[W3C] The World Wide Web Security FAQ

^ Up to Table of Contents
<< Backward to What's New
Forward to Running a Secure Server >>

3. General Questions

Q1: What's to worry about?

Unfortunately, there's a lot to worry about. There are security risks that affect Web servers, the local area networks that host Web sites, and even innocent users of Web browsers.

The risks are most severe from the Webmaster's perspective. The moment you install a Web server at your site, you've opened a window into your local network that the entire Internet can peer through. Most visitors are content to window shop, but a few will try to to peek at things you don't intend for public consumption. Others, not content with looking without touching, will attempt to force the window open and crawl in. The results can range from the merely embarassing, for instance the discovery one morning that your site's home page has been replaced by an obscene parody, to the damaging, for example the theft of your entire database of customer information.

It's a maxim in system security circles that buggy software opens up security holes. It's a maxim in software development circles that large, complex programs contain bugs. Unfortunately, Web servers are large, complex programs that can (and in some cases have been proven to) contain security holes. Furthermore, the open architecture of Web servers allows arbitrary CGI scripts to be executed on the server's side of the connection in response to remote requests. Any CGI script installed at your site may contain bugs, and every such bug is a potential security hole.

From the point of view of the network administrator, a Web server represents yet another potential hole in your local network's security. The general goal of network security is to keep strangers out. Yet the point of a Web site is to provide the world with controlled access to your network. Drawing the line can be difficult. A poorly configured Web server can punch a hole in the most carefully designed firewall system. A poorly configured firewall can make a Web site impossible to use. Things get particularly complicated in an intranet environment, where the Web server must typically be configured to recognize and authenticate various groups of users, each with distinct access privileges.

To the end-user, Web surfing feels both safe and anonymous. It's not. Active content, such as ActiveX controls and Java applets, introduces the possibility that Web browsing will introduce viruses or other malicious software into the user's system. Active content also has implications for the network administrator, insofar as Web browsers provide a pathway for malicious software to bypass the firewall system and enter the local area network. Even without active content, the very act of browsing leaves an electronic record of the user's surfing history, from which unscrupulous individuals can reconstruct a very accurate profile of the user's tastes and habits.

Finally, both end-users and Web administrators need to worry about the confidentiality of the data transmitted across the Web. The TCP/IP protocol was not designed with security in mind; hence it is vulnerable to network eavesdropping. When confidential documents are transmitted from the Web server to the browser, or when the end-user sends private information back to the server inside a fill-out form, someone may be listening in.

Q2: Exactly what security risks are we talking about?

There are basically three overlapping types of risk:
  1. Bugs or misconfiguration problems in the Web server that allow unauthorized remote users to:
  2. Browser-side risks, including:
  3. Interception of network data sent from browser to server or vice versa via network eavesdropping. Eavesdroppers can operate from any point on the pathway between browser and server including:

It's important to realize that "secure" browsers and servers are only designed to protect confidential information against network eavesdropping. Without system security on both browser and server sides, confidential documents are vulnerable to interception.

Protecting against network eavesdropping and system security are the subject of sections 1 to 5 of this document. Client-side security is covered in sections 6 and 7. Section 8 deals with security alerts for specific Web servers.

Q3: Are some operating systems more secure to use as platforms for Web servers than others?

The answer is yes, although the Unix community may not like to hear it. In general, the more powerful and flexible the operating system, the more open it is for attack through its Web (and other) servers.

Unix systems, with their large number of built-in servers, services, scripting languages, and interpreters, are particularly vulnerable to attack because there are simply so many portals of entry for hackers to exploit. Less capable systems, such as Macintoshes and MS-Windows machines, are less easy to exploit. Then again it's harder to accomplish really cool stuff on these machines, so you have a tradeoff between convenience and security.

Of course you always have to factor in the experience of the people running the server host and software. A Unix system administered by a seasoned Unix administrator will probably be more secure than a MS Windows system set up by a novice.

Q4: Are some Web server software programs more secure than others?

Again, the answer is yes, although it would be foolhardy to give specific recommendations on this point. As a rule of thumb, the more features a server offers, the more likely it is to contain security holes. Simple servers that do little more than make static files available for requests are probably safer than complex servers that offer such features as on-the-fly directory listings, CGI script execution, server-side include processing, and scripted error handling.

