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

CSC 382: Computer Security Slide #1

CSC 382: Computer Security

Secure Design Principles


Administrivia

• Can you login to Sun account (zappa)?

• How much C/C++ experience do you have?


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


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.


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.


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.


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.


Software Security

• More than just coding!• Security in every phase of development:

– Requirements– Design– Implementation– Testing


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.


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?


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


Meta Principles

1. Simplicity– Fewer components and cases to fail.– Fewer possible inconsistencies.– Easy to understand.

2. Restriction– Minimize access.– Inhibit communication.


Security Design Principles

1. Least Privilege2. Fail-Safe Defaults3. Economy of Mechanism4. Complete Mediation5. Open Design 6. Separation of Privilege7. Least Common Mechanism8. Psychological Acceptability


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.


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?


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.?


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.


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?


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?


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.


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?


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


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.


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.


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.


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.


Separation of Privilege

Require multiple conditions to grant access.– Separation of duty.– Compartmentalization.– Defence in depth.


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.


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?


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.


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.


Least Common Mechanism

• Mechanisms used to access resources should not be shared.– Information can flow along shared channels.– Covert channels.

• Contradicts Economy of Mechanism?


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.


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.


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


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?


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


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– http://www.auscert.org.au/render.html?it=2016


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.


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.


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?


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.


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


Key Points

• Categories of Security Flaws– Architecture/design– Implementation– Operational

• Secure Design Principles

• Retrofitting and Maintaining Secure Design


References1. Bishop, Matt, Introduction to Computer Security,

Addison-Wesley, 2005.2. Graff, Mark and van Wyk, Kenneth, Secure Coding:

Principles & Practices, O’Reilly, 2003.3. Howard, Michael and LeBlanc, David, Writing Secure

Code, 2nd edition, Micorosft Press, 2003.4. Viega, John, and McGraw, Gary, Building Secure

Software, Addison-Wesley, 2002.5. Wheeler, David, Secure Programming for UNIX and

Linux HOWTO, http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/index.html, 2003.

http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/index.html

http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/index.html

Documents

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