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Network Security

Introduction

  • What are we protecting?
    • Confidentiality
    • Integrity
    • Availability
  • What are assets?
    • Data and services
  • What is our objective?
    • Secure data so that it can be trusted by authorised users

Default Deny

  • By default, block all access
  • Have rules to allow certain connections
  • Stops business moving quickly
  • Doesn't work for IoT applications

Default Permit

  • Whole network exposed to outside world
  • Individual assets protected on case-by-case basis
  • Firewall removes noise + bad stuff, not trusted to be complete

Logging

  • Logging is cheap (takes up little space)
  • Can be great for debugging security failures
  • Must be complete, accurate, trustworthy, and secure
  • Good to put them away from the device generating the logs

Patching

  • Vulnerabilities found all the time
  • Important to patch if security critical
  • Patching can break machines, but the more often you update the less likely the less likely it is for patches to be massive and make big breaking changes

Service Minimisation

  • More services, more attack vectors
  • Options, higher is better:
    • Don't install service
    • Don't run service
    • Run locally
    • Run protected from network
    • Run exposed to network

Transport Attacks

  • Filter according to IP address and port rules
    • e.g. only accept traffic to the DNS server from some network
  • Doesn't work sometimes, because attacker has control over the packet it sends, and therefore can change the source address
    • However, responses will be sent to the wrong place

UDP Attacks

  • Can send packets and hope for buffer overruns
  • DoS
  • Use services that are authenticated by source
    • e.g. syslog: send logs to UDP port 514, it gets appended to a log file
  • Amplification attacks
    • Ask for a lot of data, and set return address to a target
    • Target gets sent lots of data
  • Broadcast attacks
    • e.g. send echo request to broadcast address with forged source, and forged source will recieve a packet from every machine

TCP Sequence Numbers

  • Repeated TCP sequence numbers (e.g. all start at 0) means that small connections can get confused
    • TCP checksum is per-segment, so will not detect segments being swapped
  • This means that the start sequence number is set randomly
  • But if we can predict sequence numbers, can ACK data that we can't see
  • Means we can insert arbitrary data
  • ...provided we can guess start sequence number

Handshake Offload

  • Attacker sends lots of SYNs, and no further packets
    • Resources alloc'd for the connection, and never used
  • Can forge source address, harder to track
  • Solution:
    • Do SYN, SYN/ACK, ACK in firewall
    • Firewall has own initial sequence number
    • Offset applied to all packets passing through firewall for this connection
  • Modern OSs will handle this themselves, but if you need to run old software, the above solution is good

Firewall Tasks

  • Drop packets from outside which have inside IPs
  • Drop packets from outside which have odd IPs (e.g. loopback)
  • Drop packets from inside which have outside IPs
  • Rate limit
  • SYN, SYN/ACK, ACK in firewall
  • Re-randomise ISN

Isolation

  • Each process should be run with least privileges
  • Need root access for low ports
  • Need to isolate machines from each other
    • If attacker gains access to one process, they can mess with others on the machine
    • Can be done through running different services as different users
    • Ideally different machines (including VMs/containers)

Virtualisation

  • Type 1 hypervisor
    • Host runs hypervisor
    • Usually linux
  • Type 2 hypervisor
    • OS running on host
    • This OS runs all the VMs
  • Para-virtualisation
    • OS slightly modified
    • Uses special network/graphics/other drivers to pass to VMs
  • Zones/jails/containers
    • Machine runs one kernel
    • Subset of processes are isolated with their own init, filesystem, networking
    • Lightweight
    • Easy to manage
    • Security looks good, but hard to audit

chroot()

  • chroot(/some/dir/) makes all processes onwards see /some/dir/ as /
  • Can't cd out of /some/dir/

Attack Trees

  • Build a tree, with attacker's goal at the top
  • Children are ways to achieve parent's goal
  • Should keep attack vectors separate, so that "hacking" one thing doesn't give the attacker multiple vectors
    • e.g. firewalls should be on separate machines

