Common Gateway Interface

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The official CGI logo from the spec announcement

In computing, Common Gateway Interface (CGI) is an interface specification that enables web servers to execute an external program to process HTTP or HTTPS user requests.

Such programs are often written in a scripting language and are commonly referred to as CGI scripts, but they may include compiled programs.[1]

A typical use case occurs when a web user submits a

response to the browser's request.[2]

Developed in the early 1990s, CGI was the earliest common method available that allowed a web page to be interactive. Due to a necessity to run CGI scripts in a separate process every time the request comes in from a client, various alternatives were developed.


In 1993, the National Center for Supercomputing Applications (NCSA) team wrote the specification for calling command line executables on the www-talk mailing list.[3][4][5] The other Web server developers adopted it, and it has been a standard for Web servers ever since. A work group chaired by Ken Coar started in November 1997 to get the NCSA definition of CGI more formally defined.[6] This work resulted in RFC 3875, which specified CGI Version 1.1. Specifically mentioned in the RFC are the following contributors:[2]

Historically CGI programs were often written using the

C library
routine getenv() or variable environ".

The name CGI comes from the early days of the Web, where webmasters wanted to connect legacy information systems such as databases to their Web servers. The CGI program was executed by the server and provided a common "gateway" between the Web server and the legacy information system.


Traditionally a

fully-qualified domain name, and its document collection is stored at /usr/local/apache/htdocs/ in the local file system
(its document root), then the web server will respond to a request for by sending to the browser a copy of the file /usr/local/apache/htdocs/index.html (if it exists).

For pages constructed on the fly, the server software may defer requests to separate programs and relay the results to the requesting client (usually, a Web browser that displays the page to the end user).

Such programs usually require some additional information to be specified with the request, such as query strings or cookies. Conversely, upon returning, the script must provide all the information required by HTTP for a response to the request: the HTTP status of the request, the document content (if available), the document type (e.g. HTML, PDF, or plain text), et cetera.

Initially, there were no standardized methods for data exchange between a browser, the HTTP server with which it was communicating and the scripts on the server that were expected to process the data and ultimately return a result to the browser. As a result, mutual incompatibilities existed between different HTTP server variants that undermined script portability.

Recognition of this problem led to the specification of how data exchange was to be carried out, resulting in the development of CGI. Web page-generating programs invoked by server software that adheres to the CGI specification are known as CGI scripts, even though they may actually have been written in a non-scripting language, such as C.

The CGI specification was quickly adopted and continues to be supported by all well-known HTTP server packages, such as Apache, Microsoft IIS, and (with an extension) node.js-based servers.

An early use of CGI scripts was to process forms. In the beginning of HTML, HTML forms typically had an "action" attribute and a button designated as the "submit" button. When the submit button is pushed the URI specified in the "action" attribute would be sent to the server with the data from the form sent as a query string. If the "action" specifies a CGI script then the CGI script would be executed, the script in turn generating an HTML page.


A Web server that supports CGI can be configured to interpret a

standard output is passed to the Web client instead of being shown in the terminal window that started the web server. Another popular convention is to use filename extensions
; for instance, if CGI scripts are consistently given the extension .cgi, the Web server can be configured to interpret all such files as CGI scripts. While convenient, and required by many prepackaged scripts, it opens the server to attack if a remote user can upload executable code with the proper extension.

The CGI specification defines how additional information passed with the request is passed to the script. The Web server creates a subset of the

standard input. The script can then read these environment variables or data from standard input and adapt to the Web browser's request.[8]


CGI is often used to process input information from the user and produce the appropriate output. An example of a CGI program is one implementing a wiki. If the user agent requests the name of an entry, the Web server executes the CGI program. The CGI program retrieves the source of that entry's page (if one exists), transforms it into HTML, and prints the result. The Web server receives the output from the CGI program and transmits it to the user agent. Then if the user agent clicks the "Edit page" button, the CGI program populates an HTML textarea or other editing control with the page's contents. Finally if the user agent clicks the "Publish page" button, the CGI program transforms the updated HTML into the source of that entry's page and saves it.


CGI programs run, by default, in the security context of the Web server. When first introduced a number of example scripts were provided with the reference distributions of the NCSA, Apache and CERN Web servers to show how shell scripts or C programs could be coded to make use of the new CGI. One such example script was a CGI program called PHF that implemented a simple phone book.

