How does cyclomatic complexity, defensive code constructs, and generic data types relate to security issues? Why is it that injection flaws and cross site scripting still rank in the top of OWASP top 10 despite a decade of "awareness"? Why isn't security a natural part of how we measure quality? In this talk, I will address this by showing how the use of good design principles, Domain Driven Security, and different mindset helps one to avoid mistakes that lead to security weaknesses.

1. #DevoxxPL @DanielDeogun
Designing Software with Security in Mind?
Daniel Deogun
Omegapoint
Platinum Sponsors:

2. About Me
• Daniel Deogun
• Sr Consultant, Omegapoint, Stockholm, Sweden
• Started to play with Java in 1997
• Worked in various domains;
• Medtech
• Telecom
• Gambling
• Web
• Strong advocate of DDD, TDD, and Domain Driven Security
• Passionate about high quality software and design

3. Hypothesis
“Crafting software with good design principles along
with a security mindset, significantly reduces the
number of security flaws in an application.”
- Daniel Deogun

4. Important Questions
?

5. Important Questions
?
What distinguishes secure software from insecure software?

6. Important Questions
How should one treat legacy code?
?
What distinguishes secure software from insecure software?

7. Important Questions
How should one treat legacy code?
?
What distinguishes secure software from insecure software?
Is it really possible to design secure software?

8. Important Questions
How should one treat legacy code?
Is security a quality aspect or is quality a security aspect?
?
What distinguishes secure software from insecure software?
Is it really possible to design secure software?

9. OWASP top 10
A1 - Injection
A2 - Broken Authentication and Session Management
A3 - Cross-Site Scripting (XSS)
A4 - Insecure Direct Object References
A5 - Security Misconfiguration
A6 - Sensitive Data Exposure
A7 - Missing Function Level Access Control
A8 - Cross-Site Request Forgery (CSRF)
A9 - Using Components with Known Vulnerabilities
A10 - Unvalidated Redirects and Forwards

10. OWASP top 10
A1 - Injection
A3 - Cross-Site Scripting (XSS)

11. A1 - Injection
“Injection flaws, such as SQL, OS, and LDAP injection occur
when untrusted data is sent to an interpreter as part of a
command or query. The attacker’s hostile data can trick the
interpreter into executing unintended commands or
accessing data without proper authorization.”
- OWASP top 10

12. Example
Address
Regeringsgatan
Street
56
Number
111 56
ZIP Code
Stockholm
City
Sweden
Country

13. The Code…
public void register(String street, String number, String zip, String city, String country) {
//do stuff
}

14. The Code…
public void register(String street, String number, String zip, String city, String country) {
//do stuff
}
Examples of illegal input…

15. The Code…
public void register(String street, String number, String zip, String city, String country) {
//do stuff
}
Examples of illegal input…
register(“Regeringsgatan”, “56”, “111 56”, “Sweden”, “Stockholm”)
Parameters swapped by mistakeIssue:

16. The Code…
public void register(String street, String number, String zip, String city, String country) {
//do stuff
}
Examples of illegal input…
register(“Regeringsgatan”, “or 1=1”, null, “Sweden”, “Stockholm”)
Null parameters, SQL injection, illegal dataIssues:

17. The Code…
public void register(String street, String number, String zip, String city, String country) {
//do stuff
}
Examples of illegal input…
register(StringUtils.repeat("X", 999999999), “56”, “111 56”, “Stockholm”, “Sweden”)
“Unbounded” inputIssue:

18. Treat it as a
Validation Problem
• Illegal data should be rejected through validation
• White list input validation
• Verify boundary conditions
• Input is validated at each entry point
• Create a strong security barrier
[Bodiam Castle]

19. Validation Example
public void register(String street, String number, String zip, String city, String country) {
notNull(street);
isTrue(street.length() < 101);
matchesPattern("^[a-zA-Z åäöÅäö]{3,100}$", street);
notNull(number);
isTrue(number.length() < 4);
matchesPattern("^[d]{1,3}[a-zA-Z]?$", number);
notNull(zip);
isTrue(zip.length() < 7);
matchesPattern("^d{5}|(d{3} d{2})$", zip);
//more validation…
addressRepository.add(street + “ “ + number, zip, city, country);
}

20. Treat it as a
Validation Problem
Upsides
+ Prevent illegal data from being processed
+ Detects illegal data as early as possible
+ Focuses on legal input values
[Bodiam Castle]

21. Treat it as a
Validation Problem
Upsides
+ Prevent illegal data from being processed
+ Detects illegal data as early as possible
+ Focuses on legal input values
[Bodiam Castle]
Downsides
- Validation and data are separated
- Validation logic is often duplicated
- Tends to focus on structural correctness and ignore invariants

