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.
Unix systems, with their large number of built-in servers, services, scripting languages, and interpreters, are particular 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.
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:
http://hopf.math.nwu.edu/docs/security.html
A table comparing the features of a large number of commercial, freeware and public domain servers has been put together by the WebCompare site:
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.
ftp://ftp.cert.org/pub/tools/crack/
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:
ftp://coast.cs.purdue.edu/pub/COAST/Tripwire/
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:
ftp://ftp.cert.org/pub/tools/cops/
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.
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:
ftp://ftp.cert.org/pub/cert_advisories/
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:
http://www.iss.net/sec_info/addsec.html
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: