User:Curos

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CS 155 Network Security Notes 04.12.2007

  • Types of Viruses that defeat signature detection
    • Encrypted Virus
  Has decryption code and encrypted body.
  Solution: Build signature on descrption code
    • Oligomorphic Virus
  Encrypted virus that ships with many decryption engines
      • Memorial Virus
   96 different encryption engines
   Solution: Build signature on all encryption engines
    • Polymorphic (1998)
  Slightly reencode the virus, randomize somehow.  Use a random decoding
  algorithm.
  Solution: Emulate virus and scan memory for signature on the stack.
    • Metamorphic Virus
  Entire virus code changes after each infection
      • Apparition Virus
   Carries source code with it.  Injects junk code in itself and
   recompiles itself
      • Vecha Virus
   metamorphosis by changing register allocation.  Switches registers around.
      • Zperm
   Injects jump after every instruction.
   Geometry method protects against this by looking at the structure of
   the flow graph.
  • Conclusion
 No complete detection possible.  Impossibility result.  Fred Cohen (87)
 put forth claim that no perfect detector exists.  Proof: Let A be a
 virus detector where A(F) outputs yes if virus present, no if not

 Virus P in file F:
   Run A(F)
   If "yes", exit
   otherwise, spread

 Run A on File F:
   Always produces incorrect result

 If A(F)="yes" then answer is wrong because F is benign

 If A(F)="no" then answer is wrong because F spreads


CS 155 Network Security Notes 04.17.2007

  • View access control as matrix
 Users (rows) and the files(columns) they want access to
 Complex to individualize, so groups of users or roles
    • Access Control Lists (Slice along columns)
    • Capability Systems (Slice along rows)
  • Unix Security
 Every process has numeric user id and one or more group ids.  The system
 stores the user who owns the file and the group the file is in.
 Permissions also stored for user, group and other.  `ls -l` shows: 
 user group other owner and group
 drwx rwx   rwx   stevetan  users

 Execute a directory means use files in that directory
 Bypass: convince root process to do your bidding
    • Time-of-check-to-time-of-use bug (TOCTTOU bug)
      • Clearing old files in /tmp
 `find /tmp -atime +3 -exec rm -f -- {} \;`
 Find files that are three days are older and execute rm on it.  Braces
 get replaced by the file name. `--` prevents path from being parsed as
 option.
 Problem:  You can use a symbolic link to a file or directory you don't
 own.
    • setuid problems
  Each process has real and effective uid/gid.  'Real' is user who has
  real and effective uid/gid.  'Effective' is owner/group of file, used
  in access checks.
      • xterm
   Needed to run with root privileges because of pseudo-terminal (pty)
   device - to own terminal you're typing in.  Also needed to write tmp
   and wtmp to record usesrs.  Had feature to log terminal session.  
   Problem: If you have root privileges, then the output to file can be
   written anywhere.  So have to make sure that the user also has the
   right prvileges.  
   Solution: system call `access` checks permission with real uid instead
   of effective uid.  
   BUT: Another TOCTTOU bug.  access() is a potential secuirty hole!
      • SSH configuration files
   SSH 1.1.12 login program runs as root because needs to bind to TCP
   1024 and read client private key (for host authentication).  Also
   needs to read configuration file ~/.ssh/config and record server keys
   ~/.ssh/known_hosts

   Plan: do setuid root and do root actions, then drop all permissions.
   Problem: ptrace bug.  Dropping the privileges allowed user to "debug"
   SSH: use the prvileged port to connect to anywhere, read secret host
   key from memory, overwite local user name to get privileges of other
   user.
   Fix: restructured ssh into 3 processes and communicated kind of like
   network
    • When can you send a signal and do a ptrace
  Should restrict certain signals (eg SIGALARM might mean something to a
  process) Ptrace is a very powerful tool, that can modify memory of a
  process, so recommendation is to not let a process ptrace a more
  privelaged process.  Disable setuid if executing a process thats being
  ptraced
      • Old su linux bug
   A ptraces B
   A run su user
   A, for now turns into root
   B runs su root
   B turns into root
   A can get root shell from process B

