Transcript Diapositiva 1

?What is the scenario?
An enterprise and its IT system
Dipartimento di Scienze, 16 luglio 2015
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?What are the players?
Attacker
Dipartimento di Scienze, 16 luglio 2015
Defender
2
?What is the game?
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?What is the game?
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agenda
1
2
3
4
Defence trees + indexes
Strategic games
Three novel indicators
……
1Risk Management process
1. Risk Assessment
identification of the:
 assets,
 threats and vulnerabilities,
 countermeasures
Defence
trees
Economic
Indexes
2. Risk Analysis
determination of the acceptable risk threshold.
3. Risk Mitigation
prioritize, evaluate and implement the countermeasure recommended.
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1Defence tree
Defence trees are an extension of attack trees [Schneier00].
Attack tree:
 the root is an asset of an IT system
 the paths from the root to the leaf
are the way to attack the root
 the non-leaf nodes can be:
 and-nodes
 or-nodes
root
or-nodes
and-nodes
Defence tree:
 attack tree
 a set of countermeasures
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An enterprise server is used to store
information about customers…
An attacker wants
to steal this server…
1An example:
(1)
Steal the
server
a2
a1
Break down
the door
Go out
unobserved
c1
Install a
security door
c2
Install a video
surveillance
equipment
c3
Assume
a security
guard
Dipartimento di Scienze, 16 luglio 2015
Have the
keys
c4
Install a
safety lock
Go out
unobserved
c2
Install a video
surveillance
equipment
c3
Assume
a security
guard
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1Estimate the cost of investment
 the annual loss produced by an attack
 the effectiveness of a countermeasure in mitigating the risks
 the cost of a countermeasure
Steal the
server
a2
a1
Break down
the door
c1
Install a
security door
Go out
unobserved
c2
Install a video
surveillance
equipment
c3
Assume
a security
guard
Dipartimento di Scienze, 16 luglio 2015
Have the
keys
c4
Install a
safety lock
Go out
unobserved
c2
Install a video
surveillance
equipment
c3
Assume
a security
guard
10
1Economic index: SLE
The Single Loss Exposure (SLE) represents a measure of an
enterprise's loss from a single threat event and can be computed by
using the following formula:
where:
 the Asset Value (AV) is the cost of creation, development, support,
replacement and ownership values of an asset,
 the Exposure Factor (EF) represents a measure of the magnitude of
loss or impact on the value of an asset arising from a threat event.
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1Economic index: ALE
The Annualized Loss Expectancy (ALE) is the annually
expected financial loss of an enterprise that can be ascribed to a
threat and can be computed by using the following formula:
where:
 the Annualized Rate of Occurrence, (ARO) is a number that
represents the estimated number of annual occurrences of a
threat.
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1Economic index: ROI
The Return on Investment (ROI) indicator can be computed by
using the following formula:
where:
 MR is the risk mitigated by a countermeasure and represents the
effectiveness of a countermeasure in mitigating the risk of loss
deriving from exploiting a vulnerability
 CSI is the cost of security investment that an enterprise must face
for implementing a given countermeasure.
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1Economic index: ROI
Attack
EF
ARO
a1
0,9
0,1
a2
Break down
the door
and go out
unobserved
Open the
door with
keys and go
out
unobserved
0,93
0,1
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Countermeasures
RM
CSI
c1
Install a security door
0,7
1500
c2
Install a video surveillance ...
0,1
3000
c3
Employ a security guard
0,5
12000
c4
Install a security lock
0
300
c1
Install a security door
0
1500
c2
Install a video surveillance …
0,1
3000
c3
Employ a security guard
0,5
12000
c4
Install a security lock
0,2
300
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1Economic index: ROI
AV
Asset Value
EF
Exposure Factor
SLE
Single Loss
Exposure
ARO
Annualized Rate
of Occurrence
ALE
Annualized Loss
Expectancy
RM
Risk Mitigated
CSI
Cost Security
Investment
AV=100.000 €
SLE=90.000 €
ALE=9.000 €
Break down
the door
Install a
security door
RM=70%
CSI=1.500€
ROI=3,20
EF=90%
ARO=0,10
Steal the
server
Have the
keys
Go out
unobserved
Install a video
surveillance
equipment
Assume
a security
guard
EF=93%
ARO=0,10
RM=10%
CSI=3.000€
ROI= - 0,70
RM=50%
CSI=12.000€
ROI= - 0,62
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Install a
safety lock
RM=20%
CSI=300€
ROI=5,20
SLE=93.000 €
ALE=9.300 €
Go out
unobserved
Install a video
surveillance
equipment
RM=10%
CSI=3.000€
ROI= - 0,69
Assume
a security
guard
RM=50%
CSI=12.000
€
ROI=
- 0,61
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1Estimate the cost of the attack
