Network Intrusion Detection
Systems
Randy Marchany
VA Tech Computing Center
Blacksburg, VA 24060
Marchany@vt.edu
FAQ Information
 These notes come from the Network
Intrusion Detection Systems FAQ by
Robert Graham (nidsfaq@robertgraham.com
 http://www.robertgraham.com/pubs/netw
ork-intrusion-detection.html
Introduction
 Intrusion – an attempt to compromise or
misuse a computer system or network.
 Network Intrusion Detection System (NIDS)
monitors packets on the network wire and
attempts to discover if hackers are attempting
to break into a system or cause a DOS.
 NIDS can run on the target or independent
system.
Introduction
 Network NIDS monitor many machines.
 System Integrity Verifiers (SIV) monitor
system files to detect trojan versions of
system binaries. It may log the attempt
as it occurs.
 Log File Monitors (LFM) monitor log files
generated by network services.
Introduction
 Deception Systems aka honeypots, flytraps contain pseudo-services that
emulate well-known holes in an attempt
to trap hackers.
 Intruders – outsiders or insiders
How do Intruders Get In?
 Physical Intrusion – console passwords,
disk removal, etc.
 System Intrusion – hacker has a low
privilege account on the system and
uses a tool that exploits a weakness to
gain system privilege.
 Remote Intrusion – gains access via a
remote service on the system.
Vulnerability Types
 Race Conditions – 2 programs accessing
the same data at the same time.
 Software bugs – Buffer Overflows
 Unexpected Combinations – input is
meaningless at 1 level but not at another.
 Unhandled Input – what happens when
input doesn’t match specifications.
System Configuration
 Default – vendor shipped configurations
 Lazy – sysadmins too lazy to tighten the
system.
 Hole Creation – most programs can run
in non-secure mode.
 Trust Relationships – one system trusts
another. R-commands are an example.
Password Cracking
 Trivial – names of people, places, things
 Dictionary – Unix Crack or NT/L0pht
password cracking programs
 Brute Force – programs that try all
possible combinations of characters.
Sniffers & Design Flaws
 Shared Medium – base ethernet
 Server – sniffer runs on the server. Works on
switched nets.
 Remote – SNMP based
 TCP/IP Protocol Flaws – smurf, synflood, IP
spoofing. IP allows data to be changed
anytime. IPSEC is a fix.
 System Flaws – Windows, Unix
How Do They Get
Passwords?
 Clear Text Passwords
 Encrypted Sniffing
 Replay Attack – the intruders don’t decrypt the
passwords. They use the encrypted form to
login the systems.
 Password file stealing
 Observation/Social Engineering – piece of
paper attack
Intrusion Steps
 Outside Reconnaissance – whois, DNS,
WWW, FTP
 Inside Reconnaissance – ping sweep,
inverse mapping, port scanning, rpcinfo,
showmount, snmpwalk.
 Exploit – exploiting vulnerabilities
discovered earlier.
Intrusion Steps
 Foothold – gained entrance into the
machine and now starts to hide the
evidence. Install rootkits, trojans.
 Profit – taking advantage of the entry, the
hacker now goes after the real target –
information, $$, credit card info, etc.
 Joyride – systems used in a relay attack.
Common WWW Exploits
 CGI – passing data to the command
shell via shell metacharacters, using
hidden variables, phf.
 WWW server
 IIS/RDP - ../../../../ attack to get files from
the server.
 Alternate data streams ( Win95 names).
Common WWW Exploits
 URL – fields can cause buffer overflows as it’s
parsed in the HTTP header, displayed on the
screen or saved in the cache history. Old IE
bug would execute .LNK or .URL commands.
 HTTP headers can be used to exploit bugs
because some fields are passed to functions
that expect only certain information.
Common WWW Exploits
 HTML – MIME-type overflow in Netscape
Communicator’s <EMBED> command.
 Javascript – usually tries to exploit the
“file upload” function by generating a
filename and automatically hidden the
SUBMIT button. Many fixes for this but
equal # of circumventions.
Common WWW Exploits
 Frames – part of JavaScript or Java hack
(hiding web bugs). Hackers include link to valid
site that uses frames then replace some of
those frames with bad www pages.
 Java – normal Java applets have no access to
the local system but sometimes they’d be more
useful if they did have local access.
 Active X – works purely on trust model and
runs in native mode.
Buffer Overflows & DNS
Attacks
 DNS – extra long DNS name is sent to the
server. DNS names are limited to 256 bytes.