   You can hardlink a file you don't have access to and then other user
   can never remove that file.
  • The Confused Deputy (Norm Hardy)
 Multi-user time sharing system.  Fortran compiler keeps statistics.
 /sysx/fort records to /sysx/stat.  Gave compiler "home files license"
 or "root" access.
 Problem: have fort executable compile/output to directory/file of
 choice.
  • Capabilities
 For each process store a list of objects it can access.
    • Hydra
  Invoke a method on object if you had that capability.  If you acquired
  object then you have that object's capability.
    • KeyKOS
  Pure capability system referred to as "keys" for short.  Can grant keys
  to others.  To create a new key, had to have a prvileged operation.
      • Single-level store
  No distinction between memory and disk memory.  Everything
  persistent.  So don't have to worry about memory.  No dynamic memory.
    • KeyNIX: UNIX on KeyKOS
  One kernel per process.  Each file was its own system.  A separate
  process for each file.  To read a file, you would need to have the
  capability.  Fault isolated because only can exploit that one section
  and not the whole file system.  Made some functions extremely slow;
  fork, namei, pipes
    • Self-Authenticating Capability - AMOEBA
  Pass capabilities around in a string with encrypted check value
    • Limitations of capabilities
  The CPU is optimized for common code and not for context switches.
  Requires alot of Inter-process communication.  Capability programming
  never took off.  Capabilities are an OS concept of the future and they
  always will be.
  • Discretionary Access Control Systems (DAC) vs Mandatory Access Control (MAC)
 In DAC, can pass access along at your discretion.  Can pass secret to
 friend who passes on to enemy.  Military likes MAC.
    • Bell-Lapadula Model
  Access requests and outputs are decisions on these requests.
      • Security level
   A security level is a classification and category-set.  Eg
   classified/top secret and nuclear info/cryptography.  Must have
   classification to do actions.  Secret files can never be written to
   lesser unsecret levels.  It must be the case that the file being read
   is being dominated by one that can be written.  No read up.  No write
   down.
  • Covert channels
 Can be anything, disk read, cache miss
 Solution: No sharing.

CS 155 Network Security Notes 04.19.2007

How to confine untrusted code?

Biba integrity model

DoD Orange Book

Limitations of Orange Book

Common Criteria

LOMAC Uses integrity system, but doesn't care too much about covert channels

Flask Security Architecture Military attempt to make security system Prototyped in fluke and then integrated into SElinux Policy and mechanism separated

...

  • Protecting your system, w/o changing the OS
    • firewall
    • chroot
  • System call interposition
 Enforce policy on system calls
 Problem: Symbolic links problem


  • Sandboxing code
 Protect code within an application.  Then can safely use "unsafe" code.
 Cross-address-space/context-switch calls are expensive.  Can ensure
 determinism.  Every component has its own fault domain so if it messes
 up, it doesn't mess up the whole system.
    • Segment matching
    • Address sandboxing
  Just set the top bits...

CS 155 Network Security Notes 04.20.2007 Problem Session


  • URLs
 protocol (http), username, password, hostname, port, path, query (?id=4),
 fragment (#heading1)
 very similar to URI
    • Special character encodings
  %0A = newline
  %20 or + = space
  %2b = +

