Physical Threat Modeling

physical-threat-modeling

Threat library for systems that face physical-access adversaries. Use when designing or reviewing systems that travel, are stored unattended, are operated in shared spaces, or face threat actors with momentary physical access (border crossings, hotel rooms, conferences, theft of laptops).

STRIDE and OWASP Top 10 implicitly assume a network adversary against a fixed-asset server. Their threat enumeration is silent on physical attacks — the threat model literally doesn't have categories for "evil-maid swap" or "Thunderbolt DMA". This skill fills that gap.

Triggers

• "evil maid"
• "DMA attack" / "Thunderbolt attack"
• "travel host" / "airport / border / hotel"
• "hostile peripheral" / "BadUSB"
• "physical access" / "physical adversary"
• "coercion" / "rubber-hose" / "duress"
• "cold boot" / "RAM extraction"
• "supply chain implant"
• "side channel" / "TEMPEST" / "EM emanation"

When NOT to use this skill

• Pure datacenter/cloud workloads with no portable assets (use STRIDE)
• Pure web app threat modeling (use OWASP)
• Forensics IR after a known compromise (use forensics-complete)

Section 1: Threat Library

The library lists ten canonical physical-access threats. For each: applicability conditions, attacker capabilities required, detection difficulty, and remediation patterns. The remediation patterns are references to other skills' solutions — this skill is the enumeration.

Threat 1: Evil-maid swap of bootstrap / boot media

Description: Adversary with brief physical access modifies code that runs before the integrity chain is established (bootloader, init scripts on writable media, USB bootstrap files).

Applicability:

• Asset is portable AND left unattended (hotel room, hotel safe, baggage claim, customs)
• Bootstrap code lives on writable media (FAT32 partition, USB MBR, unsigned ESP)
• Boot chain has no signature verification before user data is touched

Attacker capability: ~5–30 minutes physical access, basic technical skill, ability to write to the device or swap it.

Detection difficulty: Hard — the swap is functionally transparent until the operator runs something that reveals tampering, which the modified bootstrap can prevent.

Remediation: chain-of-trust-design skill — Pattern A (signed bootstrap blob) + Pattern D (printed hash card), or Pattern B (signed live image so the host code itself isn't on the writable media). Maps to review finding B3.

Threat 2: Thunderbolt / USB-C DMA attack

Description: Adversary plugs a hostile DMA-capable peripheral into a Thunderbolt or USB-C port (with PCIe tunneling) and reads RAM via direct memory access, bypassing OS protections.

Applicability:

• Host has Thunderbolt 3+, USB-C with PCIe-tunnel, or FireWire
• Asset is in S0 (running) or S3 (sleep) state with secrets in RAM
• IOMMU is disabled or not properly configured

Attacker capability: 30–60 seconds, commodity attack tool (PCILeech, Inception), direct port access.

Detection difficulty: Detection-only via post-hoc memory analysis. Real-time detection requires DMAR/IOMMU event logging.

Remediation:

• Disable Thunderbolt DMA in BIOS (Thunderbolt Security Level: User Authorization or DisplayPort Only)
• Enable IOMMU (intel_iommu=on iommu=pt Linux kernel)
• Lock screen → S5 (shutdown), not S3 (suspend), in hostile environments
• Use only known peripherals; physically block ports if possible (port blockers)

Maps to review finding H4.

Threat 3: Hostile USB peripheral (BadUSB, keystroke injection)

Description: Adversary hands or plants a USB device that presents itself as a HID (keyboard) and runs commands, OR presents itself as a network adapter and intercepts traffic, OR presents as a mass storage device and exploits autorun.

Applicability:

• Host accepts USB devices without user authorization per device
• USB authorization (/sys/bus/usb/.../authorized) not enforced
• Operator plugs in unverified devices (gifted, found, public charging)

Attacker capability: Pre-attack hardware preparation; one-shot usage; deniability.

Detection difficulty: Hard at runtime; possible via USB authorization logs, USBGuard rules.

Remediation:

• USB authorization daemon (USBGuard) with allowlist
• Disable USB classes you don't use (mass storage, HID-from-storage)
• Use USB data blockers ("USB condoms") when charging from untrusted sources
• BIOS-level USB boot disabled when not provisioning

Threat 4: Travel-host root compromise

Description: The host the operator uses to access the secret system is itself compromised at the OS or firmware level. Anything that runs on it — including the secrets pipeline — is observed by the adversary.

Applicability:

• Host is shared, public, hotel-provided, or has been out of operator's continuous custody
• Operator brings the secret system to the host (USB plug-in)
• Host runs an unsigned, unattested OS

Attacker capability: Persistent root malware; potentially firmware persistence.

Detection difficulty: Hard — root malware can hide from standard tools.

Remediation: This is the single highest-leverage mitigation for portable secret systems.

• Boot the host from a signed live image (Pattern B in chain-of-trust-design). The host stops being a trust dependency.
• Measured boot with TPM attestation against a known-good PCR set
• Forbid Docker fallback in production (Docker shares the host kernel; against kernel-resident adversary it's not a boundary)

Maps to review finding H4.

Threat 5: Coercion of operator (rubber-hose attack)

Description: Adversary compels the operator (legally, threateningly, or via deception) to authenticate against the secret system.

