CNIT 152: 9 Network Evidence
- 4. Types of Network Monitoring
• Event-based alerts
• Snort, Suricata, SourceFire, RSA NetWitness
• Require rule sets
• Provides real-time notification
- 5. Types of Network Monitoring
• Headers or full packets
• Helps to identify scope of data theft
• Capture actions done with interactive shells
• Closely monitor malware communicating with
remote sites
- 6. Types of Network Monitoring
• High-level statistics showing type and number
of packets
• Can reveal interesting information on
activities that are not otherwise detectable
- 7. Event-Based Alert
Monitoring
• Most common type
• Based on rules or thresholds
• Events are generated by Network Intrusion
Detection Systems (NIDS)
• Or by software that monitors traffic patterns
and flows
• Standard tools: Snort and Suricata
- 8. Indicators (or Signatures)
• Matched against traffic observed by the network
sensor
• Simple indicators
• Such as IP address + port
• "Cheap" (small load on sensor)
• Complex indicators
• Session reconstruction or string matching
• Can burden the sensor so much it drops
packets
- 9. Example Snort Rule
• This rule detects SSH Brute Force attacks
• Depth: how many bytes of packet to read
• Links Ch 9a, 9b
- 10. alert_fast
• Put this in Snort configuration file
• output alert_fast alerts.txt
• Simplest output module for Snort
• Puts text into a file
- 11. Detect Fake SSL Certificate
• Detects a specific fake certificate used by an
APT group identified by Mandiant's in 2003
• Written by Emerging Threats
• Matches serial number and Issuer string
- 12. Header and Full Packet
Logging
• Two distinct purposes
• To help IR team generate signatures, monitor
activity, or identify stolen data
• Collect evidence for an administrative or legal
matter
• Consider whether to treat packet captures as
evidence and generate a chain of custody
- 13. Thoroughness
• IDS systems can retain the full session that
generated an alert
• But for targeted collection against specific
subjects, use tcpdump or Wireshark
- 14. tcpdump
• Complete packet capture of an HTTP request
• Done with "tcpdump -X"
• Limiting capture to 64 bytes captures only the
headers (called "trap and trace" by law
enforcement)
- 17. flow-tools and argus
• Open-source
• Convert pcap file (from tcpdump) to Argus format
• Graph all packets > 68 bytes from server1 by port
number
- 22. Simple Method
• Deploy laptops or 1U servers
with hardware network taps
• Snort + tcpdump works
• Best if you are setting up
monitoring after an incident
is detected--fast & easy
- 23. IDS Limitations
• IDS platforms cannot reliably perform both
intrusion detection and network surveillance
simultaneously
• If you set an IDS to capture full-content, its
effectiveness as a sensor will diminish
- 25. Hardware
• Difficult to collect and store every packet
traversing high-speed links
• Recommended:
• 1U servers from large manufacturers
• Linux-based network monitoring distributions
• Linux now outperforms FreeBSD
• For best performance, use NTOP's PF_RING
network socket, not the default AF_PACKET
interface
- 26. Before an Incident
• If your organization plans ahead
• Commercial solutions that combine Snort-style
alerting wth storage
• Solera Network's DeepSea appliance
• RSA's NetWitness platform
- 27. Security Onion
• Free Linux distribution, with kernel patched
installed (securityonion.net)
• Includes analysis tools
- 29. Major Network Changes
• May facilitate network surveillance
• Ex: route all company locations through a
single Internet connection with MPLS
(Multiprotocol Label Switching), not a
separate ISP for each office
- 30. Secure Sensor Deployment
• Place network sensor in a locked room, to
maintain chain of custody
• Patch the OS, keep it up to date
• Protect it from unauthorized access
• Document everything
• Review logs
• Use Tripwire to ensure integrity of OS
- 31. Evaluating Your Network
Monitor
• Is it receiving the traffic you want to monitor?
• Is the hardware responsive enough to
achieve your goals?
• Create signatures to detect test traffic and
test your monitor
• Such as a nonexistent URL
• Performance metrics in logs will tell you if the
sensor is dropping packets
- 33. General Principles
• Wireshark is excellent
• Especially with custom decoders, written in
Lua or C
• Don't hunt through large packet captures
looking for something new
• Limit the scope
• Use targeted queries that follow your leads and
answer investigative questions
- 34. Data Theft Scenario
• On Dec. 3, 2013, your investigation starts
• Two days ago, an attacker accessed a user's
desktop system
• Ran rar.exe and ftp.exe once each
• You have complete packet capture data
- 35. Prefetch
• Shows exact date and time ftp.exe was executed
• Dec. 1, 2013 at 00:57 UTC
• Interviews tell you that RAR and FTP are not
used normally on that workstation
- 36. PCAP File
• 73 FTP sessions on the date in question
• 2 are active during the time of interest
• Download PCAP files from link Ch 9e
- 39. Stream 1: FTP-Data (Port 20)
• A RAR file
being
transferred
• Show data as
"Raw"
• Save the file
as file.rar
with "Save
as"
- 41. Password
• The RARs are password-protected
• We can see the names of files and folders, but
not extract them
• A forensic examiner could search for command
lines using RAR.exe on the system, which might
contain the password
• Password cracking tools might help, but they
are slow
- 42. Is the Process Automated?
• Look for typographical errors
• Look at timing between steps of the attack
• Timing below indicates a human user
- 44. Webshell Reconnaissance
Scenario
• IDS detects a port scan coming from your DMZ
• From an Apache & MySQL server, on Windows,
at 203.0.113.101
• Interviews: no authorized port scan was run at
that time
• Login history shows no user logged in to the
server at that time
- 45. Apache Server Logs
• Large number of requests at the time of interest
• From an external IP address you don't recognize
• Many different pages requested
• Then many requests of the "/apps/login.php"
page
- 46. PHP Shell
• Many POST requests to "/apps/login.php"
• Then GET requests to "/tmpbkxcn.php"
• Containing strings such as
• cmd=netstat
• cmd=tasklist
- 48. SSL Encryption
• New versions of TLS have Forward Secrecy
• A different key for each session, using a
"session master secret"
• Older versions of TLS
• All data can be decrypted with the RSA private
key on the server
- 49. Importing the Key
• In Wirehark
• Wireshark,
Preferences,
Protocols,
SSL
• In "RSA keys
list" line,
click Edit
- 50. • "Decrypted SSL data" tab appears at bottom
• User-Agent: sqlmap (a common hacking tool)
- 59. Network-Based Logs
• Server-based logs are files on the individual
systems
• May be altered or deleted by the attacker
• Network-based logs may be more reliable
• Especially if network devices are physically
and electronically secured
- 60. Log Aggregation
• Log aggregation is difficult because:
• Logs are in different formats
• Originate from different operating systems
• May require special software to access and
read
• May have inaccurate timestamps