Web Server Administration
Chapter 10
Securing the Web Environment
Overview
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Identify threats and vulnerabilities
Secure data transmission
Secure the operating system
Secure server applications
Overview
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Authenticate Web users
Use a firewall
Use a proxy server
Use intrusion detection software
Identifying Threats and
Vulnerabilities
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Focus is on threats from the Internet
Hackers sometimes want the challenge of
penetrating a system and vandalizing it –
other times they are after data
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Data can be credit card numbers, user names and
passwords, other personal data
Information can be gathered while it is being
transmitted
Often, operating system flaws can assist the
hacker
Examining TCP/IP
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Hackers often take advantage of the intricacy
of TCP/IP
The following are parts of the IP header most
relevant to security
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Source address
Destination address
Packet identification, flags, fragment offset
Total length
Protocol – TCP, UDP, ICMP
TCPDelivering Data to Applications
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Important header fields
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Source and destination ports
Sequence number, data offset
Flags, such as SYN, ACK, FIN
Establishing a TCP connection
Vulnerabilities of DNS
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Historically DNS has had security
problems
BIND is the most common
implementation of DNS and some older
version had serious bugs
BIND 9, the current version, has been
more secure
Vulnerabilities in Operating
Systems
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Operating systems are large and complex
which means that there are more
opportunities for attack
Although Windows has had its share of
problems, often inattentive administrators
often fail to implement patches when
available
Some attacks, such as buffer overruns, can
allow the attacker to take over the computer
Vulnerabilities in Web servers
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Static HTML pages pose virtually no
problem
Programming environments and
databases add complexity that a hacker
can exploit
Programmers often do not have time to
focus on security
Vulnerabilities of E-mail
Servers
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By design, e-mail servers are open
E-mail servers can be harmed by a series of
very large e-mail messages
Sending an overwhelming number of
messages at the same time can prevent valid
users from accessing the server
Viruses can be sent to e-mail users
Retrieving e-mail over the Internet often
involves sending your user name and
password as clear text
Securing Data Transmission
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To secure data on a network that is
accessible to others, you need to
encrypt the data
SSL is the most common method of
encrypting data between a browser and
Web server
Secure Shell (SSH) is a secure
replacement for Telnet
Secure Sockets Layer (SSL)
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A digital certificate issued by a certification
authority (CA) identifies an organization
The public key infrastructure (PKI) defines
the system of CAs and certificates
Public key cryptography depends on two keys
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A public key is shared with everyone
The public key can be used to encrypt data
Only the owner of the public key has the
corresponding private key which is needed to
decrypt the data
Establishing an SSL
Connection
Using SSH for Tunneling
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Tunneling allows you to use an unsecure
protocol, such as POP3, through a secure
connection, such as SSH
To set up tunneling
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Configure the SSH client so the local port is 55555
(or another port between 1024 and 65535)
Configure the SSH client to connect to POP3 port
110
Log in to the SSH client
Direct the e-mail client to port 5555 and log in to
the e-mail server
Securing the Operating
System
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Use the server for only necessary tasks
Minimize user accounts
Disable services that are not needed
Make sure that you have a secure password
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In addition to using upper case, lower case
numbers and symbols, hold down the ALT key on
a number (on the numeric keypad) from 1 to 255
Check a table of ALT values to avoid common
characters
The use of the ALT key will thwart most hackers
Securing Windows
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There are many services that are not needed in
Windows for most Internet-based server applications
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Alerter
Computer browser
DHCP client
DNS client
Messenger
Server
Workstation
Also, the registry can be used to alter the
configuration to make it more secure such as
disabling short file names
Securing Linux
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As with Windows, make sure that you only
run daemons (services) that you need
Generally, daemons are disabled by default
The command netstat -l gives you a list of
daemons that are running
Use chkconfig to enable and disable daemons
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chkconfig imap on would enable imap
Securing E-mail
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You have already seen the ability to
tunnel POP3 which would prevent data
from being seen
Exchange 2000 can also use SSL for the
protocols it uses
