Secure Design Principles. CSC 482/582: Computer Security Slide #1

Size: px

Start display at page:

Download "Secure Design Principles. CSC 482/582: Computer Security Slide #1"

Louisa Lester
5 years ago
Views:

1 Secure Design Principles CSC 482/582: Computer Security Slide #1

2 Topics Categories of Security Flaws Architecture/Design Implementation Operational Software Security: More than Just Coding Secure Design Principles Design Issues in Legacy Code Case Study: Sendmail vs. Postfix CSC 482/582: Computer Security Slide #2

3 Categories of Security Flaws 1. Architectural/design-level flaws: security issues that original design did not consider or solve correctly. 2. Implementation flaws: errors made in coding the design. 3. Operational flaws: problems arising from how software is installed or configured. CSC 482/582: Computer Security Slide #3

4 Architecture/Design Flaws Race Condition Application checks access control, then accesses a file as two separate steps, permitting an attacker to race program and substitute the accessible file for one that s not allowed. Replay Attack If an attacker can record a transaction between a client and server at one time, then replay part of the conversation without the application detecting it, a replay attack is possible. Sniffing Since only authorized users could directly access network in original Internet, protocols like telnet send passwords in the clear. CSC 482/582: Computer Security Slide #4

5 Implementation Flaws Buffer overflow Application with fixed-size buffer accepts unlimited length input, writing data into memory beyond buffer in languages w/o bounds checking like C/C++. Input validation Application doesn t check that input has valid format, such as not checking for../ sequences in pathnames, allowing attackers to traverse up the directory tree to access any file. Back door Programmer writes special code to bypass access control system, often for debugging or maintenance purposes. CSC 482/582: Computer Security Slide #5

6 Operational Flaws Denial of service System does not have enough resources or ability to monitor resources to sustain availability under large number of requests. Default accounts Default username/password pairs allow access to anyone who knows default configuration. Password cracking Poor passwords can be guessed by software using dictionaries and permutation algorithms. CSC 482/582: Computer Security Slide #6

7 How can design securely? What about using checklists? Learn from our and others mistakes. Avoid known errors: buffer overflow, code injection, race conditions, etc. Too many known problems. What about unknown problems? CSC 482/582: Computer Security Slide #7

8 How can design securely? Think about security from the beginning. Evaluate threats and risks in requirements. Once we understand our threat model, then we can begin designing an appropriate solution. Apply Secure Design Principles Guidelines for security design. Not a guarantee of security. Tradeoffs between different principles CSC 482/582: Computer Security Slide #8

9 Security Design Principles 1. Least Privilege 2. Fail-Safe Defaults 3. Economy of Mechanism 4. Complete Mediation 5. Open Design 6. Separation of Privilege 7. Least Common Mechanism 8. Psychological Acceptability CSC 482/582: Computer Security Slide #9

10 Meta Principles Simplicity (Minimization) Fewer components and cases to fail. Fewer possible inconsistencies. Easy to understand. Restriction (Isolation) Minimize access. Inhibit communication. CSC 482/582: Computer Security Slide #10

11 Least Privilege A subject should be given only those privileges necessary to complete its task. Function, not identity, controls. Rights added as needed, discarded after use. Minimal protection domain. Most common violation: Running as administrator or root. Use runas or sudo instead. CSC 482/582: Computer Security Slide #11

12 Least Privilege Example Problem: A web server. Serves files under /usr/local/http. Logs connections under /usr/local/http/log. HTTP uses port 80 by default. Only root can open ports < Solution: Web server runs as root user. How does this solution violate the Principle of Least Privilege and how could we fix it? CSC 482/582: Computer Security Slide #12

13 How do we run with least privilege? List required resources and special tasks Files Network connections Change user account Backup data Determine what access you need to resources Access Control model Do you need create, read, write, append, etc.? CSC 482/582: Computer Security Slide #13

14 Fail-Safe Defaults Default action is to deny access. When an action fails, system must be restored to a state as secure as the state it was in when it started the action. CSC 482/582: Computer Security Slide #14

15 Fail Safe Defaults Example Problem: Retail credit card transaction. Card looked up in vendor database to check for stolen cards or suspicious transaction pattern. What happens if system cannot contact vendor? Solution: No authentication, but transaction is logged. How does this system violate the Principle of Fail-Safe Defaults? CSC 482/582: Computer Security Slide #15

16 Fail Safe Defaults Example Problem: MS Office Macro Viruses. MS office files can contain Visual Basic code (macros.) MS Office automatically executes certain macros when opening a MS Office file. Users can turn off automatic execution. Don t mix code and data! Solution: MS Office XP has automatic execution of macros turned off by default. While the solution is a fail-safe default, does it follow least privilege too? CSC 482/582: Computer Security Slide #16