Stop 8:08

CS 155 Network Security Notes 04.24.2007

  • Cryptography
 want communication security and cryptography can help
  • Symmetric Encryption
  Using a shared Key
  Message M and key K
  M is plaintext
  Encrypt(K, M) -> C ciphertext
  Decrypt(K, C) -> M
    • One-time pad
  XOR message with K and decrypt by XOR with K
  Advantage: Information theoretically secure
  Disadvantage: K must be as long as M
  • Types of Encryption
    • Stream ciphers
  Encrypt one bit at a time with pseudo-random stream.  Similar to
  one-time pad but isn't
    • Block cipher
  Encrypt in chunks 64 bit, 128 bit
  • RC4 security hole
 Second byte of RC4 is 0 twice expected probability
 Bad to use many related keys
 Should discard first 256 bytes of RC4 output
  • Bad use of stream cipher
 Know c1, know c2 and know c2's M then can get m = c1 XOR c2 XOR m2
 Never reuse keys with stream cipher
  • ECB (Electronic Codebook)
 Message is divided into blocks and each block is encrypted
 separately. Attacker will know repeat plaintext blocks.  Sees same block
 again.
  • CBC (Cipher-block chaining)
 Use initialization vector and then xor with previous block.  So unique
 each time.
  • Integrity Problem
 attacker can tamper message
 Encryption does not guarantee integrity
 Solution: Message authentication codes
  • Cryptographic hashes
 Hash arbitrary length input to fixed size output
  • Order of Encryption and Macs
 Better to encrypt and then MAC then encrypt Mac and message together.
  • Public key Encryption
 Need opposite key to decrypt

CS 155 Network Security Notes 04.26.200

Web Security

Firewall IDS Application firewall (WAF)

  • Web Application firewalls
 Prevents common web attacks.  At a high web server level instead of the
 link layer
  • Common attacks
 Denial of Service
 Web server attack
 Worms
  • Cookies
 Common fields include domain name, expiration data and secure (use SSL
 to send this cookie only)
    • Browser limitation on cookies
  Browser can only store 20 cookies/site and 3kb/cookie
  • Same Origin Principle
 Like OS and using different threads.  Shouldn't interact with each
 other.  Two origins are the same iff domain name, port and protocol are
 the same.
  • Web Application Vulnerabilities
 Cross site scripting
 SQL injection	
 HTTP splitting
 Broken session management
 Insecure storage - store credit card data in clear

CS 155 Network Security Notes 05.01.2007


CS 155 Network Security Notes 05.03.2007

User Authentication

  • Password based authentication
 typically done over SSL
    • Problems
  1) humans are terrible at generating passwords; 
     low entropy secrets ~2^23 words 
  2) compromise password file on website
  3) Phishing
  • How to store passwords
 Don't store in clear
    • Store Hash(pwd) MD5 SHA1
  Benefit for using slow hash functions because dictionary attack can be
  used
    • Public Salt with hash
  Attacker must hash for each salt
    • Pepper (secret salt)
  Make server do secret salt and brute force authentication
  • Single sign on service
 Microsoft's Passport
    • Problems
  cookie used for authentication is compromised
  Phishing attacks
  • Phishing Solutions
 Phishing URLS
 Phishing using VoIP, a phone call with emulation of voice system
 Check SSL certificates
 IE has Extended Validation Certificates which shows the name of the
 certificate in the address bar and a green bar
    • Other attacks
  Picture in picture attacks.  Simulate what they should see in picture.
  IE7 now disables the disabling of the toolbar
  Man in the middle toolkits available to exploit one-time passwords
  • PAKE Password Authentication Key Exchange
 Partial solution to phishing attacks where both server and client must
 have password.  They compare themselves and only get key to talk to each
 other if passwords are the same.CS 155 Network Security Notes

05.08.2007

  • IP
  • Medium Access Control (MAC)
  • Address Resolution Protocol (ARP)
 ARP tables kept on computer.  Where is 128.12.61.2?
  • Promiscuous mode
 See all packets going over network.  802.11b still does this
  • Ethernet switches
 Sends packets only to machines that are listening on that network.
 Fixes promiscuous mode, however can do ARP spoofing
  • Attacks
    • ARP spoofing
  get incorrect ARP entry into other peoples ARP tables.
    • MAC table overflow
  send a bunch of packets to old routers that then die
    • ICMP redirect abuse
  redirect packets, can affect routers routing tables
    • BGP routing protocol abuse
  used between ISPs, no secure binding between ips
    • DHCP abuse
  setup competing DHCP server
  • UDP
 user datagram protocol.  Provides checksum on whole packet and
 demultiplexing.
  • TCP
 transmission control protocol.  provides flow control and congestion
 control.  TCP is supposed to be reliable, so retransmits
  • Access control based on IP
 bad assumption.
  • Spoofing IP
 Must be able to guess SEQ to use, can guess and predict based on real
 interaction
 RST from real client, so can flood server with SYN packets
  • Desynchronize TCP connection
 Send NULL data to both client and server and drive WINDOW up
 Inject data as needed and an ACK storm occurs as everyone is
 transmitting wrong window section.
  • Domain name sytem (DNS) Port 53
 represented as resource records
 name - domain name
 TTL - time to live in seconds
 class - usually internet
 type - type of record
 rdata - data dependent on type
    • DNS RR types
  A - Internet Address
  NS - name server
  MX - mail exchanger