To prevent someone from sending large
e-mail messages until the disk is full,
set a size limit for each mailbox
Securing the Web Server
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Enable the minimum features
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If you don't need a programming
language, do not enable it
Make sure programmers understand
security issues
Implement SSL where appropriate
Securing the Web Server
Apache Directories
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You can restrict access to directories by using
"allow" and "deny"
The following only allows computers with the
two IP addresses to access the directory
<Directory "/var/www/html/reports">
order allow, deny
allow from 10.10.10.5 192.168.0.3
deny from all
</Directory>
Securing the Web ServerIIS
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The URLScan utility blocks potentially harmful page
requests
The IIS Lockdown utility has templates to ensure that
you only enable what you need
Change NTFS permissions in \inetpub\wwwroot from
Everyone Full Control to Everyone Execute
In IIS 5, delete \samples \IISHelp and \MSADC
folders
Delete extensions you do not use, such as .htr, .idc,
.stm, and others
Authenticating Web Users
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Both Apache and IIS use HTTP to
enable authentication
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HTTP tries to access a protected directory
and fails
Then it requests authentication from the
user in a dialog box
Accesses directory with user information
Used in conjunction with SSL
Configuring User
Authentication in IIS
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Four types of authenticated access
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Windows integrated authentication
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Digest authentication for Windows domain servers
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Works with proxy servers
Requires Active Directory and IE
Basic authentication
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Most secure – requires IE
User name and password in clear text
Works with IE, Netscape, and others
Passport authentication
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Centralized form of authentication
Only available on Windows Server 2003
User Authentication in Apache
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Basic authentication is most common
User names and passwords are kept in
a separate file
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Create password file
-c creates the users file
-b adds a password when creating user
htpasswd –c users mnoia
htpasswd users fpessoa
htpasswd users lcamoes –b lusiades
Apache
User Authentication Directives
Directive
Description
AuthName
Specifies descriptive text for user authentication that appears on the
user’s browser when the request is made to log on. Example:
AuthName Internal Product Information
AuthType
Specifies the authentication type. Digest not supported so use
Basic. Example: AuthType Basic
AuthUserFile
Specifies the complete path to the user authentication file.
Example: AuthUserFile /var/www/users
AuthGroupFile
Specifies the complete path to the text file that associates users with
groups.
require
Defines which users in the user authentication file are allowed access
to the directory. Examples:
require user fpessoa lcamoes
require group developers designers
require valid-user
Apache
User Authentication
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Assume you want to restrict the
/newprods directory to any user in the
users file
<Location /newprods>
AuthName "New Product Information"
AuthType Basic
AuthUserFile /var/www/users
require valid-user
</Location>
Using a Firewall
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A firewall implements a security policy
between networks
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Our focus is between the Internet and an
organization's network
You need to limit access, especially
from the Internet to your internal
computers
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Restrict access to Web servers, e-mail
servers, and other related servers
Types of Filtering
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Packet filtering
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Circuit-level filtering (stateful or dynamic filtering)
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Looks at each individual packet
Based on rules, it determines whether to let it pass through
the firewall
Controls complete communication session, not just individual
packets
Allows traffic initialized from within the organization to
return, yet restricts traffic initialized from outside
Application-level
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Instead of transferring packets, it sets up a separate
connection to totally isolate applications such as Web and email
A Packet-filtering Firewall
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Consists of a list of acceptance and denial
rules
A firewall independently filters what comes in
and what goes out
It is best to start with a default policy that
denies all traffic, in and out
We can reject or drop a failed packet
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Drop – (best) thrown away without response
Reject – ICMP message sent in response
Firewall on Linux - iptables
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Connections can be logged
Initializing the firewall
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Remove any pre-existing rules
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Set default policy to drop packets
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iptables --flush
iptables --policy INPUT DROP
