Spring 2006 CS 155 Computer Security Dan Boneh and John Mitchell
What’s this course about? Some challenging fun projects n n Learn about attacks Learn about preventing attacks Lectures on many topics n n Application security Operating system security Network security not a course on Cryptography (take CS 255)
General course info (see web) Prerequisite: Operating systems (CS 140) Textbook: none – reading online Coursework n n 3 projects, 2 homeworks, final exam grade: 0. 3 H + 0. 5 P + 0. 2 F Teaching assistants n n Colin Jackson, Eu-Jin Goh Office hours: see web site Optional section n Friday, 3: 15 - 4: 05, Gates B 01 (live on E 3)
Why computer security? People attack systems and do damage n Why do they do it? w Bored teenagers, Ukrainian criminals, rogue states, industrial espionage, angry employees, … n How do they do it? w Physical access, network attacks w Exploit vulnerabilities in applications and security mechanisms n How can we prevent attacks and/or limit their consequences? w No silver bullet; buggy code is serious problem w Large collection of specific methods for specific purposes
Security concepts and terms Secure n Guarantee specific properties against a class of possible attacks Sample security properties n Confidentiality, integrity, availability, … Threat models n n n Access to network, keyboard, memory bus Can install code on system? As what user? Password dictionary, timing information, …
How big is the security problem? CERT Vulnerabilities reported http: //www. cert. org/stats/
Why does this happen? Lots of buggy software. . . n n Why do programmers write insecure code? Awareness is the main issue Some contributing factors n n n n Few courses in computer security Programming text books do not emphasize security Few security audits C is an unsafe language Programmers are lazy Legacy software (some solutions, e. g. Sandboxing) Consumers do not care about security Security is expensive and takes time
Ethical use of security information We discuss vulnerabilities and attacks n n n Most vulnerabilities have been fixed Some attacks may still cause harm Do not try these at home Purpose of this class n n Learn to prevent malicious attacks Use knowledge for good purposes
Law enforcement Sean Smith n Melissa virus: 5 years in prison, $150 K fine Ehud Tenenbaum (“The Analyzer”) n n Broke into US Do. D computers 6 mos service, suspended prison, $18 K fine Dmitry Sklyarov n n Broke Adobe ebooks Prosecuted under DMCA
Example: voting machine Standard hardware Commercial OS n Many run Win. CE Programmable n Specify election Smartcard authentication n Invalidate card when done Data output n n Network, or Place disk in another computer
Basic security analysis What is voting system supposed to do? n n n Correctly count votes One person, one vote Voter privacy Prohibit vote selling Allow recount, provide confidence in results Who might attack system? n n n Voter wants to vote twice Election worker Programmer working for voting machine company
Assurance Testing n “Testing can reveal the presence of bugs but not their absence” Follow design and coding process n Many certification processes involve process but not quality of results Code analysis n n Third-party code walkthroughs Automated tools
T. Kohno, A. Stubblefield, A. Rubin, D. Wallach Diebold Case Study Proprietary system n Certification mandated by election laws w Without public review: Security through obscurity Diebold system leaked n n Accu. Vote-TS DRE system, Oct 2000 - April 2002 Available on open ftp server Identified by activist Bev Harris Some zip files, cvs repository w DMCA concern over zip “encryption” w Available on New Zealand site
Some problems Encrypted votes and audit logs n #define DESKEY ((des_key*)"F 2654 h. D 4") No authentication of smartcard to voting terminal Insufficient code review
Sample comment in code // // // LCG - Linear Conguential Generator used to generate ballot serial numbers A psuedo-random-sequence generator (per Applied Cryptography, by Bruce Schneier, Wiley, 1996) Unfortunately, linear congruential generators cannot be used for cryptography” Page 369 Applied Cryptography, by Bruce Schneier - Ballot. Results. cpp Diebold Election Systems
Other problems Smartcards use no cryptography Votes kept in sequential order Several glaring errors in cryptography Inadequate security engineering practices Default Security PINs of 1111 on administrator cards Windows Operating System n n tens of millions of lines of code new “critical” security bugs announced frequently
Difficult problem: insider threat Easy to hide code in large software packages Virtually impossible to detect back doors Skill level needed to hide malicious code is much lower than needed to find it Anyone with access to development environment is capable Requires n n n background checks strict development rules physical security slides: Avi Rubin
Example insider attack Hidden trap door in Linux, Nov 2003 n n n Allows attacker to take over a computer Practically undetectable change Uncovered by anomaly in CVS usage Inserted line in wait 4() if ((options == (__WCLONE|__WALL)) && (current->uid = 0)) retval = -EINVAL; n n Looks like a standard error check Anyone see the problem? See: http: //lwn. net/Articles/57135/
Example #2 Rob Harris case - slot machines n an insider: worked for Gaming Control Board Malicious code in testing unit n when testers checked slot machines w downloaded malicious code to slot machine n n was never detected special sequence of coins activated “winning mode” Caught when greed sparked investigation n $100, 000 jackpot
Example #3 Breeder’s cup race n n n Upgrade of software to phone betting system Insider, Christopher Harn, rigged software Allowed him and accomplices to call in w change the bets that were placed w undetectable n Caught when got greedy w won $3 million http: //horseracing. about. com/library/weekly/aa 110102 a. htm
Software dangers Software is complex n top metric for measuring number of flaws is lines of code Windows Operating System n n tens of millions of lines of code new “critical” security bug announced every week Unintended security flaws unavoidable Intentional security flaws undetectable
Ken Thompson What code can we trust? n n n Consider "login" or "su" in Unix Is Red. Hat binary reliable? Does it send your passwd to someone? Can't trust binary so check source, recompile n n Read source code or write your own Does this solve problem? Reflections on Trusting Trust, http: //www. acm. org/classics/sep 95/
Compiler backdoor This is the basis of Thompson's attack n n Compiler looks for source code that looks like login program If found, insert login backdoor (allow special user to log in) How do we solve this? n Inspect the compiler source
C compiler is written in C Change compiler source S compiler(S) { if (match(S, "login-pattern")) { compile (login-backdoor) return } if (match(S, "compiler-pattern")) { compile (compiler-backdoor) return }. . /* compile as usual */ }
Clever trick to avoid detection Compile this compiler and delete backdoor tests from source n Someone can compile standard compiler source to get new compiler, then compile login, and get login with backdoor Simplest approach will only work once n n Compiling the compiler twice might lose the backdoor But can making code for compiler backdoor output itself w (Can you write a program that prints itself? Recursion thm) Read Thompson's article n Short, but requires thought
Social engineering Many examples n n We are not going to talk about social engineering a lot, but good to remember that there are many attacks that don't use computers Call system administrator Dive in the dumpster Online version w send trojan in email w picture or movie with malicious code
Questions?
Скачать презентацию Spring 2006 CS 155 Computer Security Dan Boneh
866b96c031627a8da46078b6d29bf6e4.ppt