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Bypassing IDS and firewall

Packet fragmentation​

  • Also known as IP fragment scanning or IP fragmentation
  • 📝 Splitting up TCP header to several smaller (fragmented) packets when sending
  • Server then reassembles them once all packets are received.
  • Usually ignored by IDSes as processing them requires a lot of computer resources
  • Any IP datagram can be fragmented: including UDP, TCP, ICMP, etc.
  • See also session splicing for HTTP header variant.
  • Tools
    • Nmap: -f flag e.g. nmap -f <ip-or-host>
      • splits the packets into 8 bytes or less after the IP header
      • Alternatively can use --mtu option allows to set bytes e.g. --mtu 16
    • fragroute
      • Usage: fragroute <domain-name>
      • Intercept, modify, and rewrite egress traffic to use fragments

Source routing​

  • Also called path addressing
  • Specifying which path the malformed packet will take to get to the target host.
  • Used to skip routes (routers/gateways) where firewalls exist
    • Disregards what route tables say
  • ❗ Almost always blocked
  • Done by modifying IP address field in IP Options field
    • IP datagram
  • Using Nmap:
    • Loose routing
      • Specifying packet to be loose source routed through given IP way points
      • E.g. --ip-options "L 192.168.0.7 192.168.30.9"
    • Strict routing
      • You will have to specify every single hop along the path.
      • E.g. --ip-options "S 192.168.0.7 192.168.0.9 .. 192.168.30.9"
  • See also IP address spoofing through source routing | Session hijacking.

IP address decoy​

  • Also known as decoy scan
  • All packets originate from the scanning machine, but some have spoofed source addresses.
  • Pros: Helps to confuse port scan detection
  • Cons
    • Does not offer any information beyond a regular scan.
    • Slows down the scanning process
  • Using Nmap:
    • nmap -D decoy1,decoy2,ME,decoy3... <target>: Manual list with custom positioned ME
    • or nmap -D RND:10 <target> to randomize 10 decoy IP addresses

IP address spoofing​

  • Used to make packet appear to come from someone else
  • Done by changing address information in IP packet header field
  • Replies go back to the spoofed address not to the attacker
  • Mostly used for DoS attacks
  • Tools:
    • hping: hping3 <target> -a <spoofed-ip>
    • Nmap: nmap <target> -S <spoofed-ip>

IP address spoofing detection techniques​

  • Direct TTL probes
    • Ping to suspect IP
    • If TTL in reply is not same as suspect packet, it may be spoofed
  • IP Identification Number
    • Ping to suspect IP
    • IPID should be close to but higher than suspect packet
    • OS increases IP incrementally for each packet sent
  • TCP Flow Control Method
    • Sliding window protocol
      • In TCP, for each packet ACK is expected back before sending another packet.
      • Window packets are allowed to be sent without having to wait for an ACK.
      • Allows packets to arrive out of order
      • Window size field in TCP header
        • Tells maximum amount of data sender can transmit without waiting for ACK
        • Windows update packet is used to negotiate a different window size.
    • Attacker that uses spoof IPs do not receive window size information
    • If victims receives data packets beyond the window size, they are spoofed packets

IP address spoofing countermeasures​

  • Use encryption
    • Best prevention against IP spoofing attacks
  • Avoid trust relationships
    • Also known as no trust architecture
    • Do not rely on IP-based authentication
    • Test all packets, even when they come from one of your trusted hosts
    • E.g. through password authentication, OAuth etc.
  • Use firewalls and filtering mechanisms
  • Use random initial sequence numbers (ISN)
    • As SYN is ISN+1 it allows malicious connections.
    • E.g. if it's based on timed counter it's predictable
  • Ingress filtering
  • Egress filtering against insider attacks
    • Blocking outcoming traffic
    • Good against insider attacks where e.g. malware can send information
  • SYN flooding countermeasures

Encryption​

  • 📝 Encryption over e.g. SSL/TLS or SSH is a good evasion as it "cripples" payload inspection.
  • One way to be able to inspect is through MITM attacks
    1. The server-side encryption is terminated at the inspecting firewall
    2. The firewall re-encrypts the client-side connection and passes data in between.
  • Other solution is to use Network Traffic Analysis (NTA)
    • It focuses more on unencrypted metadata rather than encrypted payload.
    • It uses behavioral analysis to sort through network traffic and identify dangerous or suspicious traffic based on what the traffic/host does on the network.

Proxy servers​

  • Acts as an intermediary for requests from clients and servers
  • client <--> proxy <--> server
  • You can use free public proxies for educational purposes
    • ❗ Careful with public proxies as they can be malicious e.g. inject stuff into your traffic.
    • 💡 Have your own proxies set-up, see steps for creating a proxy server

Usage by defenders​

  • As a firewall to protect internal network
  • As a IP address multiplexer hiding IP addresses of internal servers.
  • Anonymize web surfing to some extend e.g. with VPN services or Tor
    • Mobile tools include Shadowsocks (cross-platform sock5 proxy), proxydroid (http / sock4 / sock5 on Android)
  • Filtering unwanted content such as ads
  • Save bandwidth on local networks

Usage by attackers​

  • To conceal the real source of the scan.
  • To remotely access intranets/corporate resources using victim as proxy
  • To do man-in-the-middle attacks by redirecting user traffic to the proxy
  • To sniff user data
  • Tools include

Proxy chaining​

  • Helps to increase anonymity by using two or more proxies in chain:
    • like your_host <--> proxy1 <--> proxy2 <--> target_host
ProxyChains​

  • 📝 Open source unix tool to setup a proxy.

  • Download a file using e.g. proxychains4 wget <fileurl>

  • Configure through its configuration file vim /etc/proxychains.conf

    • E.g.

        sock4 127.0.0.1 9050
        http 159.65.133.175 3128
        http 50.198.134.65 80

Anonymizer​

  • Proxy server that acts as an intermediary and privacy shield between a client computer and Internet provider
  • Use-cases include
    • Bypassing internet censors
    • Ensuring privacy
    • Having protection against online attacks
    • Bypassing IDS and firewall rules (of e.g. companies or schools)

Types of anonymizers​

  • Networked anonymizers
    • Information is transferred through network of computers
    • Pros: Harder to analyze traffic
    • Cons: Nodes in the sequence can be compromised
  • Single-point anonymizers
    • Information is transferred through single website
    • Pros: Protects IP and related information
    • Cons: Less resistance to sophisticated analysis

Anonymizer tools​

Tor​
  • Free and open-source software for enabling anonymous communication
  • 📝 Stronger privacy than single node proxies or proxy chains without encryption
  • ❗ Anonymous as long as guard and exit nodes are not the same party and the traffic is over HTTPs
  • VPN / Proxies (non-chained): Single point of failure
    • Provider knows who you are and what you browse
    • Provider may inject packets (e.g. ads) into traffic or record personal details.
  • Tor traffic goes through at least 3 different servers (up to 5000) before destination
    • Provides separate encryption layer for each server
    • An observer cannot read or modify packets
    • Second server (middle relay) passes encrypted data without knowing more than its predecessor and descendant
    • Weaknesses
      • ❗ A bad first server (guard node) knows your IP but not what you sent or where.
      • ❗ A bad third server (exit node) sees your traffic but cannot know who sent the traffic.
        • ❗ Exit node can eavesdrop if the packet is not using HTTPS but HTTP
      • ❗ Will be no privacy if both the entry and exit nodes are hijacked by same hostile party.
I2P (Invisible Internet Project)​
  • Peer-to-peer alternative to Tor
  • Each participant acts both as a client and as a router