computer system engineering, lecture 20 20: introduction ... · 6.033 spring 2018 lecture #20 •...

6.033 Spring 2018Lecture #20

• Introduction to security• Threat models, policy• Guard model

6.033 | spring 2018 | Katrina LaCurts 1

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what makes computer securityspecial?

6.033 | spring 2018 | Katrina LaCurts 10

why is security difficult?

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steps towards building amore secure system:

1. be clear about goals (policy)

2. be clear about assumptions(threat model)

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complete mediation: every request forresource goes through the guard

server

request guard resource principal

(identifies client on server)

authentication: is the principal who they claim to be?

authorization: does principal have access to perform request on resource?

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what can go wrong with the guardmodel?

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sql injection demo

username | email | public? karen | karen@fake.com | yes peter | peter@fake.com | yes katrina | no

SELECT username, email FROM users WHERE username='<username>' AND public='yes'

Let <username> = katrina' OR username='

6.033 | spring 2018 | Katrina LaCurts 15

sql injection demo

username | email | public? karen | karen@fake.com | yes peter | peter@fake.com | yes katrina | no

SELECT username, email FROM users WHERE username='katrina' OR username='' AND

public='yes'

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> cd /mit/bob/project> cat ideas.txtHello world....> mail alice@mit.edu < ideas.txt

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what can go wrong with the guardmodel?

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• Adversarial attacks are different from “normal” failures.They’re targeted, rarely random, and rarely independent.Just one successful attack can bring down a system.

• Securing a system starts by specifying our goals (policy)and assumptions (threat model).

• The guard model provides complete mediation. Eventhough things can still go wrong, systems that use thismodel avoid common pitfalls.

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6.033 Computer System EngineeringSpring 2018

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