 the expected gain from the successful attack on the target
 the cost sustained by the attacker to succeed,
 the additional cost brought by a possible countermeasure
Steal the
server
a2
a1
Break down
the door
c1
Install a
security door
Go out
unobserved
c2
Install a video
surveillance
equipment
c3
Assume
a security
guard
Dipartimento di Scienze, 16 luglio 2015
Have the
keys
c4
Install a
safety lock
Go out
unobserved
c2
Install a video
surveillance
equipment
c3
Assume
a security
guard
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1Economic index: ROA
Return On Attack (ROA)
measures the gain that an attacker expects from a successful attack
over the losses that he sustains due to the adoption of security
measures by his target
 GI is the expected gain from the successful attack on the specified
target
 costa is the cost sustained by the attacker to succeed,
 costac is the additional cost brought by the countermeasure
c adopted by the defender to mitigate the attack a.
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1Economic index: ROA
Attack
Costa
Countermeasures
a1
4000
c1
Install a security door
2000
c2
Install a video surveillance equip.
1000
c3
Employ a security guard
1500
c4
Install a security lock
0
c1
Install a security door
0
c2
Install a video surveillance equip.
1000
c3
Employ a security guard
1500
c4
Install a security lock
a2
Break down the
door and go out
unobserved
Open the door
with keys and go
out unobserved
4200
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Costac
200
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1Economic index: ROA
Install a
security door
costac= 2.000 €
ROA=5,00
RM
Risk Mitigated
costac Additional cost
produced by a
countermeasure
Steal the
server
Break down
the door
Asset Value
costa Cost of the attack
GI=30.000 €
costa=4.000 €
GI
costa=4.200 €
Have the
keys
Go out
unobserved
Install a video
surveillance
equipment
Assume
a security
guard
costac=1.000€
ROA=6
costac= 1.500 €
ROA=5,45
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Go out
unobserved
Install a
safety lock
Install a video
surveillance
equipment
costac= 1.000 €
ROA=5,77
costac=200€
ROA=6,82
Assume
a security
guard
costac= 1.500 €
ROA=5,26
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1Evaluation
Steal the
server
a2
a1
Break down
the door
ROI=3.20
ROA=0.50
Go out
unobserved
c1
c2
Install a
security door
Install a video
surveillance
equipment
Have the
keys
c4
ROI=-0.70
ROA=4.40
c3
Assume
a security
guard
Dipartimento di Scienze, 16 luglio 2015
ROI=5.20
ROA=4.45
Install a
safety lock
Go out
unobserved
c2
Install a video
surveillance
equipment
ROI=-0.69
ROA=4.19
c3
ROI=-0.63
ROA=1.73
Assume
a security
guard
ROI=-0.61
ROA=1.63
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
Future Works: attack graphs
Steal the
server
a2
a1
Break down
the door
Go out
unobserved
Have the
keys
c1
c2
Install a
security door
Install a video
surveillance
equipment
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c3
Assume
a security
guard
Go out
unobserved
c4
Install a
safety lock
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
Future Works: journal version?
New version of ROI
Old ROI
 1 attack 1 countermeasure
 1 attack n countermeasures
where f is fC=max(c) or fC=sum(c) and CRMc 1
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
Future Works: journal version?
New version of ROI
Old ROI
 m attacks 1 countermeasure
where g is gA=sum(a) and gA AV
 m attacks, n countermeasures
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
Future Works: journal version?
New version of ROA
Old ROA
 1 attack 1 countermeasure
 1 attack n countermeasures
where f is fC=max(c) or fC=sum(c) and CRMc 1
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
Future Works: journal version?
New version of ROA
Old ROA
 m attacks 1 countermeasure
where g is gA=sum(a) and
 m attacks, n countermeasures
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
Future Works: min set cover
c1
a1
c1
c2
a1
c2
a2
a2
c3
a3
a3
c4
c3
c4
RM=[max(c1,c2), min(1, c1+c2)]
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
Future Works: intervals
Intervals to represent the possible values of the exposure
factor (EF), and risk mitigated (RM)
20%
40%
20%
40%
30%
80%
Devo ridefinire tutte le formule considerando adesso gli intervalli!
Ad se x<EF<y  AV ottengo che anche SLE è un intervallo! E
quindi anche ALE e anche ROI
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1Paper
Defense trees for economic
evaluation of security investments
S. Bistarelli, F. Fioravanti, P. Pamela
In: 1st International Conference on Availability,
Reliability and Security (ARES 2006).
Vienna, Austria, April 20-22 2006.
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2Strategic game
We consider a strategic game:
2 players: the defender and the attacker of a system.
 Sd: the set of defender's strategies (the countermeasures)
 Sa: the set of attacker's strategies (the vulnerability)
 ROI and ROA: payoff functions for the defender and the attacker

