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Threat Modeling for Hostile Client Systems

Threat Modeling for Hostile Client Systems. Avni Rambhia. Outline. Goals of threat modeling Quick overview of generic threat modeling Quirks of a hostile user system Threat modeling for hostile user systems Interactive threat model for PKI signature verification Summary

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Threat Modeling for Hostile Client Systems

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  1. Threat Modeling for Hostile Client Systems Avni Rambhia

  2. Outline • Goals of threat modeling • Quick overview of generic threat modeling • Quirks of a hostile user system • Threat modeling for hostile user systems • Interactive threat model for PKI signature verification • Summary • Questions and discussion

  3. What is threat modeling? • Discover and document threats to a system or application • Threat = system is compromised in a certain way • Fake transaction is created • Determine severity of each threat • Severity = measure of danger if the threat were successfully executed. • 20,000 dollar car shipped to different address • Determine vulnerabilities or mitigations for each threat path • Threat path = way in which threat is executed • Session number can be guessed and fake cookie created

  4. Conventional Threat Modeling • Create a threat model • APIs or entry points (trusted/untrusted) • Data flows • Assets • Determine the severity of each threat • Decompose application to discover vulnerabilities • Document external dependencies and assumptions

  5. PKI-based Authentication System Quick and Dirty overview R E Q U E S T E R V E R I F I E R 1 Random Challenge = n 2 Certificate + signed n Verify Certificate 3 Verify Signed n 4 Authentication success or failure 5

  6. Conventional Threat Modeling Integer overflow Buffer overflow Repeated or predictable n R E Q U E S T E R V E R I F I E R Random Challenge = n M I M Certificate + signed n Verify Certificate Private key discovered using known plaintext Verify Signed n M I M Authentication success or failure

  7. Hostile User Threat Modeling • What is hostile user scenario? • Administrator of system has reason to attack the system to subvert it. • virus running in kernel or with admin privilege • user of multimedia management system • More effective to threat model based on • Assumptions • Assets

  8. Some assumptions in PKI Verifier • Root public key used to verify the certificate is correct • Local time used to check certificate expiry is correct • Cryptography functions correctly perform operations and correctly report results • Authentication does not succeed unless certificate and signature are correctly verified

  9. Some assets in Verifier • Challenge • Random number generator • Certificate verification • Root public key (certificate chain) • System clock • Cryptography routines (signature verification)

  10. Hostile Threat Modeling - Verifier Replace random no. generator V E R I F I E R Random Challenge = n Change code to not call random no. fn. Certificate + signed n Bypass verification routines Tamper clock Verify Certificate Replace public key! Verify Signed n Replace hash calculation function Replace hash calculation function success or failure Tamper (hash == hash) condition to if (1)

  11. You have the threats. What next? • Determine severity/risk of each threat • Relative to the security requirements of the system (penny sale auditing v/s NSA auditing) • DREAD (conventional) • Damage Potential, Reproducibility, Exploitability, Affected Users, Discoverability • FACE (For hostile user systems) • Feasibility, Asset value, Collateral Damage, Execution Difficulty • In general, don’t consider will-do mitigations while calculating risks.

  12. Mitigating Risk • For all threats with unacceptable risk, determine mitigations • Some mitigations for hostile user systems: • Obfuscation/Fragilization • Integrity verification • In-lining critical functions • Server-based security • Stack checking • Privileged execution • (more useful for safety against virus)

  13. Summary • Hostile user threat modeling has a much larger threat surface than conventional threat modeling • Asset and Assumption based investigation is best approach for hostile user systems • Systems needing resistance to hostile users have unique mitigation needs • Breach of crypto algorithms is the least of your worries for such systems

  14. Key Takeaway • Design of hostile user systems is challenging. Whenever you design a system, identify all portions which must be resistant against hostile users, and ensure that your design can achieve the requisite level of security

  15. Resources/Contact • Avni Rambhia, Principal, Security Matters • www.avnirambhia.com • avni@avnirambhia.com • Conventional Threat Modeling resources • Writing Secure Code, Second Edition • Mike Howard, Dave LeBlanc (MS Press) • Threat Modeling • Frank Swiderski, Window Snyder (MS Press)

  16. Questions/Discussion

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