CSC 382/582: Computer SecuritySlide #1 CSC 382/582: Computer Security Denial of Service

CSC 382/582: Computer Security Slide #1

CSC 382/582: Computer Security

Denial of Service


Topics

1. Introduction

2. History of DoS

3. Modes of Attack

4. Technical Attacks

5. Distributed DOS (DDOS)

6. Computational Attacks

7. Defending against DoS


Denial of Service

• Attacks against availability of services.• Common attacks:

– CAIDA conservatively estimated 4000/week.– Attacks have increased since 2001 estimate.

• Most DoS vs. small site for short duration.• However, DoS attacks can be very big

– Millions of packets/second.– Many thousands of hosts involved.

• DoS attacks are very difficult to stop.


Denial of Service Motives

• Attacker Competitions– Take over IRC channels.– Fame: take down a well-known target.– Revenge for being DoSed.

• Political– Take down computers of political opponents.

• Financial– Take down competitor web site.– Extortion: pay attacker or be DoSed.


History of Denial of Service1988: Morris Worm brings down most of Internet.1996: SYN flood attacks take out IRC networks.1997: Fragment attacks used to DoS Windows systems.1998: Distributed DoS attack tools appear.1999: Heavy use of DDoS tools.2000: DDoS attacks takes down Amazon, eBay, Yahoo.2001: Reflected DDoS attacks appear.2002: DDoS takes down 9 of 13 root DNS servers.2003: DDoS takes down Al-Jazeera after broadcasting pictures

of captured American soldiers.2003-: Financially motivated DDoS. Spammers DDoS anti-

spam organizations. Extortionists DDoS small sites.


DoS: Modes of Attack

1. Technical Attacks• Fragment attacks.• SYN Floods• Smurf.

2. Bandwidth Consumption• DDoS, amplifier-based attacks.

3. Other Resource Consumption• Email bombs.• Disk filling by syslog spoofing/anonymous ftp.• Computational attacks.


Packet Fragmentation

Occurs at IP layer

Each fragment has own IP header

Characteristics:– Each fragment of a packet has same identification field.

– More Fragments flag set (except on final frag).

– Fragment Offset is offset (8-byte units) of fragment from beginning of original datagram.

– Total Length field is length of fragment.


Fragment Security IssuesLarge Datagrams

– Use multiple fragments that will be re-assembled into a packet larger than the maximum IP packet size of 64KB.

– example: ping of deathIncomplete Datagrams

– Jolt2: stream of fragments, with no first fragment.Overlapping Fragments

– Fragment offsets overlap, so during reassembly, second packet is copied over part of TCP header, allowing true header to be hidden in second packet while firewall reads misleading header data from first packet.

– Denial of Service: Teardrop attack uses overlapping fragments to overflow integer in memory copy to crash Windows 95/NT and Linux <2.0.32 hosts.


SYN Floods

Create many half-open connections to target– Send SYN packet from spoofed, invalid IP address.– Ignore SYN+ACK response

Target connection table fills up– all new TCP connections refused

SYN x

SYN y

SYN z

SYN a, ACK x

SYN b, ACK y

SYN c, ACK z

Attacker Target

SYN_RECV x from IP n

SYN_RECV y from IP n

SYN_RECV z from IP n


SYN Flood Defences• Micro-connections: Allocate few resources (~16

bytes) micro-record until ACK recv’d• RST Cookies:

– Server sends incorrect SYN+ACK to first client connection request, eliciting RST as response. Thereafter, connections from that client are accepted.

• SYN Cookies: Store state in ISN, not on server. – Compute ISN using hash of src + dst IP addresses and

ports.– Valid clients will respond with ISN+1, allowing server

to compute connection table entry.


Smurf Attack

Attacker Target

SRC=Target IP,DST=10.0.0.255

10.0.0.*

Reflector Network


Smurf Attack

Build special ICMP/UDP echo packetForge IP source address to be that of target.Destination address is a broadcast address.

Each host that receives broadcast will respond to the spoofed target address with an echo packet, overwhelming target host.

Most current routers refuse to pass on directed broadcast packets.


DDoS Attacks

• Use valid protocols

• Spoof source IP

• Massively distributed

• Variety of attacks

Entire Data Center:• Servers, security devices, routers• Ecommerce, web, DNS, email,…

Provider Infrastructure:• DNS, routers, and links

Access Line

Attack zombies:


Timeline of a DDOS Attack

1. Set up DDOS Network (Botnet)1. Manual compromise by group of crackers.2. Automated comprise by a worm.

2. Launch Attack3. Victim networks become unresponsive

Identification difficult due to router/host failures and lack of logging of packets.

4. Third party effectsVictim responses sent to spoofed IP addresses.


DDoS Attack Architecture


DDOS Machines

Intruder: One or more intruders access master machines to control network, typically through intermediate proxies.

Master: The command and control servers that control agents. Multiple masters are used for redundancy and scalability. Often communication is via the IRC protocol.

Agent: Infected hosts that launch DDOS attack and recruit more agents, aka zombies or bots.


Level of Automation

• Manual– Manual compromise and attack

• Semi-automated– Use of automated exploit to compromise hosts,

then DDoS agent (Stacheldraht, TFN2k, Trinoo)

• Automated– Worm such as W32/Blaster


Agent Lifecycle

1. Old agent scanner selects target host.2. Old agent attempts exploit against target.3. If exploit ok, agent transfers self to target.4. Agent begins running on target.5. Agent registers itself with master server.6. Master accepts agent as member w/ pw.7. Agent logs into communication channel

with provided password.8. Agent waits for commands from master.


Agent Capabilities

• DDOS Attacks– SYN floods

– Smurf

– UDP floods

– ICMP floods

– Mixed

• Recruitment– Scanning engine.