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2Strategic game: an example
Sa={a1, a2}
 Sd={c1, c2, c3}
 payoff: ud(ci,ai) and ua(ci,ai)

a1
a2
Ud=1
Ua=1
Ud=0
Ua=2
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c2
c3
c3
c1
Ud=1
Ua=2
Ud=1
Ua=0
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2Nash equilibrium
Nash Equilibrium
The combination of strategy (s1*,s2*) with s1* S1 and s2* S2 is a
Nash Equilibrium if and only if, for each player i, the action si* is the
best response to the other player:
This game admits two
different Nash Equilibrium:
the couple of strategies
{c1,a1} and {c3,a2}.
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2Mixed strategy: an example
If a player does not know the
behaviour of the other player?
pa1
pa2
Mixed
strategies
1
pc1
½
pc2
½
pc3
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2Our game
 Selection of a single countermeasure/attack
Steal the
server
a2
a1
The set of strategies for
the defender and the
attacker is composed by
a single action.
Break down
the door
c1
Install a
security door
Go out
unobserved
c2
Install a video
surveillance
equipment
c3
Assume
a security
guard
Dipartimento di Scienze, 16 luglio 2015
Have the
keys
c4
Install a
safety lock
Go out
unobserved
c2
Install a video
surveillance
equipment
c3
Assume
a security
guard
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2Our game
 Selection of a single countermeasure/attack
The set of strategies for
the defender and the
attacker is composed by
a single action.
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2Our game
 Selection of a single countermeasure/attack
31
52
There is one
Nash Equilibrium
with mixed
strategies.
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52
205
769
564
769
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2Our game
 Selection of a set of countermeasures/attack
Steal the
server
a2
a1
Each player can
play any set of
countermeasures
attacks together.
Break down
the door
c1
Install a
security door
Go out
unobserved
c2
Install a video
surveillance
equipment
c3
Assume
a security
guard
Dipartimento di Scienze, 16 luglio 2015
Have the
keys
c4
Install a
safety lock
Go out
unobserved
c2
Install a video
surveillance
equipment
c3
Assume
a security
guard
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2Our game
 Selection of a set of countermeasures/attack
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2Our game
 Selection of a set of countermeasures/attack
5
21
There is one
Nash Equilibrium
with mixed
strategies.
16
21
39
55
16
55
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
Future Works
Considerare giochi con 1 attaccante e n-1 difensori
Cooperazione tra attaccanti
Tipi di attaccanti (giochi bayesiani)
Giochi dinamici, giochi ripetuti
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2Papers
Strategic game on defense trees
S. Bistarelli, M. Dall’Aglio, P. Pamela
In: 4th International Workshop on Formal
Aspects in Security and Trust (FAST2006).
Hamilton, ON, Canada, August 26-27 2006.
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3Three novel indicators
 Critical time
 Retaliation
 Collusion
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3Critical time
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3Critical time
Exposure Factor during Critical Time
expresses the influence that the criticality of a specific time instance
plays on the EF as follows:
CTF being the Critical Time Factor that expresses the percentage of
criticality of a specific time instance.

