CSC 382 Computer Security Secure Design Principles CSC

CSC 382: Computer Security Secure Design Principles CSC 382: Computer Security 1

Administrivia • Can you login to Sun account (zappa)? • How much C/C++ experience do you have? CSC 382: Computer Security 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 382: Computer Security 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 382: Computer Security 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 382: Computer Security 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 382: Computer Security 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 382: Computer Security 7

Software Security • More than just coding! • Security in every phase of development: – Requirements – Design – Implementation – Testing CSC 382: Computer Security 8

Why is Software Security poor? 1. Security is seen as something that gets in the way of software functionality. 2. Security is difficult to assess and quantify. 3. Security is often not a primary skill or interest of software developers. 4. Time spent on security is time not spent on adding new and interesting functionality. CSC 382: Computer Security 9

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 382: Computer Security 10

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 382: Computer Security 11

Meta Principles 1. Simplicity – Fewer components and cases to fail. – Fewer possible inconsistencies. – Easy to understand. 2. Restriction – Minimize access. – Inhibit communication. CSC 382: Computer Security 12

Security Design Principles 1. 2. 3. 4. 5. 6. 7. 8. Least Privilege Fail-Safe Defaults Economy of Mechanism Complete Mediation Open Design Separation of Privilege Least Common Mechanism Psychological Acceptability CSC 382: Computer Security 13

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 382: Computer Security 14

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 < 1024. • Solution: – Web server runs as root user. – How does this solution violate the Principle of Least Privilege and how could we fix it? CSC 382: Computer Security 15

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 382: Computer Security 16

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 382: Computer Security 17

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 382: Computer Security 18

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 382: Computer Security 19

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 382: Computer Security 20

Economy of Mechanism Example • Problem: SMB File Sharing Protocol. – Used since late 1980 s. – 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 382: Computer Security 21

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 382: Computer Security 22

Open Design Security should not depend on secrecy of design or implementation. – Popularly misunderstood to mean that source code should be public. – “Security through obscurity” – Refers to security policy and mechanism, not simple user secrets like passwords and cryptographic keys. CSC 382: Computer Security 23

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 225, which takes only a few seconds. CSC 382: Computer Security 24

Closed Source • Security through obscurity. • Assumes code in binary can’t be read – what about disassemblers? – what about decompilers? – what about debuggers? – what about strings, lsof, truss, /proc? • Reverse engineering. CSC 382: Computer Security 25

Open Source • Linus’ Law: Given enough eyeballs, all bugs are shallow. • Not so effective for security – More incentives to add features than security. – Few people have skills to find security holes. • Having source eliminates a barrier to entry for crackers. CSC 382: Computer Security 26

Separation of Privilege Require multiple conditions to grant access. – Separation of duty. – Compartmentalization. – Defence in depth. CSC 382: Computer Security 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 382: Computer Security 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 382: Computer Security 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 382: Computer Security 30

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 382: Computer Security 31

Least Common Mechanism • Mechanisms used to access resources should not be shared. – Information can flow along shared channels. – Covert channels. • Contradicts Economy of Mechanism? CSC 382: Computer Security 32

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 382: Computer Security 33

Psychological Acceptability Security mechanisms should not add to the difficulty of accessing a resource. – Hide complexity introduced by security mechanisms. – Ease of installation, configuration, and use. – Human factors critical here. CSC 382: Computer Security 34

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 382: Computer Security 35

Psychological Acceptability Example • Problem: Your workstation is myws, but you log into green every day to do other tasks and don’t want to type your password. • Solution: Let green trust myws. – Create ~/. rhosts file on green that lists myws as trusted host, then rlogin green will allow access without a password. – Does this solution violate other principles? – Is there a more secure alternative solution? CSC 382: Computer Security 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 382: Computer Security 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: Aus. CERT overflow_wrapper – http: //www. auscert. org. au/render. html? it=2016 CSC 382: Computer Security 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 382: Computer Security 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 382: Computer Security 40

Legacy Issues: Maintenance • How can you avoid adding new security flaws when performing code maintenance? • Before looking at a code maintenace procedure, what design principles could have prevented the Sun tar flaw? CSC 382: Computer Security 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 382: Computer Security 42

Case Study: Postfix vs Sendmail • Sendmail – monolithic program with root privileges • Postfix – separate programs with different privileges – smptd: listens to network (port 25) – sendmail: accepts local mail – postdrop: setgid drops in maildrop directory – pickup: retrieves mail from maildrop CSC 382: Computer Security 43

Key Points • Categories of Security Flaws – Architecture/design – Implementation – Operational • Secure Design Principles • Retrofitting and Maintaining Secure Design CSC 382: Computer Security 44

References 1. 2. 3. 4. 5. Bishop, Matt, Introduction to Computer Security, Addison -Wesley, 2005. Graff, Mark and van Wyk, Kenneth, Secure Coding: Principles & Practices, O’Reilly, 2003. Howard, Michael and Le. Blanc, David, Writing Secure Code, 2 nd edition, Micorosft Press, 2003. Viega, John, and Mc. Graw, Gary, Building Secure Software, Addison-Wesley, 2002. Wheeler, David, Secure Programming for UNIX and Linux HOWTO, http: //www. dwheeler. com/secureprograms/Secure-Programs-HOWTO/index. html, 2003. CSC 382: Computer Security 45