 RPC – statd, ttdbserverd, cmsd, snmpXdmid
 DNS Cache Poisoning – Every DNS packet
contains a Question/Answer section.
Vulnerable servers will believe and cache
Answer you provide.
Common Reconnaissance
Scans and DOS Attacks
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Ping Sweeps
TCP/UDP Scans
OS identification
Account Scans
Ping of Death
SYN Flood
Land
DDoS
How Do NIDS Detect
Intrusions?
 Anomaly detection – measures a baseline of
stats like CPU utilization, disk activity, user
logins, file activity. NIDS triggers when a
deviation from this baseline occurs.
 Signature recognition – pattern matching
attack probes. Uses large databases to detect
the attack. Antiviral software uses this. Works
only for known attacks.
Matching Signatures with
Incoming Traffic
 NIDS consists of special TCP/IP stack that
reassembles datagrams and TCP streams. It
uses:
 Protocol Stack Verification – search for
protocol violations (SYN/FIN, etc.)
 Application Protocol Verification
 New Event Creation – log all application layer
protocols for later correlation.
NIDS Detect the Attack
 Firewall reconfiguration to block IP address.
 Chime – “Danger, Will Robinson!” alarm. Email
or page admins.
 SNMP trap – send trap datagram to console.
 Syslog – record it in NT Event log or Unix
syslog
 Save Evidence.
 Launch Program to handle the event.
 Terminate the TCP connection by sending a
FIN.
Other Countermeasures
 Firewalls – should be considered as the LAST
line of defense.
 Authentication – password policies, single
signon, removing cleartext protocols.
 VPN – secure connection for remote access.
However, they decrease corporate security
because both ends of the pipe are wide open.
Where to locate IDS
 Network hosts
 Network Perimeter
 WAN/LAN Backbone
 Server farms
 Need to be on low-bandwidth nets to
keep up with traffic.
Fitting IDS with Security
Framework
 Put firewalls between networks with different
security requirements.
 Use scanners to check for exploits.
 Set host policy to conform with standards.
 Use NIDS to see what is actually happening.
 Use Host based IDS to flag intrusions.
 Create effective IRP.
Implementing IDS
 OS – enable logging/auditing features
 Services – build/enable security in WWW
servers, Email Servers, DB servers.
 NIDS – install in appropriate places.
 Firewalls – enable detection facilities.
 Install SNMP traps (Openview, Tivoli)
Some NIDS Products
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BlackIce Defender (Network Ice)
CyberCop Monitor (Network Associates)
RealSecure (ISS)
NetRanger (WheelGroup/Cisco)
eTrust Intrusion Detection (CA)
NetProwler (Axent)
Centrax (CyberSafe)
NFR (Network Flight Recorder)
Dragon (Security Wizards)
Network Grep System
 Based on raw packet capture and
searching for patterns using a ‘grep’ tool.
 Extract the suspect string and compare
to attack database.
 Libpcap ( library for packet capture) is
the library used by Unix-based IDS.
 Feed output from libpcap to grep filters.
Network Grep System
 Advantage – easy to update. Largest DB
of signatures, fastest time-to-market for
detecting attack scripts.
 Disadvantage – they detect the fewest #
of serious intrusions. Example: scanning
for default BO passwords. Can set off
false positives.
Network Grep System
 Advantages – system based on protocol
analysis result in fewer false positives.
Able to fully diagnose a problem.
Example: BO PING is harmless. BO
compromise is more serious.
Sample IDS Placement
IDS #1
INTERNET
INTERNAL
NETWORK
FIREWALL
IDS #3
IDS #2
IDS #4
IDS #1 – FW don’t produce enough info to effectively detect hits.
IDS #2 – detects attacks that penetrate the FW
IDS #3 – detects attacks attempted against the FW
IDS #4 – Insider attacks will be detected
Attacks Against the NIDS
 Blind the sensor with high traffic rates.
 Blind the event storage. Use decoy
scans to fill up log space.
 DOS
 Packet Fragmentation
 Slow Scan
 Coordinated low-bandwidth attacks
Attacks Against the NIDS
 Address spoofing
 Pattern Change
 Ptacek’s Paper on NIDS evasion.
Questions to ask IDS
Vendors
 How Much?
 What do signature updates cost?
 What traffic level blinds the IDS?
 How easy to evade?
 How scalable is it?
 How many signatures does it support?
 What IR features are included?