Network Elements

Spectrum of Boxes

  • Hubs
    • Copy ethernet packets from one interface to all interfaces
    • Done electronically
  • Simple Bridges
    • Copy ethernet packets, dropping malformed ones
    • Provide no protection apart from flooding attacks
  • Switches
    • Copy packets if source and host are on other sides
    • Stops attackers snooping on data on other side of switch
  • Filtering switches
  • Routers
    • Looks at IP header
    • Chooses correct interface
    • Doesn't propagate broadcast packets
  • Filtering routers/firewalls
    • Look at TCP/UDP ports and block/pass based on those values
    • Minimum requirement to be a security component
  • Stateful firewalls
    • Look at the state of TCP connections
    • Tracks sequence numbers, sensible responses, etc.
    • Blocks complex attacks on TCP
  • Deep packet inspecting firewall
    • Looks at whole packet
    • Virus scanner
    • Block encryption it doesn't know about
    • Unethical?

Intrusion Detection/Prevention Systems (IDS/IPS)

  • Gets copy of data from switches/routers
  • Examines whole packets for worrying conditions
  • Reports attacks, or tells routers to drop traffic
  • Will do things like checking SMTP commands

Switch Authentication

  • Configure switch to only accept certain host
  • e.g. set MAC address, block other ones
  • Does not scale well
  • Can be bypassed through MAC filtering, but does require attacker to proactively do something suspicious
  • Can also run 802.1x over ethernet, not widely done

Firewalls

  • Looks at packet headers, and blocks/passes based on some policy
  • Not based on packet contents
  • Usually live on:
    • Routers
    • Dedicated firewall machines
    • Computers acting as routers
    • Computers looking after themselves
  • Keep unusual/unwanted packets away from applications
  • They can not:
    • Detect malware etc.
    • Detect misuse of legitimate protocols
  • Don't do anything that an OS can't do, but
    • Provide smaller attack vector
    • Only have to audit one machine for it
    • Use hardened OSs
  • Track TCP state
    • Discard packets that don't line up, should happen anyway but can help prevent untested code paths being run etc.
    • Helps with DoS attacks

Proxies

  • Make request to stub server
  • Stub server puts in queue
  • Stub client reads from queue
  • Stub cilent sends request to real server
  • Issues with VOIP, too much delay

Wired/Wireless Security

Wired Networks

  • You know where the wires are
  • Tapping wires requires effort and is detectable
  • Tempest attacks are hard
    • Looking for radiation from a wire
    • Fibre makes this risk essentially zero
  • Can do MAC filtering
    • Easily bypassed

802.1x Authentication

  • Client supplies username and password
    • Or proves ownership of certificate
  • Pre-auth, port is restricted to auth only, or VLAN
  • Can do "posture analysis", checking how patched a device is
  • Problems
    • Difficult to generalise to printers/servers/webcams
    • Who has the keys? Device or user? Can malicious user copy keys to other devices?
    • Gets complicated with multiple supplicant support
    • Trivially bypassed through carefully timed switching of MAC addresses
  • Benefits
    • Posture analysis is useful
    • Requires attacker to do something overt

Wireless Networks

  • None of the inherent security of wired networks

Wired Equivalent Privacy (WEP)

  • Everyone has shared key (40 bits, later 104 bits)
  • Encrypt packets using RC4
  • All stations with key can see all packet contents
  • Broken
    • Attacker can attempt to obtain key
    • Generating random IVs very difficult in embedded devices
  • RC4
    • Uses key and IV to generate evolved keystream of bytes, which are XOR'd with message to form cipher text
    • Can either recover key, or recover evolved keystream
    • Inject unencrypted packets into the network, and then see how the network encrypts them

Wi-Fi Protected Access (WPA)

  • Emergency interim fix to WEP
  • Add sequence numbers
  • Better key mixing
  • Integrity checks
  • Sill weak

WPA2

  • Initial secret (PSK, or per-user login)
    • Generates ephemeral key used for one session (~1hr)
  • Uses AES128
  • Needs new hardware
  • Can't see other's traffic
    • But can use special key for broadcast traffic

WPA2 PSK

  • Use a pre-shared key
  • Mixes in SSID to help prevent dictionary attacks
  • No forward secrecy