17 Economy of Mechanism Keep it as simple as possible (KISS). Use the simplest solution that works. Fewer cases and components to fail. Reuse known secure solutions i.e., don t write your own cryptography. CSC 482/582: Computer Security Slide #17

18 Economy of Mechanism Example Problem: SMB File Sharing Protocol. Used since late 1980s. Newer protocol version protects data integrity by employing packet signing technique. What do you do about computers with older versions of protocol? Solution: Let client negotiate which SMB version to use. How does this solution violate economy of mechanism? CSC 482/582: Computer Security Slide #18

19 Complete Mediation Check every access. Usually checked once, on first access: UNIX: File ACL checked on open(), but not on subsequent accesses to file. If permissions change after initial access, unauthorized access may be permitted. bad example: DNS cache poisoning CSC 482/582: Computer Security Slide #19

20 Open Design Security should not depend on secrecy of design or implementation. Popularly misunderstood to mean that source code should be public. It means avoiding Security through obscurity Refers to security policy and mechanism, not simple user secrets like passwords and cryptographic keys, e.g. it follows Kerchoff s Principle. CSC 482/582: Computer Security Slide #20

21 Open vs. Closed Source Is open-source software secure? Open: Some people might look at security of your application (if they care) may or may not tell you what they find Closed: not making code available does not hide much need diverse security-aware code reviews A business decision: Not a security one!

22 Open Design Example: Problem: MPAA wants control over DVDs. Region coding, unskippable commercials. Solution: CSS (Content Scrambling System) CSS algorithm kept secret. DVD Players need player key to decrypt disk key on DVD to descript movie for playing. Encryption uses 40-bit keys. People w/o keys can copy but not play DVDs. What happened next? CSS algorithm reverse engineered. Weakness in algorithm allows disk key to be recovered in an attack of complexity 2 25, which takes only a few seconds. CSC 482/582: Computer Security Slide #22

23 Flaws in the Approach What assumptions to make about adversary? Knows algorithms? Or not? Algorithms in binary secret? Attackers can probe for weaknesses reverse engineer executables observe behavior in normal vs. aberrant conditions (fault injection) Fuzzing: trying random input strings to find an exploit blackmail insiders

24 SWS Obscurity Distributing Java bytecode of SWS (and not source code) does not provide security. Tools like strings can search binary for passwords, keys, etc. Bytecode can be decompiled (see Mocha, Jad) to produce source code, including class and public member names. Machine code can be disassembled into assembly by tools like IDA Pro and even decompiled into rough C code. Debuggers and reflection tools can examine a running program. Code obfuscators offer some protection Make code harder to read by replacing readable names with meaningless ones, re-organizing code, etc. But reverse engineers can work through any obfuscation given enough time. CSC 482/582: Computer Security Slide #24

25 Disassembling SWS public void processrequest(java.net.socket); throws java/lang/exception Code: 0: new 25; //class BufferedReader 3: dup 4: new 26; //class InputStreamReader 7: dup 8: aload_1 9: invokevirtual 27; 12: invokespecial 28; 15: invokespecial 29; 18: astore_2 19: new 30; //class OutputStreamWriter 22: dup 23: aload_1 24: invokevirtual 31; 27: invokespecial 32; 30: astore_3 31: aload_2 32: invokevirtual 33; 35: astore 4 37: aconst_null 38: astore 5 40: aconst_null 41: astore 6 99: astore 8 101: aload_3 102: invokevirtual 44; 105: return 43: new 34; //class StringTokenizer 46: dup 47: aload 4 49: ldc 35; //String 51: invokespecial 36; 54: astore 7 56: aload 7 58: invokevirtual 37; 61: astore 5 63: aload 7 65: invokevirtual 37; 68: astore 6 70: aload 5 72: ldc 38; //String GET 74: invokevirtual 39; 77: ifeq 90 80: aload_0 81: aload_3 82: aload 6 84: invokevirtual 40; 87: goto 90: aload_3 91: ldc 41; 93: invokevirtual 42; 96: goto 101

26 Separation of Privilege Require multiple conditions to grant access. Separation of duty Compartmentalization (encapsulation) Defence in depth CSC 482/582: Computer Security Slide #26

27 Separation of Duty Functions are divided so that one entity does not have control over all parts of a transaction. Example: Different persons must initiate a purchase and authorize a purchase. Two different people may be required to arm and fire a nuclear missile. CSC 482/582: Computer Security Slide #27

28 Compartmentalization Problem: A security violation in one process should not affect others. Solution: Virtual Memory Each process gets its own address space. In what ways is this solution flawed? i.e., how can the compartments communicate? How could we improve compartmentalization of processes? CSC 482/582: Computer Security Slide #28