Version 1.3 of NCSA's Unix server contains a serious known security hole. Discovered in March of 1995, this hole allows outsiders to execute arbitrary commands on the server host. If you have a version 1.3 httpd binary whose creation date is earlier than March 1995 don't use it! Replace it with the patched 1.3 server (available at http://hoohoo.ncsa.uiuc.edu/) or with version 1.4 or higher (available at the same site). The Apache plug-in replacement for NCSA ( http://www.hyperreal.com/apache/info.html) is also free of this bug.

Servers also vary in their ability to restrict browser access to individual documents or portions of the document tree. Some servers provide no restriction at all, while others allow you to restrict access to directories based on the IP address of the browser or to users who can provide the correct password. A few servers, primarily commercial ones (e.g. Netsite Commerce Server, Open Market), provide data encryption as well.

The WN server, by John Franks, deserves special mention in this regard because its design is distinctively different from other Web servers. While most servers take a permissive attitude to file distribution, allowing any document in the document root to be transferred unless it is specifically forbidden, WN takes a restrictive stance. The server will not transfer a file unless it has been explicitly placed on a list of allowed documents. On-the-fly directory listings and other "promiscuous" features are also disallowed. Information on WN's security features can be found in its online documentation at:


A table comparing the features of a large number of commercial, freeware and public domain servers has been put together by the WebCompare site:


Q5: Are CGI scripts insecure?

CGI scripts are a major source of security holes. Although the CGI (Common Gateway Interface) protocol is not inherently insecure, CGI scripts must be written with just as much care as the server itself. Unfortunately some scripts fall short of this standard and trusting Web administrators install them at their sites without realizing the problems.

Q6: Are server-side includes insecure?

Server side includes, snippets of server directives embedded in HTML documents, are another potential hole. A subset of the directives available in server-side includes instruct the server to execute arbitrary system commands and CGI scripts. Unless the author is aware of the potential problems it's easy to introduce unintentional side effects. Unfortunately, HTML files containing dangerous server-side includes are seductively easy to write.

Some servers, including Apache and NCSA, allow the Web master to selectively disable the types of includes that can execute arbitrary commands. See Q10 for more details.

Q7: What general security precautions should I take?

For Web servers running on Unix systems, here are some general security precautions to take:
  1. Limit the number of login accounts available on the machine. Delete inactive users.

  2. Make sure that people with login privileges choose good passwords. The Crack program will help you detect poorly-chosen passwords:


  3. Turn off unused services. For example, if you don't need to run FTP on the Web server host, physically remove the ftp daemon. Likewise for tftp, sendmail, gopher, NIS (network information services) clients, NFS (networked file system), finger, systat, and anything else that might be hanging around. Check the file /etc/inetd.conf for a list of daemons that may be lurking, and comment out the ones you don't use.

  4. Remove shells and interpreters that you don't absolutely need. For example, if you don't run any Perl-based CGI scripts, remove the Perl interpreter.

  5. Check both the system and Web logs regularly for suspicious activity. The program Tripwire is helpful for scanning the system logs and sensitive files for break in attempts:


    More on scanning Web logs for suspicious activity below.

  6. Make sure that permissions are set correctly on system files, to discourage tampering. The program COPS is useful for this:


Be alert to the possibility that a _local_ user can accidentally make a change to the Web server configuration file or the document tree that opens up a security hole. You should set file permissions in the document and server root directories such that only trusted local users can make changes. Many sites create a "www" group to which trusted Web authors are added. The document root is made writable only by members of this group. To increase security further, the server root where vital configuration files are kept, is made writable only by the official Web administrator. Many sites create a "www" user for this purpose.

Q8: Where can I learn more about general network security measures?

Good books to get include:

A source of timely information, including the discovery of new security holes, are the CERT Coordination Center advisories, posted to the newsgroup comp.security.announce, and archived at:


A mailing list devoted specifically to issues of WWW security is maintained by the IETF Web Transaction Security Working Group. To subscribe, send e-mail to www-security-request@nsmx.rutgers.edu. In the body text of the message write:

SUBSCRIBE www-security your_email_address

A series of security FAQs is mainted by Internet Security Systems, Inc. The FAQs can be found at:


The main WWW FAQ also contains questions and answers relevant to Web security, such as log file management and sources of server software. The most recent version of this FAQ can be found at:


^ Up to Table of Contents
<< Backward to What's New
Forward to Running a Secure Server >>

Lincoln D. Stein (lstein@w3.org)
WWW Consortium

Last modified: Sat Nov 1 15:41:16 EST 1997