WPA2 Enterprise

  • Per-user key is negotiated
  • Compromising key only compromises that key's user

Wirless Security Objectives

  • Authentication
    • Don't want neighbours using up bandwidth
  • Confidentiality
    • Don't want others on my network to see my data
  • Integrity
    • Don't want MITM attacks

Wireless Protected Setup

  • Bad
  • 8 digit pin
  • Split into 2 4s, each verified separately

Intrusion Detection Systems (IDSs)

  • Look at all traffic passing through a network
  • Access to headers, payloads, timing, etc.
  • Triggered by:
    • Patterns in packets, i.e. matching payloads of packets against signatures of known malware
    • Behaviour: matching against known behaviours of malware, e.g. lots of probing
    • Heuristic: matching against changes in network performance, or obviously dubious activity
  • Possible set ups
    • Behind firewall (monitors traffic that makes it through the firewall)
    • In front of firewall (for research purposes)
    • Mirror port: most switches has ability to pass all traffic to a mirror port, used for debugging
  • Lots of false positives, many rules are for old irrelevant attacks

Host IDSs

  • Tripwire
    • Take hashes of critical files
    • Compare with files on disk periodically
    • How do you secure the hashes/kernel/etc?
  • OSSEC
    • Similar to tripwire
    • Log analysis, filtering and response

TLS And Friends

  • Transport Layer Security (TLS)
  • Standardisation and extension of Secure Sockets Layer (SSL)
  • Encrypted layer over TCP
  • Basic idea:
    • Asymmetric encryption to agree on a key
    • Zero knowledge proof to show possession of private key for certificate
    • Symmetric encryption to encrypt data, HMAC to prove integrity
  • Some combinations are weak, some implementations have flaws
  • Huge range of negotiated cipher suites

Certificates

  • Public key, signed by certification authority (CA)
    1. Server sends cert
    2. Client checks cert using CA key
    3. Client sends random to server using public key
    4. Random is used to set up symmetric encryption

Some Initial Problems

  • Clients are rubbish at generating random numbers
    • So the nonces will be insecure, especially on portable devices
  • Communication starts with session key encrypted by server public key
    • So if server private key is recovered in future, then all communications can be decrypted, provided they were recorded
    • AKA no forward secrecy
    • Can do Diffie-Hellman key exchange to try and fix this
      • But as weak as the weakest RNG

Implementations

  • OpenSSL
    • Old, lots of portability support
  • LibreSSL

Hardware Security Modules (HSM)

  • Put key in HSM, and HSM provides interfaces for encryption decryption etc.
  • HSM is secured, so harder to get key

TLS Benefits

  • Confidentiality, integrity
  • Protects against MitM attacks if certificates are properly checked

One Time Passwords (OTP)

  • Secure hardware shares secret with server
  • Token combines secret with counter/clock, displays result
  • Used as password
  • Server accepts expected password plus some next n expected passwords, just incase the button was pressed without the code being entered
  • Secret is hard to extract, so secure
  • Can be done with texts as well (OTP over the air)
  • Protects against key loggers

OAuth

Denial of Service (DoS) Attacks

  • Killing applications/machines/networks
  • Can be done in variety of ways
    • Just sending lots of packets
      • Need high speed connection/lots of low speed connections
    • Exploiting stuff like allocating on SYNs
    • Relying on more complicated knowledge of applications
  • If attacker is idiot, can just filter them out
  • Cloudfare will proxy traffic through them
    • Can make attacker run complex Javascript before getting to your server
    • If there's HTTPS, requires Cloudfare to have certificate
  • Host your side on a CDN, rely on them to withstand the DoS
  • Anycasting
    • Advertise of BGP multiple locations hosting that IP
    • Attacker then has to take out each of those locations
  • Amplification attacks
    • Small request, big response
    • Forge source IP
    • To avoid being the middle man:
      • Default deny, rate limiting, paged responses (?)
  • Ingress filtering
    • All packets leaving network should originate in the network
  • Egress filtering
    • All packets entering the network should not originate in the network

