Ethical Hacking - Denial of Service - notes

Notes from a video course


These are just my notes from the Pluralsight course: “Ethical Hacking: Denial of Service” by Troy Hunt.

HTTP flood attack

  • HTTP flood attack
    • it’s a “volumetric” attack - they’re normal HTTP requests
    • it can saturate web server resources (“service request flood”)
    • can saturate bandwidth (“bandwidth attack”)
  • Attack effectives can be amplified via certain app features:
    • They use resources (login page, api endpoint) that takes a lot of resources or time to process (like slow hashing function on login page)
    • requests that result in SMTP connections (sending email after registering dummy accounts, password reset page etc)
    • long-running requests (like DB heavy requests)
    • download of large files to flood bandwidth
  • Requests can adhere to legitimate patterns which makes defence difficult

TCP attacks

These are the layer 4 (OSI) attacks

TCP handshake:


SRC ←---------------------------- TRGT


SYN flood attack


SRC ←---------------------------- TRGT

    SRC does not respond with ACK

SYN flood attack with spoofed source IP address

    SYNchronize (spoofed src IP address)
SRC ----------------------------→

3rd ←---------------------------- TRGT

UDP and ICMP attacks

  • UDP (User Datagram Protocol)
    • usually it’s a flood of UDP packets send to random ports on the target
    • the target consumes resources responding with “destination unreachable” packets
    • No initial handshake means source IP can be spoofed for anonymization
  • ICMP (Internet Control Message Protocol)
    • Supports utilities such as ping using echo request and echo replay
    • source IP in a ping can be forged to create a “Smurf” attack
    • can be used to send a “Ping of Death”, a large packet causing a buffer overlow

DNS Amplification attack

  • sometimes called DNS Reflection attack
  • attacker sends a smal request for e.g. all DNS records for particular zone which generates a large response
           Small spoofed DNS request
Attacker ----------------------------→ DNS Server
                                             Large amplified response
                                          ----------------------------→ Target

Other forms of amplification attack

  • SNMP (Simple Network Management Protocol)
    • can amplify by a factor of 650x
  • NTP (Network Time Protocol)
    • can return a large resul via the monlist command
    • runs over UDP
    • src IP can be spoofed
    • amplification factor can be up to 206x
  • SSDP (Simple Service Discovery Protocol)
    • Attacker query home devices with uPnP services enabled
    • Responses are larger than the incoming request
    • src IP can be spoofed

P2P Attacks

  • works in DC++ networks
  • uses flaw in the implementation to ask p2p peers to disconnect from the network and send requests to external target.


  • “Low and slow” attack tool
    • uses a low volume of trafiic to generate a slow rate
  • Sends HTTP requests without a termination sequence
    • Causes website to leave the connection open
    • Resources are allocated to wait for the termination sequence
  • Can originate from a single origin
    • Partial requests are sent and then resent and then resent …
          Uncompleted HTTP GET request
          Uncompleted HTTP GET request
Attacker  Uncompleted HTTP GET request    Target
          Uncompleted HTTP GET request

Permanent DoS and Phlashing

  • PDoS attack renders a device unusable, usually by replacing firmware (flashing)
  • also referred to as “bricking
  • exploits weaknesses in the target device (e.g. update process)

Interesting attacks:

  • GitHub’s Man on the Side Attack
  • Incapsula Client’s DDoS attack by botnet called “Cutwail"/"PushDo


  • LOIC a C# tool written in 2004, frequently used by hacktivists
  • JS LOIC JS version of Loic
  • DoSHTTP - DoS HTTP Flood DoS testing tool
  • DoS booters - tools you can rent for performing a DoS
    • eg.,
  • a legit stress testing tool by SendGrid


  • do you have multiple defence types depending on the attack?
  • are you able to adapt as the attack does?
  • Can you do all this without neglecting other security imperatives?

Discovering the attack pattern

  • Is it a lyaer 4 (UDP, TCP, ICMP) or layer 7 (HTTP) attack?
  • What’s the source and the target
  • are there any unique signatures like headers or target resource
  • What’s the intent? is it definitely malicious or unitentional DDoS caused by .e.g. misconfiguration?

Absorbing the attack

If your Capacity > Attack then:


  • No distruption for legitimate users
  • Auto-scale is simplified when using cloud services
  • Scaling buys time to determine other countermeasures


  • can be very expensive
  • may not be practical withing a short timeframe
  • may not practically feasible for a large enough attack

Establishing Traffic Reputation

  • When does normal traffic look like?
    • where do people come from and what do they do?
  • How much of a deviation constitute potentially malicious traffic?
  • What layers of defence are available when repoutation thresholds are exceeded?
  • For example:
    • what does a normal login process look like?
    • e.g. if user attempts more than 3 times to login, then present them captcha or other challange
    • this won’t stop the attack, but will prevent the site from doing a CPU or IO sensitive work

Network Level Defences

  • Hardware appliances
    • like first line of defence places before a firewall
    • firewalls
    • IPS - Intrusion Prevention System
    • SLB - Software Load Balancer
    • WAF - Web Application Firewall
  • Layer 4 countermeasures
    • Disable all unused ports
    • block unnecessary protocols (i.e. ICMP)
    • Filter ingress traffic
      • Repotation bsased blocking
    • Implemented “blackholing” or “tarpitting”
      • Terminate traffic upstream of the origin server
      • No response is sent, the traffic just “disappears”
      • can route traffic to a different segment of network for further analysis.
    • This only works if the network isn’t saturated with data
  • Anycast
    • network addressing and routing methodology, in which you can stand up a single IP address and then depending on where the traffic originates from route it to the nearest possible node. So from the outside world your service is available via only 1 IP address.

Application level defences

  • In case there’s not anomalous layer 4 traffic
  • but there’s a lot of HTTP traffic
  • then you can use WAF (Web Application Firewall) which can inspect HTTP traffic
    • make sure WAF can also inspect HTTPS traffic
  • Minimize the attack surface area
    • take any resources (admin panels, reporting pages etc) from the publicly available network and place then in the private subnet
  • Design the application for DDoS resiliency by Sandboxing Application Features
    • run application features (modules) on separate nodes/servers
    • so that when one module is under attack, then only this module is unavailable and remaining modules work fine.

Preparing for DoS Resiliency

  • Implement a DoS strategy well in advance of an attack
  • Test your defences
  • Do you have a crisis management plan (DDoS Runbook)?
    • check “DDoS RunBook” created by Robert Handson of Whitehat security
  • Do you know how much a successful DoS attack will cost you?
    • this will justify the costs involved with improving your defence