If CTF=0, then EFCT = EF
If CTF=1, then EFCT = 1
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
If EF=0, then EFCT=CTF
If EF=1, then EFCT=1
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3Critical time: the indicators
 Annualized Rate of Occurrence, AROCT, is the rate of occurrence
of an attack at a specific CTF per year.
 Single Loss Exposure, SLECT, is the cost of a single attack at a
specific CTF:
 Annualized Loss Expectancy, ALECT, is the cost per year of an
attack at a specific CTF:
 Return On Investment, ROICT, is the economic return of an
enterprise's investment against an attack mounted at a specific CTF:
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3Critical time: an example
Asset
Demo machine
Simulation Infrastructure
Researcher's machine
AV
EF
ARO
SLE
ALE
5000 $
30%
55%
1500 $
825$
30000 $
40%
60%
12000 $
7200$
3000 $
15%
20%
450 $
90$
Asset
Demo machine
Simulation Infrastructure
Researcher's machine
AV
CTF
EFCT
AROCT
SLECT
ALECT
5000 $
95%
96,5%
25%
4825 $
1206,25 $
30000 $
98%
98,8%
60%
29640 $
17784 $
3000 $
90%
91,5%
20%
2745 $
549 $
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3Retaliation
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3Retaliation
Exposure Factor under Retaliation
expresses the influence that the chance of retaliating an attack to an
asset plays on the EF as follows:
RF being the Retaliation Factor that expresses the percentage of
retaliation that can be performed.

If RF=0, then EFR = EF
If RF=1, then EFR = 0
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
If EF=0, then EFR=0
If EF=1, then EFR=1-RF
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3Retaliation: the indicators
 Annualized Rate of Occurrence, AROR, is the rate of occurrence per
year of an attack that can be retaliated.
 Single Loss Exposure, SLER, is the cost of a single attack that can
retaliated:
 Annualized Loss Expectancy, ALER, is the cost per year of an
attack that can be retaliated:
 Return On Investment, ROIR, is the economic return of an
enterprise's investment against an attack that can be retaliated:
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3Retaliation : an example
Asset
Demo machine
Simulation Infrastructure
Researcher's machine
AV
EF
ARO
SLE
ALE
5000 $
30%
55%
1500 $
825$
30000 $
40%
60%
12000 $
7200$
3000 $
15%
20%
450 $
90$
Asset
Demo machine
Simulation Infrastructure
Researcher's machine
AV
RF
EFR
AROR
SLER
ALER
5000 $
25%
23%
15%
1150 $
172,50 $
30000 $
25%
30%
60%
9000 $
5400 $
3000 $
130%
-4,5%
20%
-135 $
-27 $
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3Collusion
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3Collusion
Mitigated Risk against Collusion
expresses the influence that collusion of attackers plays on the MR
(mitigated risk) as follows:
CF being the Collusion Factor that expresses the percentage of
collusion of the attackers.

If CF=0, then MRC = MC
If CF=1, then MRC = 0
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
If MR=0, then MRC=0
If MR=1, then MRC=1-CF
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3Collusion: the indicators
The Return On Investment against Collusion
is the economic return of an enterprise's investment against an attack
mounted by one or more colluding attackers:
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3Collusion: an example
Asset
Demo machine
Simulation Infrastructure
Researcher's machine
AV
ALE
CSI
MR
ROI
5000 $
825 $
600 $
85%
16,87%
30000 $
7200 $
4500 $
75%
20%
3000 $
90 $
70 $
90%
15,71%
AV
ALE
CSI
CF
MRC
ROIC
5000 $
825 $
600 $
45%
46,75%
-35,71%
30000 $
7200 $
4500 $
35%
45%
-22%
3000 $
90 $
70 $
10%
81%
4,14%
Asset
Demo machine
Simulation Infrastructure
Researcher's machine
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3Paper
Augmented Risk Analysis
G. Bella, S. Bistarelli, P. Peretti, S. Riccobene
In: 2nd Workshop in Views On Designing
Complex Architectures (VODCA2006).
Bertinoro (FC), September 16-17 2006.
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
Future Works
 ….
 …..
 ….
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
CP-nets
Sv > Su
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Su  Wr
S
Su  Ww
W
Sv  Ww
Sv
Wr > W w
Su
Ww > Wr
Sv  Wr
57

