriate programing language to read the information, format, and share it with the client.

Virtual Documents

Virtual document creation is the most important part of CGI. While virtual documents are created as per the user’s request, they can vary from virtual HTML, images, plain text to sometimes audio.

Working Applications of Common Gateway Interface

Some of the impressive CGI programs on the web that describes the powerful aspects of CGI are listed below.

Lycos World Wide Web Search

This server helps users to browse for particular documents. In response to the user’s request, Lycos returns the hypertext document matching the user’s Web search criteria. The links are www.lycos.com.

Coloring Book

This amazing application is designed for users who enjoy coloring. The link is www.ravenna.com/coloring.

Japanese to English Dictionary

It is a virtual document-based application. The cutting-edge CGI program inquires for an English word from the user and returns an equivalent Japanese word with appropriate images.

Guestbook with World Map

It uses a form-based application, which enables users to leave messages that are read by everyone.

Security Concerns of Common Gateway Interface

A CGI script implemented in the user’s browser may contain bugs. Each bug is the potential to create security issues. The possible security issues of CGI scripts are presented in two ways –

  • Leaking the information intentionally or unintentionally regarding host system helps hackers to easily break-in
  • CGI scripts processing remote user inputs can be vulnerable to attacks wherein the remote user deludes them to execute commands


1). Define the common gateway interface.

The Common Gateway Interface (CGI) is described as a set of rules for exchanging information between a web server and a custom script

2). Name some of the programming languages supported by CGI

C, C++, Java, Perl, Python, or VB (Visual Basic)

3). Give one feature of CGI

It utilized to create simple shell scripts and interactive application

4. Give one advantage and disadvantage of CGI

Advantage – CGI enables to use of the already created code and users can avoid writing their own code again

Disadvantage – Reduces the functioning of the server and consumes lots of processing time

5). What are the forms of CGI?

The form is a subset of HTML and is used for collecting the data and interactive communication

6). What is CGI overhead?

CGI incurs overhead when the HTTP becomes a stateless protocol. This means that the CGI process is initialized for every ‘hit’ on the browser.

7). Why is Perl used by many people for CGI?

Perl is an easy-to-use tool. It has powerful features to extract information from text files, scanning arbitrary text files, and print reports based on that information.

In this article, we have explained the definition of the Common Gateway Interface. Also, we discussed in detail the working principle of common gateway interface, advantages and disadvantages, uses, working applications of CGI across the globe, and possible security threats.