22. Second Order
Injection Flaw
“An attack vector with structurally correct data
that exploits missing invariant checks between
input parameters or application state”
- Daniel Deogun

23. Second Order
Injection Flaw
“An attack vector with structurally correct data
that exploits missing invariant checks between
input parameters or application state”
- Daniel Deogun
register(“Regeringsgatan”, “56”, “111 56”, “Gothenburg”, “Sweden”)
Structurally correct but violates city - zip code invariantIssue:

24. Domain Driven Security
• Founded by Dan Bergh Johnsson and Dr. John Wilander
• Introduced at JavaPolis (later named Devoxx) in 2006
• Brings in the power of Domain Driven Design into the
realm of security
• All input data is mapped to value objects
• Combines structural correctness and invariant checks
• Validation and data are kept together
[Matrix Code]

25. What’s the Meaning of
the Address Data?
111 56
City reference in ZIP code
Geographical
Location
56
House number
Street name
Country

26. Conceptual
Address Model
Address
City
StreetName
Number
Country
ZIP Code

27. Conceptual
Address Model
Address
City
StreetName
Number
Country
ZIP Code
• Invariants
- City must exist in Country
- Street must exist in City
- Building with number must exist on street
- ZIP code and City must match

28. public void register(String street, String number, String zip, String city, String country) {
notNull(street);
isTrue(street.length() < 101);
matchesPattern("^[a-zA-Z åäöÅäö]{3,100}$", street);
notNull(number);
isTrue(number.length() < 4);
matchesPattern("^[d]{1,3}[a-zA-Z]?$", number);
notNull(zip);
isTrue(zip.length() < 7);
matchesPattern("^d{5}|(d{3} d{2})$", zip);
//more validation…
addressRepository.add(street + “ “ + number, zip, city, country);
}
Modeling Example

29. Modeling Example
public class ZipCode {
private final String value;
public ZipCode(final String value) {
this.value = validated(value);
}
private static String validated(final String value) {
notNull(value);
isTrue(value.length() < 7);
matchesPattern("^d{5}|(d{3} d{2})$", value);
return value;
}
//methods with domain logic…
}

30. Modeling Example
public class Address {
private final Street street;
private final BuildingNumber buildingNumber;
private final ZipCode zipCode;
private final City city;
private final Country country;
public Address(final Street street, final BuildingNumber buildingNumber,
final ZipCode zipCode, final City city, final Country country) {
this.street = notNull(street);
this.buildingNumber = notNull(buildingNumber);
this.zipCode = notNull(zipCode);
this.city = notNull(city);
this.country = notNull(country);
validateInvariant(city, country);
validateInvariant(street, city);
validateInvariant(buildingNumber, street);
validateInvariant(zipCode, city);
}

31. Modeling Example
public void register(final Street street, final BuildingNumber number, final Zip zip,
final City city, final Country country) {
notNull(street);
notNull(number);
notNull(zip);
notNull(city);
notNull(country);
addressRepository.add(new Address(street, number, zip, city, country));
}

32. Treat it as a
Modeling Problem and use DDS
Upsides
+ Tight coupling between data and validation
+ Promotes white list validation and invariant checks
+ Use of domain objects reduces duplicated validation
+ Helps to prevent second order security weaknesses

33. Treat it as a
Modeling Problem and use DDS
Upsides
+ Tight coupling between data and validation
+ Promotes white list validation and invariant checks
+ Use of domain objects reduces duplicated validation
+ Helps to prevent second order security weaknesses
Downsides
- Involves business and domain experts
- May seem less flexible
- May cause problems with test data

34. What About
Prepared Statements?
• Are prepared statements redundant when using validated
domain objects?
Application

35. What About
Prepared Statements?
• Are prepared statements redundant when using validated
domain objects?
Application
• So, yes we definitely should use prepared statements
when interacting with a persistence context
• Prepared statements separate syntactical constructs and data

36. A3 - XSS
Cross Site Scripting
“XSS flaws occur whenever an application takes untrusted
data and sends it to a web browser without proper
validation or escaping. XSS allows attackers to execute
scripts in the victim’s browser which can hijack user
sessions, deface web sites, or redirect the user to malicious
sites.”
- OWASP top 10

37. DDD - Bounded Contexts
Shipping Context
Customer Context
Support Context

38. DDD - Bounded Contexts
Shipping Context
Customer Context
Support Context
• Bounded context is a central pattern in DDD
• Trying to create one unified model of a large
domain is bound to fail
• Bounded contexts are DDD’s way of dealing with large domains
• Mapping interrelationships between contexts is imperative since it
reveals how data flows in the application

39. Example - Coder’s Blog
• Let’s say we’re running a website where anyone can ask
questions about code
• Is it possible to avoid XSS?