In common with a number of other scripts at the time, this script made use of a function: escape_shell_cmd(). The function was supposed to sanitize its argument, which came from user input and then pass the input to the Unix shell, to be run in the security context of the Web server. The script did not correctly sanitize all input and allowed new lines to be passed to the shell, which effectively allowed multiple commands to be run. The results of these commands were then displayed on the Web server. If the security context of the Web server allowed it, malicious commands could be executed by attackers.

This was the first widespread example of a new type of Web based attack called code injection, where unsanitized data from Web users could lead to execution of code on a Web server. Because the example code was installed by default, attacks were widespread and led to a number of security advisories in early 1996.[9]


For each incoming HTTP request, a Web server creates a new CGI

by a virtual machine. For a high number of HTTP requests, the resulting workload can quickly overwhelm the Web server.


computational overhead
involved in CGI process creation and destruction can be reduced by the following techniques:

  • CGI programs precompiled to machine code, e.g. precompiled from C or C++ programs, rather than CGI programs executed by an interpreter, e.g. Perl, PHP or Python programs.
  • Web server extensions such as
    plugins which allow long-running application processes handling more than one request and hosted within the Web server.
  • FastCGI, SCGI, and AJP which allow long-running application processes handling more than one request to be hosted externally; i.e., separately from the Web server. Each application process listens on a socket; the Web server handles an HTTP request and sends it via another protocol (FastCGI, SCGI or AJP) to the socket only for dynamic content, while static content is usually handled directly by the Web server. This approach needs fewer application processes so consumes less memory than the Web server extension approach. And unlike converting an application program to a Web server extension, FastCGI, SCGI, and AJP application programs remain independent of the Web server.
  • threads. It also exposes the programmer to the library that comes with Java SE
    on which the version of Jakarta EE in use is based.
  • Standalone HTTP Server
  • Python programming language. It is defined by PEP 3333[10] and implemented via various methods like mod_wsgi (Apache module), Gunicorn web server (in between of Nginx & Scripts/Frameworks like Django), UWSGI
    , etc.

The optimal configuration for any Web application depends on application-specific details, amount of traffic, and complexity of the transaction; these trade-offs need to be analyzed to determine the best implementation for a given task and time budget. Web frameworks offer an alternative to using CGI scripts to interact with user agents.

See also

  • – Perl module for programming Common Gateway Interface (CGI) web applications
  • DOS Gateway Interface
     – Graphical web browser for DOS and Linux
  • Perl Web Server Gateway Interface
     – web application framework
  • Rack (web server interface) – API specification for web applications in programming language Ruby
  • Server Side Includes – Interpreted server-side scripting language


  1. from the original on 11 February 2007. Retrieved 16 February 2021.
  2. ^
    doi:10.17487/RFC3875. Archived from the original on 19 April 2021. Retrieved 25 February 2012. {{cite journal}}: Cite journal requires |journal= (help
  3. ^ McCool, Rob (November 14, 1993). "Server Scripts". www-talk (Mailing list). Retrieved 2019-05-15.
  4. ^ "The Common Gateway Interface". National Center for Supercomputing Applications (NCSA). Archived from the original on 27 January 2010.
  5. ^ "CGI: Common Gateway Interface". World Wide Web Consortium. Archived from the original on 19 December 2009. Retrieved 2019-05-15.
  6. ^ "Common Gateway Interface RFC Project Page". Archived from the original on 25 August 2013.
  7. ^ "Mapping URLs to Filesystem Locations Apache HTTP Server Version 2.2". Archived from the original on 15 July 2014. Retrieved 16 July 2014.
  8. ^ Nelson, Anne Fulcher, and Nelson, William Harris Morehead. (2001). Building Electronic Commerce with Web Database Constructions. Boston, MA: Addison Wesley.
  9. ^ "phf CGI Script fails to guard against newline characters". Software Engineering Institute CERT Coordination Center. Archived from the original on 28 July 2020. Retrieved 21 November 2019.
  10. ^ "PEP 3333 – Python Web Server Gateway Interface v1.0.1 |". Python Enhancement Proposals (PEPs). Retrieved 5 April 2024.

External links