General

1
Writing Secure Code
CS 6262 Fall 02
2
Outline

• Designing secure systems
• Secure coding techniques

3
Time to Think About Building Secure Software

• Common mistakes
• Forget to make the software secure
• Adding security as an afterthought
• Why these mistakes
• Security is boring
• Security gets in the way
• Security is difficult to measure
• Security is not the primary skill or interest of
  designers and developers

4
Security Principles to Live By

• Establish a security process
• Define the product security goals
• Consider security as a product feature
• Learn from mistakes
• Use defense in depth
• Assume external systems are insecure

5
Security Principles to Live By (contd)

• Plan on failure
• Fail to a secure mode
• Employ secure defaults
• Remember that security features ! secure
  features
• Never depend on security through obscurity

6
Security Design by Threat Modeling

• Use a taxonomy of threats, e.g., STRIDE
• Spoofing identity
• Tampering with data
• Repudiation
• Information disclosure
• Denial of service
• Elevation of privilege
• Consider target, chance, criticality, attack
  techniques, mitigation techniques, etc.

7
Open Source and Closed Source

• Security by obscurity is no panacea
• Reverse engineering
• Disassembler machine code to assembly
• Discomplier machine code to high-level language
• Code obfuscation make the code difficult to
  understand, raise the bar a little

8
Some Basic Obfuscation Techniques

• Add code that never executes, or that does
  nothing
• E.g., make computations far more complex than
  they need be
• Move code around
• Spread related information as far apart as
  possible
• E.g., Copy and rename the same function, or put
  multiple functions into a single function
• Encode your data oddly

9
Open Source and Closed Source (contd)

• Open-source software potentially more secure
• The many-eyeballs phenomenon
• Given enough eyeballs, all bugs are shallow
• Need incentives
• Many people dont know and dont think much about
  security
• Vulnerability detection in source code is hard
• E.g., which function has buffer overflow problem

10
Buffer Overflow

• Safe string handling, pay attention to
• strcpy, strncpy
• sprintf, snprintf
• gets, fgets
• strcat, strncat
• etc.