40. Example - Coder’s Blog
Stored XSS
<script>alert(’42’)</script> Browser

41. Stored XSS &
Broken Context Mapping
<script>alert(’42’)</script>
Write Context Read Context
Browser

42. Preventing XSS
• Context maps reveals how data flows between contexts
• One cannot rely on white list validation since “evil input” is
valid input
• Proper escaping is needed in the read context to prevent
data from the write context to be interpreted as code

43. The Reactive Manifesto
• The Reactive Manifesto (http://www.reactivemanifesto.org/)
• Reactive Systems are
• Responsive
• Resilient
• Elastic
• Message Driven

44. Responsive
“The system responds in a timely manner if at all possible. …
Responsive systems focus on providing rapid and consistent
response times, establishing reliable upper bounds so they
deliver a consistent quality of service. …”
Reactive Systems

45. Resilient
“The system stays responsive in the face of failure. … any
system that is not resilient will be unresponsive after a failure.
Resilience is achieved by replication, containment, isolation
and delegation. Failures are contained within each
component, isolating components from each other and thereby
ensuring that parts of the system can fail and recover without
compromising the system as a whole. …”
Reactive Systems

46. Reactive Systems
Elastic
“The system stays responsive under varying workload.
Reactive Systems can react to changes in the input rate by
increasing or decreasing the resources allocated to service
these inputs. This implies designs that have no contention
points or central bottlenecks, resulting in the ability to shard
or replicate components and distribute inputs among them. …”

47. It’s All About Feedback
• The hacker needs feedback
• A system that behaves strangely due to some input reveals its
internal vulnerabilities
• Let’s look at some good craftsmanship patterns…

48. Circuit Breakers
“Circuit Breaker is the fundamental pattern for protecting your
system from all manner of Integration Points problems.”
- Michael T. Nygard, Release It!, April 2007

49. Bulkheads
“Bulkheads are effective at maintaining service, or partial service,
even in the face of failures. They are especially useful in service-
oriented architectures, where the loss of a single service could
have repercussions throughout the enterprise.”
- Michael T. Nygard, Release It!, April 2007

50. Sensitive Value -
One Time Read Only
public final class SensitiveValue {
private final AtomicReference<String> value;
public SensitiveValue(final String value) {
this.value = new AtomicReference<>(validated(value));
}
private static String validated(final String value) {
//Validation of input value
return value;
}
public String value() {
return notNull(value.getAndSet(null), "Sensitive value has already been consumed");
}
@Override
public boolean equals(final Object o) {
throw new UnsupportedOperationException("Not allowed on sensitive value");
}
@Override
public int hashCode() {
throw new UnsupportedOperationException("Not allowed on sensitive value");
}
@Override
public String toString() {
return "SensitiveValue value = *****";
}
}

51. “Evil” Test
@RunWith(Theories.class)
public class ValueObjectTest {
private interface IllegalValue {String value();}
@DataPoints
public static IllegalValue[] input() {
return new IllegalValue[]{
() -> null,
() -> "",
() -> " ",
() -> "A",
() -> "AA",
() -> " AA ",
() -> "1234567890",
() -> "<script>alert('42')</script>",
() -> "' or '1'='1"
};
}
@Rule
public ExpectedException exception = ExpectedException.none();
@Theory
public void should_be_illegal(final IllegalValue illegal) {
exception.expect(ValidationException.class);
new ValueObject(illegal.value());
}

52. Stack Traces
java.sql.SQLException: Closed Connection at oracle.jdbc.driver.DatabaseError.throwSqlException(DatabaseError.java:112)
at oracle.jdbc.driver.DatabaseError.throwSqlException(DatabaseError.java:146)
at oracle.jdbc.driver.DatabaseError.throwSqlException(DatabaseError.java:208)
at oracle.jdbc.driver.PhysicalConnection.rollback(PhysicalConnection.java:1170)
at org.apache.tomcat.dbcp.dbcp.DelegatingConnection.rollback(DelegatingConnection.java:368)
at org.apache.tomcat.dbcp.dbcp.PoolingDataSource$PoolGuardConnectionWrapper.rollback(PoolingDataSource.java:323)
at net.sf.hibernate.transaction.JDBCTransaction.rollback(JDBCTransaction.java:86)
at org.springframework.orm.hibernate.HibernateTransactionManager.doRollback(HibernateTransactionManager.java:529)
at org.springframework.transaction.support.AbstractPlatformTransactionManager.processRollback(AbstractPlatformTransact
at org.springframework.transaction.support.AbstractPlatformTransactionManager.rollback(AbstractPlatformTransactionMana
at org.springframework.transaction.interceptor.TransactionAspectSupport.completeTransactionAfterThrowing(TransactionAs