DNS Attacks

  • Kaminsky attack
    • Pretend to be name server, and reply with fake DNS response
    • Guess the query ID number, which is only 16 bit, so doable
    • "Solved" by doing over several ports, so that adds entropy to QID
    • Don't NAT DNS!
  • Cache poisoning
    • Get corrupt data into DNS cache

VPNs

  • IP-in-IP to get packets between parts of the virtual network
    • IP packet becomes payload in new IP packet
  • Not secure by itself
  • Can use IPsec
  • Can use SSL-VPN
  • Networks address space should not clash

IPsec

  • Two modes of encapsulation
    • Tunnel, transport
  • Two modes of use
    • AH (authentication), ESP (encryption)
  • Two mechanisms for keying
    • Static and dynamic (IKE)
  • Layers
    • Data plane transformation has to happen at wire-speed
      • Big focus on speed
    • Management layer, i.e. key exchange, much less demanding
    • Transformation in kernel, key exchange in user-space
  • Problems
    • Implementation is complex
    • Key management issues
  • Implementations: KAME, FreeS/WAN
  • AH vs. ESP
    • AH (authentication header)
      • Integrity of payload and immutable parts of header
      • Keyed hash
      • Ensures it comes from where it claims to have come from
    • ESP (encapsulated security payload)
      • Provides encryption and optionally payload integrity
    • AH+ESP
      • Full integrity and encryption
      • Dropped in favour of other mechanisms that do the same basic job
  • Transport vs. Tunnel
    • Transport
      • When both nodes speak IPsec
      • Packets transformed directly
      • IP header, ESP/AH header, payload
    • Tunnel
      • Communication between routers that carry traffic
      • Packets are encapsulated (IP-in-IP)
      • IP header transformed too
      • IP header, ESP/AH header, IP header for end system, payload
  • Security Parameters Index (SPI)
    • SP defines algs/keys in use for security associations
    • Lookup in SPI table when handling packets
    • AH packets contain SPI for looking up key
  • There are sequence numbers to prevent replay

Internet Key Exchange

  • Can manage keys manually, or use IKE
  • Based on Diffie-Helman exchange
  • Can have MitM
  • Basic IKE flow
    • Create an Internet Security Association Key Management Protocol Security Association (ISAKMP SA (jesus))
    • Use the ISAKMP SA to protect negotiation of keys for traffic
  • Can have main mode, involving five exchanges to set up keys
  • Can have aggressive mode, involving three exchanges, but also reveals identities
  • Poor standard for implementations
  • Config is difficult

VPNs 2.0: This Time Its Personal

SSL VPN

  • Client-less option
    • Systems provide access to HTTP resources via an SSL connection between browser and VPN server, proxying through to original server
    • Not technically a VPN
    • Can have problems, due to all resources coming from single domain
      • XXS, cookies issues, etc, if any one of the resources is compromised
  • SSL forwarding
    • Run a local app listening on 127.0.0.1:port, and are tunnelled to server:port
  • OpenVPN
    • Uses auth mechanisms from SSL (OpenSSL)
    • Runs over TCP/UDP
      • Means it's ok for firewalls

Point-to-Point Protocol (PPP)

  • Layer 2 Tunnelling Protocol (L2TP)
    • Sends packets as UDP containing tunnelled data
    • Used to tunnel PPP
  • PPP derived from HDLC
  • Sends IP packets down serial lines
  • Has its own auth mechanism
    • Allows username/password
    • Can negotiate MTU, IPs
  • No encryption in PPP and L2TP
    • But we can use IPsec
    • Complex to set up

Bring Your Own Desktop (BYOD)

  • People will do it anyway, regardless of company policy
  • How do we deal with it?
    • Need to consider islanding, separating devices from each other for minimal interaction
    • TFA?

Legal

  • EU Laws
    • European Convention on Human Rights
    • European Data Protection Directive
    • European Data Retention Direction
  • Most countries have something like Regulation of Investigatory Powers Act
  • Telecoms slang yo:
    • Licensed operators (LOs)
    • Other LOs (OLOs)
    • Mobile LOs (MOLOs)