dig www.stanford.edu

    • PTR record
  stores name given address
  • Glue Records
 dig +norec www.stanford.edu @a.root-servers.net
 Can fake glue records

CS 155 Network Security Notes 05.10.2007

  • Port usage
 well known ports below 600
 priveleged ports < 1024 (proof they are root)
 /etc/services has well known ports
  • Insecure network services
 NFS - if can guess file handle, can read write entire filesystem
 Portmap - another layer of indirection for port numbers.  Can make
 network requests appear to be from localhost
 FTP - client can specify third machine for bounc attack
 YP/NIS - serves password file
  • Firewall
 separate network into zones (finances and SS#'s) topology
    • packet filtering
  ip source/destination, protocol, TCP flags, ICMP message type
    • block forgeries
  packets that don't originate from in local network (spoofing)
    • egress filtering
  block forged outgoing packets
    • Disallowing servers
  block incoming packets with SYN flag (that means someone on the
  internal network is running server)
    • Complications from fragmentation
  Eg. Send two packets, second one for restricted port, but since allowed
  first, will allow second (connection established) then when reassemble
  packets, it goes to bad port
    • keeping state in firewall
  allow flows that have already existed
  • NAT
 translates private IP address to public
    • application level proxies
  policies for specific protocols FTP, http, etc.  Can examine more
  in-depth
  • IPsec
 new IP protocol, embed one IP packet in another IP packet
 avoids replays with sequence numbers
 pad packets so don't know how big files you are sending
  • Intrusion detection
  • Subterfuge attacks
 Change TTL field.  and exploit fragmentation.  So some packets don't
 even make it through
  • Kerberos

CS 155 Network Security Notes 05.10.2007

  • DoS
 Denial of Service attacks.  Make stuff not work.  Take out
 infrastructure with little work.
    • Amplification
  technique of using small number of packets
      • Amplifications due to bugs
      • Amplification due to bot-net
  • 80211b Network
 NAV - reserve bandwidth for NAC seconds.  So keep reserving.
 Doesn't work since most cards don't follow NAV standard
  • Dauthentication bug
 Deauth packet is a "I'm done using wireless slot, someone else use"  Can
 send deauth packets on behalf of others.
  • SMURF attack
 Send ICMP to gateway with many computers and broadcast and make ICMP
 echo go to target.
  • DNS Amplification
 Similar to SMURF attack.  EDNS response is much bigger.  x40
 amplification
  • SYN Flood
 Too much SYN to store
 Detect SYN floods with backscatter
    • SYN-cookies
  Have client store info.  Only start connection once client echos back
  • Take out a site's DNS server
  • Puzzles

CS 155 Network Security Notes 05.10.2007

Worms CS 155 Network Security Notes 05.22.2007

Surf Anonymously

  • How to surf anonymously
    • Anonymizer
  All users go through anonymizer
  Anonymizer knows who you are
  Traffic Analysis can determine things
  Protocol issues (http not meant to keep your privacy and anonymizer may
  fail certain things)
    • Mix Nets
  aka onion routing
  Use multiple onion routers.  Pick random route each time.  Onion levels
  of encryption.  Include return onion for server to respond.
  This protocol is too high latency
  No forward secrecy
    • TOR Network
  TOR Network is stateful onion routing (a circuit-based method)

CS 155 Network Security Notes 05.24.2007

  • Cryptographic file systems
 Initialization vector to change cipher text each time
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