53. Stack Traces
java.sql.SQLException: Closed Connection at oracle.jdbc.driver.DatabaseError.throwSqlException(DatabaseError.java:112)
at oracle.jdbc.driver.DatabaseError.throwSqlException(DatabaseError.java:146)
at oracle.jdbc.driver.DatabaseError.throwSqlException(DatabaseError.java:208)
at oracle.jdbc.driver.PhysicalConnection.rollback(PhysicalConnection.java:1170)
at org.apache.tomcat.dbcp.dbcp.DelegatingConnection.rollback(DelegatingConnection.java:368)
at org.apache.tomcat.dbcp.dbcp.PoolingDataSource$PoolGuardConnectionWrapper.rollback(PoolingDataSource.java:323)
at net.sf.hibernate.transaction.JDBCTransaction.rollback(JDBCTransaction.java:86)
at org.springframework.orm.hibernate.HibernateTransactionManager.doRollback(HibernateTransactionManager.java:529)
at org.springframework.transaction.support.AbstractPlatformTransactionManager.processRollback(AbstractPlatformTransact
at org.springframework.transaction.support.AbstractPlatformTransactionManager.rollback(AbstractPlatformTransactionMana
at org.springframework.transaction.interceptor.TransactionAspectSupport.completeTransactionAfterThrowing(TransactionAs

54. Hide It!
Oops!
That’s embarrassing.
We’ve seem to have made an error.
Sorry for the inconvenience…
[Head in Hands]

55. Legacy Code
[Room in a mess]

56. Cyclomatic Complexity
• Published in 1976 by Thomas J. McCabe “A Complexity Measure” i
IEEE Transactions on Software Engineering, Vol. SE-2 No. 4
• A measurement of the number of linearly independent paths through a
program's source code.

57. public void reserveRoomFor(String meeting, String owner, String roomName,
Calendar start, Calendar end, String... invitees) {
final List<Booking> bookings = repository.getBookingsFor(roomName);
if(bookings != null && !bookings.isEmpty()) {
for(Booking booking : bookings) {
if(booking.collidesWith(new Booking(start, end, meeting, roomName, owner))) {
throw new AlreadyReservedException(start, end, roomName, meeting, owner);
}
}
}
repository.store(new Booking(start, end, meeting, roomName, owner));
if(dispatcher == null) {
dispatcher = Platform.instance().eventDispatcher();
}
dispatcher.notify(invitees, new Booking(start, end, meeting, roomName, owner));
}
Cyclomatic Complexity

58. Cyclomatic Complexity
public void reserveRoomFor(final Meeting meeting, final Room room) {
notNull(meeting);
notNull(room);
repository.store(booking(meeting, room));
dispatcher.notify(meeting.invitees, booking(meeting, room));
}
private Booking booking(final Meeting meeting, final Room room) {
return new Booking(meeting, room);
}

59. Key Take Aways
• Designing software is difficult
• Security spans across the entire application while development tends to be
from “top to bottom” in an application
• By using good design principles, security becomes a natural part of
development. Not a separate task.

60. Hypothesis
“Crafting software with good design principles along
with a security mindset, significantly reduces the
number of security flaws in an application.”
- Daniel Deogun

61. Hypothesis
“Crafting software with good design principles along
with a security mindset, significantly reduces the
number of security flaws in an application.”
- Daniel Deogun
Plausible

62. Thanks
@DanielDeogun

63. References
• [Bodiam Castle - https://flic.kr/p/5BjV8] by Phillip Capper under license https://creativecommons.org/licenses/by/2.0/
• [Matrix Code - https://flic.kr/p/2Poor] by David.Asch under license https://creativecommons.org/licenses/by-nc-nd/2.0/
• [OWASP top 10] https://www.owasp.org/index.php/Top_10_2013-Top_10
• [Reactive Manifesto] http://www.reactivemanifesto.org/
• [Head in Hands - https://flic.kr/p/7p7raq] by Alex Proimos under license https://creativecommons.org/licenses/by-nc/2.0/
• [Questions - https://flic.kr/p/9ksxQa] by Damián Navas under license https://creativecommons.org/licenses/by-nc-nd/2.0/
• [Room in a mess - https://flic.kr/p/FSrZU] by yan yan under license https://creativecommons.org/licenses/by/2.0/
• [Historic Log Books - https://flic.kr/p/bjViT7] by PSNH under license https://creativecommons.org/licenses/by-nd/2.0/