iptables --policy OUTPUT DROP
At this point nothing comes in and nothing
goes out
Describing the Packets to
Accept
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-A (Append rule)
INPUT or OUTPUT
-i eth0 (input interface) or –o eth0 (output)
-p tcp or -p udp (protocol type)
-s , -d (source, destination address)
--sport, --dport (source, destination port)
-j ACCEPT (this is a good rule)
Allowing Access to Web Server
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Allow packets from any address with an
unprivileged port to the address on our server
destined to port 80
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The following should be on a single line
iptables –A INPUT –i eth0 –p tcp --sport 1024:65535 –d
192.168.1.10 --dport 80 –j ACCEPT
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Allow packets to go out port 80 from our
server to any unprivileged port at any
address
iptables –A OUTPUT –o eth0 –p tcp –s 192.168.1.10
--sport 80 --dport 1024:65535 –j ACCEPT
Allowing Access to DNS
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DNS uses port 53
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UDP for resolving, TCP for zone transfers
iptables –A INPUT –i eth0 –p udp --sport
1024:65535 –d 192.168.1.10 --dport 53 –j ACCEPT
iptables –A OUTPUT –o eth0 –p udp –s 192.168.1.10
--sport 53 --dport 1024:65535 –j ACCEPT
iptables –A INPUT –i eth0 –p tcp --sport
1024:65535 –d 192.168.1.10 --dport 53 –j ACCEPT
iptables –A OUTPUT –o eth0 –p tcp –s 192.168.1.10
--sport 53 --dport 1024:65535 –j ACCEPT
Allowing Access to FTP
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Port 21 for data, port 20 for control
Data is transferred through unprivileged ports
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Opening unprivileged ports can be a problem
iptables -A INPUT -i eth0 -p tcp --sport 1024:65535 -d
192.168.1.10 --dport 21 -j ACCEPT
iptables -A OUTPUT -o eth0 -p tcp -s 192.168.1.10 --sport 21 -dport 1024:65535 -j ACCEPT
iptables -A INPUT -i eth0 -p tcp --sport 1024:65535 -d
192.168.1.10 --dport 20 -j ACCEPT
iptables -A OUTPUT -o eth0 -p tcp -s 192.168.1.10 --sport 20 -dport 1024:65535 -j ACCEPT
iptables -A INPUT -i eth0 -p tcp --sport 1024:65535 -d
192.168.1.10 --dport 1024:65535 -j ACCEPT
iptables -A OUTPUT -o eth0 -p tcp -s 192.168.1.10 --sport
1024:65535 --dport 1024:65535 -j ACCEPT
Using a Proxy Server
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A proxy server delivers content on behalf of a user or
server application
Proxy servers need to understand the protocol of the
application that they proxy such as HTTP or FTP
Forward proxy servers isolate users from the Internet
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Users contact proxy server which gets Web page
Reverse proxy servers isolate Web server
environment from the Internet
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When a Web page is requested from the Internet, the proxy
server retrieves the page from the internal server
Using Intrusion Detection
Software
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Intrusion detection is designed to show
you that your defenses have been
penetrated
With Microsoft ISA Server, it only
detects specific types of intrusion
In Linux, Tripwire tracks changes to files
Tripwire
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Tripwire allows you to set policies that allow
you to monitor any changes to the files on
the system
Tripwire can detect file additions, file
deletions, and changes to existing files
By understanding the changes to the files,
you can determine which ones are
unauthorized and then try to find out the
cause of the change
Tripwire
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After installing Tripwire, you configure the
policy file to determine which files to monitor
A default list of files is included but it will take
time to refine the list
A report can be produced to find out which
files have been added, changed, and deleted
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Usually, it runs automatically at night
Intrusion Detection in ISA
Server
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The following intrusions are tracked
Windows out-of-band (WinNuke)–A specific type of Denial-of-
Service attack
Land–A spoofed packet is sent with the SYN flag set so that the source
address is the same as the destination address, which is the address of
the server. The server can then try to connect to itself and crash.
Ping of death –The server receives ICMP packets that include large
files attachments, which can cause a server to crash.
IP half scan –If a remote computer attempts to connect to a port by
sending a packet with the SYN flag set and the port is not available,
the RST flag is set on the return packet. When the remote computer
does not respond to the RST flag, this is called an IP half scan. In
normal situations, the TCP connection is closed with a packet
containing a FIN flag.
UDP bomb –A UDP packet with an illegal configuration.
Port scan –You determine the threshold for the number of ports that
are scanned (checked) before an alert is issued.
Summary
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Every computer connected to the Internet
represents a potential target for attack
Hackers can gather data and modify systems
SSL can secure data transmission
Keep each server to a single purpose such as
Web server or e-mail
Keep applications and services to a minimum
Summary
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User authentication controls access to one or
more Web server directories
Firewalls control access policies between
networks
A proxy server delivers content on behalf of a
user or server application
Intrusion detection software identifies
intrusions but typically does not prevent them