– Many exploits.

– Use other bot backdoors.

• File transfer• Proxying• E-mail harvesting• Sniffing• Spamming• Self-defence

– Anti-virus

– Polymorphism

– Rootkit.


Botnet Families

• Agobot/Phatbot– Over 500 variants.– Modular structure: easy to add features.– Some use WASTE p2p communication.

• SDBot– Simpler than Agobot, but common.

• mIRC-based bots– Script mIRC client for Windows.


Distributed Reflection DOS

Combine SYN Flood + DDOS attacks– Forge target’s IP address in SYN flood on multiple

reflection servers.

– Amplification: most OSes send multiple SYN+ACK responses to SYN packet.

– Concealment: packets come from multiple reflection servers, not actual attacker host.

– Difficult to block: attacker may rotate attacks from large pool of reflection servers, many of which may be important hosts which you need to receive traffic from.


Computational Attacks

• Goal: Exhaust CPU resources of target.• Target: Application Layer• Examples:

– 2002. BBCode markup language of phpBB could consume 100% CPU in infinite expansion of multiply nested tags.

– 2003. Crosby and Wallach show how to select input to reduce hash (O(1) performance) to linked list (O(n) performance). Attacks vs. Bro IDS, perl.

– 2005. Tomcat 5.5 directory listing of large directories could use large amounts of CPU.


Why DDoS is Hard to Stop

1. Attacker ease of useAttackers can d/l user friendly advanced tools.

2. IP spoofingDDoS attacks can always be spoofed.

3. Numerous agentsAttacker agents spread across networks.

4. Variety of trafficDDoS attacks can be launched with any protocol.

5. High-volume trafficAt DDoS traffic volumes, firewalls and IDS must analyze

each packet quickly, i.e. without deep packet inspection or cross-packet correlations.


Filterable and non-Filterable Attacks

Filterable Attacks– Attack non-essential services (ICMP echo) or

ports (random UDP flood.)

Non-filterable Attacks– Attack essential services (email or web.)– Packets may be partially valid for targeted

protocol.


DDOS Prevention

• Apply patches against technical attacks.• Firewall rules to drop known attacks at perimeter.• Overprovisioning

– Network bandwidth, NICs

– Memory, disk

– CPU

• Load balancing• Caching

– Create static page frequently instead of dynamically creating page whenever accessed.


DDOS Detection

• Detecting zombie hosts– DDOSPing– Zombie Zapper

• Detecting a DDOS attack– Performance monitoring: network and host– Network intrusion detection systems


DDOS Reaction

• Filter out identifiable DDOS traffic.• Contact ISP for help with

– Filtering and rate limiting.– Tracking down source of attack.

• Move the target– White House avoided by changing IP address.

• Use Alternate Infrastructure– Use caching to decrease server load.– Use anti-ddos provider like Akamai, Prolexic.


Key Points

• Types of denial of service attack– Technical attacks (SYN flood, smurfing)– Bandwidth attacks (DDOS)

• DOS attacks are a serious threat– Can take down large sites (often for money.)– Difficult to prevent and react to.

• Botnets make DDOS easy and effective– Agents evolve rapidly, offer auto-updating.


References1. “Al-Jazeera Hobbled by DDOS Attack,” InfoWorld,

http://www.infoworld.com/article/03/03/26/HNjazeera_1.html, March 26, 2003.2. Scott Berinato, “How a Bookmaker and a Whiz Kid Took On an Extortionist—and Won,” CSO

Online, http://www.csoonline.com/read/050105/extortion_pf.html, Nov 22 2003.3. CERT Coordination Center, “Results of the Distributed-Systems Intruder Tools Workshop,”

December 1999, http://www.cert.org/reports/dsit_workshop-final.html4. David Dittrich, DDOS Archives, http://staff.washington.edu/dittrich/misc/ddos/, 2006.5. Greene, “ISP Security 101 Primer,” http://www.nanog.org/mtg-0602/greene.html, 2003.6. The Honeynet Project, “Know Your Enemy: Tracking Botnets,”

http://www.honeynet.org/papers/bots/, 2005.7. Steve Gibson, “Distributed Reflection Denial of Service,” http://www.grc.com/dos/drdos.htm, Feb

22 2002.8. Stuart McClure, Joel Scambray, and George Kurtz, Hacking Exposed, 5/e, McGraw-Hill, 2005.9. Jelena Mirkovic, Sven Dietrich, David Dittrich, and Peter Reiher, Internet Denial of Service:

Attack and Defense Mechanisms, Prentice Hall, 2005.10. C. Nuttall, “Crime Gangs Extort Money with Hacking Threat,” The Financial Times, 11 Dec 2003,

http://www.rense.com/general44/hack.htm11. Joel Scambray, Mike Shema, Caleb Sima, Hacking Exposed Web Applications, Second Edition,

McGraw-Hill, 2006.12. Ed Skoudis, Counter Hack Reloaded, Prentice Hall, 2006.13. Scott A Crosby and Dan S Wallach, “Denial of Service via Algorithmic Complexity Attacks,”

USENIX Security 2003, http://www.cs.rice.edu/~scrosby/hash/, 2003.

http://www.infoworld.com/article/03/03/26/HNjazeera_1.html

http://www.csoonline.com/read/050105/extortion_pf.html

http://staff.washington.edu/dittrich/misc/ddos/

http://www.nanog.org/mtg-0602/greene.html

http://www.honeynet.org/papers/bots/

http://www.grc.com/dos/drdos.htm

http://www.cs.rice.edu/~scrosby/hash/

Documents

CSC 382/582: Computer SecuritySlide #1 CSC 382/582: Computer Security Denial of Service