29 Defence in Depth Diverse defensive strategies Different types of defences. Protection Detection Reaction Different implementations of defences. If one layer pierced, next layer may stop. Avoid crunchy on the outside, chewy on the inside network security. Contradicts Economy of Mechanism Think hard about more than 2 layers. CSC 482/582: Computer Security Slide #29

30 Avoid M&M Architectures Inherently insecure system protected by another system mediating access to it Ex: Firewalls guard vulnerable systems within Ex: Death Star strong outer defense but vulnerable Hard outer shell should not be sole defense

31 Defence in Depth Example Problem: Bank. How to secure the money? Solution: Defence in depth. Guards inside bank. Closed-circuit cameras monitor activity. Tellers do not have access to vault. Vault has multiple defences: Time-release. Walls and lock complexity. Multiple compartments. CSC 482/582: Computer Security Slide #31

32 Least Common Mechanism Mechanisms to access resources should not be shared. Information can flow along shared channels. Covert channels. Contradicts Economy of Mechanism? CSC 482/582: Computer Security Slide #32

33 Least Common Mechanism Problem: Compromising web server allows attacker access to entire machine. Solution: Run web server as non-root user. Attacker still gains other access to filesystem. Run web server in chroot jail. CSC 482/582: Computer Security Slide #33

34 Psychological Acceptability Security mechanisms should not add to the difficulty of accessing a resource. Usability: Ease of installation, configuration, and use. Hide complexity introduced by security mechanisms. Principle of Least Astonishment: Design should match user s experience, expectations, and mental models. Follow UI conventions.

35 Psychological Acceptability Users will not read documentation. Make system secure in default configuration. Users will not read dialog boxes. Don t offer complex choices. example: Mozilla/IE certificate dialogs. Privacy vs Usability example: one-click shopping CSC 482/582: Computer Security Slide #35

36 Legacy Issues How can you design security into legacy applications without source code? Wrappers Interposition What is the best way to fix security flaws in an existing application? Code Maintenance Techniques CSC 482/582: Computer Security Slide #36

37 Retrofitting: Wrappers Move existing application to special location. Replace old application with wrapper that: Performs access control check. Performs input checks. Secures environment. Logs invocation of application. Invokes legacy application from new location. Example: AusCERT overflow_wrapper CSC 482/582: Computer Security Slide #37

38 Retrofitting: Interposition Interpose software between two programs we cannot control. Add access control. Filter communication. Example: Network proxy Router blocks all direct client/server connections. Client connects to proxy server, who makes connection to remote server on behalf of client. Access Control: disallow certain clients and/or servers. Filtering: scan for viruses, worms, etc. Auditing: all connections can be logged. CSC 482/582: Computer Security Slide #38

39 Maintenance: Sun tar flaw 1993: Every tar file produced under Solaris 2.0 contained fragments of /etc/passwd file. Tar reads and writes fixed size blocks. Last block written has contents of memory block that were not overwritten by disk read. Tar reads /etc/passwd to obtain user info. Immediately before it allocates the block read buffer. Heap allocation doesn t zero out memory. In earlier versions, other memory allocations were between reading passwd and block read alloc. CSC 482/582: Computer Security Slide #39

40 Legacy Issues: Maintenance How can you avoid adding new security flaws when performing code maintenance? Before looking at a code maintenance procedure, what design principles could have prevented the Sun tar flaw? CSC 482/582: Computer Security Slide #40

41 Legacy Issues: Maintenance 1. Understand security model and mechanisms already in place. 2. Learn how the program actually works. Read design docs, code, and profile the program. 3. When designing and coding the fix: 1. Don t violate the spirit of the design. 2. Don t introduce new trust relationships. CSC 482/582: Computer Security Slide #41

42 Case Study: Postfix vs Sendmail Traditional sendmail architecture. Single process runs as root. qmail architecture with application of separation of privilege and least privilege design principles. Slide #42

43 Key Points Categories of Security Flaws Architecture/design Implementation Operational Secure Design Principles Least Privilege Compartmentalization Psychological Acceptability Retrofitting and Maintaining Secure Design CSC 482/582: Computer Security Slide #43

44 References 1. Bishop, Matt, Introduction to Computer Security, Addison-Wesley, Graff, Mark and van Wyk, Kenneth, Secure Coding: Principles & Practices, O Reilly, Howard, Michael and LeBlanc, David, Writing Secure Code, 2 nd edition, Microsoft Press, Viega, John, and McGraw, Gary, Building Secure Software, Addison-Wesley, Wheeler, David, Secure Programming for UNIX and Linux HOWTO, programs/secure-programs- HOWTO/index.html, CSC 482/582: Computer Security Slide #44

Introduction to Assurance

Introduction to Assurance Overview Why assurance? Trust and assurance Life cycle and assurance April 1, 2015 Slide #1 Overview Trust Problems from lack of assurance Types of assurance Life cycle and assurance