CP-nets
a4>a3>a5>a6>a1>a2
A
C
a1
a1
c1>c2>c3
a2
c5>c3>c4
a3
a2
a3
a4
a5
c1
c3
c6
c8
c6>c7
c2
c4
c7
c9
a4
c8>c9
c3
c5
a5
c11>c10
a6
c13>c12
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c1
0
c1
1
a6
c1
2
c1
3
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
CP-nets
Steal data
stored in a
server
Attack the
system with a
remote login
Obtain root
privileges
a1
a2
Steal access
to a user with
root priv.
c2
Change the
password
periodically
c3
Log out the
pc after the
use
c3
Add an
identification
token
Corrupt a
user with
root priv.
c3
Add an
identification
token
c4
Distribute
responsibilities
among users
Steal the
server
a3
a4
Exploit an
on-line
vulnerability
c6
Update the
system
periodically
c7
Separate the
contents on
the server
Exploit a
web server
vulnerability
c8
Use an
anti-virus
software
c9
Stop
suspicious
attachment
a5
a6
Access to
the server’s
room
c10
Install a
security door
c11
Install a
safety lock
Go out
unobserved
c12
Install a video
surveillance
equipment
c13
Employ a
security
guard
c5
Motivate
employees
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
CP-nets: and-composition
The and-composition of the preference tables described by the
partial orders (D(xi), fui) and (D(xi), fvi), is described by the partial
order (D(xi), fu  vi) where fu  vi represents the conditional
preference of the instantiations of variable xi given an instantiation u
 v. So given a,b  D(xi) and xj=Pa(xi):
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
CP-nets: and-composition
a
a
b
a
b
c
c
x
y
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z
a
a
a
b

b
c

c
c
y>x>z
x>z>y
b

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
CP-nets: or-composition
Given two sets of countermeasure C={c1,…,ck} and C'={c'1…,c'k'}
covering the attacks u1, …, uk, the or-composition conditional
preference table (D(x),fu1 …  uk) is defined as follows:
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
CP-nets: or-composition
a
a
b
a
b
c
c
x
y
Dipartimento di Scienze, 16 luglio 2015
a
a,b
a,b,c
a,c
b,c
z
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Orange book
A system can be used to simultaneously store:
 unclassified information (U),
 secret information (S),
 top-secret information (T).
The information may flow from U to T
CST
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Red book: level of assurance
Considering the type of information stored into a system
we have different level of assurance
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Quantitative level of assurance
We want to define a quantitative
level of assurance as a function of:
f(data; device; environment)
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Quantitative level of assurance
Cost of compromise:
.
The costs associated to a system depend on the type of
attack and the type of countermeasure:
Cost(attack; countermeasures).
The asset value, AV[info], is the value of the information
stored in a system.
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Quantitative level of assurance
 The asset value, AV[info], is the value of the information
stored in a system.
 Given an information flow a<b the cost of a flow (Cf) is:
NOTICE: the cost of a flow can be reduced considering the
percentage of risk mitigated by a countermeasure.
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Quantitative level of assurance
The level of assurance:
Given a defence tree, the level of assurance of a system depends on:
 the asset's value, AV[info],
 the damage produced by an attack (flow),
 the type of countermeasure, Cost(attack, countermeasures).
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Quantitative level of assurance
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Cascade?
Se due sistemi A e B hanno un livello di sicurezza economicamente
accettabile, cosa succede se li collego tra loro?
Il nuovo sistema così creato può essere ancora considerato sicuro?
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Confronto
Data una configurazione di sistema A, come faccio a dire che una
nuova configurazione B non è economicamente meno vantaggiosa
della precedente?
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Analisi
Quando costruisco l’albero e cerco di raggruppare le contromisure,
devo stare attenta che non si creino conflitti!!
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