Applicability:

• Operator is identifiable as a target
• Operator can be intercepted in person (border crossing, raid, kidnapping)
• System has no "what you know" factor (biometric + hardware = both available under duress)

Attacker capability: Physical access to operator AND device.

Detection difficulty: After-the-fact; sometimes never.

Remediation: auth-factor-design skill —

• FIDO2 PIN as a "what you know" factor (review finding H2)
• Duress codes (PIN that triggers wipe/silent-alarm)
• Time-locks, witness requirements, geographic fences

Maps to review finding H2.

Threat 6: Cold-boot RAM extraction

Description: Adversary cools and removes RAM modules (or freezes the host with liquid nitrogen) to preserve the contents long enough to read out secrets that were unlocked in memory.

Applicability:

• Host can be physically opened
• Secrets are in plaintext in RAM
• Operating system/host doesn't crypto-protect RAM at rest

Attacker capability: Physical access; cooling spray (commodity); 5–10 minutes uninterrupted.

Detection difficulty: Impossible at runtime; possible via post-hoc inventory check.

Remediation:

• Reduce window: lock to S5 (full shutdown), not S3 (suspend), in hostile environments
• Use hardware features when available: Intel TME (Total Memory Encryption), AMD SME, Apple Silicon's secure enclave for sensitive computations
• Don't store keys in long-lived process memory — derive on demand and zeroize
• Accept residual risk: cold-boot is hard to fully eliminate without specialized hardware

Threat 7: Hardware implant in supply chain

Description: A device is intercepted in supply chain (during shipment, at customs, by a malicious supplier) and a hardware implant is added — modified firmware, added microcontroller on motherboard, modified USB cable.

Applicability:

• High-value target
• Hardware sourced from untrusted supplier or shipped without tamper-evident packaging
• No hardware verification post-receipt

Attacker capability: Resourced adversary — nation-state, well-funded criminal group.

Detection difficulty: Very hard. Requires hardware teardown + comparison to known-good.

Remediation:

• Procurement controls: known vendor, tamper-evident shipping, randomized purchase patterns
• Out-of-band verification: photograph hardware on receipt; compare to known-good
• Firmware verification (where feasible): re-flash to known-good firmware on receipt
• Diversity: don't depend on a single hardware vendor for critical-path crypto
• Accept residual risk; document explicitly

Threat 8: Side-channel observation (acoustic, EM, power, thermal)

Description: Adversary observes physical emanations from the device — fan noise, EM radiation, power consumption, thermal output — and infers cryptographic operations, key bits, or content.

Applicability:

• Adversary is physically near the device (within meters for EM, room for acoustic, electrical-network for power)
• Cryptographic operations are not constant-time
• Hardware emanations are not shielded

Attacker capability: Specialized equipment for advanced attacks; acoustic and timing attacks have been demonstrated with consumer microphones.

Detection difficulty: Detection-only via specialized monitoring.

Remediation:

• Use constant-time cryptographic libraries (libsodium, RustCrypto, BearSSL)
• Don't allow user-supplied data to influence timing of cryptographic operations
• Physical: shielded room (SCIF) for the most sensitive operations; otherwise accept and document

Threat 9: Visual / shoulder-surfing / camera observation

Description: Adversary observes the screen or keyboard during operation — direct shoulder-surf, hidden camera, reflection in glass.

Applicability:

• Operations involve typing secrets (passwords, recovery phrases)
• Public or semi-public location
• Display visible to others

Attacker capability: Trivial — anyone with line of sight or a phone camera.

Detection difficulty: Easy if you're paying attention; hard otherwise.

Remediation:

• Privacy filter on screen
• Secrets via hardware token (no display required)
• Operate in privacy-respecting locations for high-value steps
• Recovery phrases: write once, on paper, in a private location

Threat 10: Lost or stolen device (no other compromise)

Description: Adversary obtains the device but does not have keys, bio, or operator. Pure theft scenario.

Applicability: Always relevant for portable assets.

Attacker capability: Has the device; can attempt offline brute force, can preserve for later authenticated access.

Detection difficulty: Operator typically notices loss within hours; some scenarios (luggage skimming) may not be noticed.

Remediation:

• LUKS / full-disk encryption on operator media
• No plaintext secrets at rest (review finding B4 anti-pattern)
• Hardware factors (YubiKey, Bio key) are physically separate from media — loss of media doesn't include loss of keys
• Remote-wipe / rotation procedure when loss is detected

This is the expected scenario the system is designed for. If the design fails this scenario, the design is broken.

Section 2: Threat applicability decision tree

Not every system needs to consider every threat. Walk this tree to identify which threats apply:

Is the asset portable (travels with operator)?
├── YES → Threats 1, 2, 3, 4, 5, 6, 9, 10 apply
│         If high-value target: + 7
│         If TEMPEST-relevant operation: + 8
└── NO (datacenter / fixed)
    ├── Is the asset accessible to non-employees (cleaning, contractors)? 
    │   ├── YES → Threats 1, 3, 6 apply (limited)
    │   └── NO (controlled DC) → Threats 7, 8 apply (resourced adversary only)

For each applicable threat, document in physical-threat-scenarios.md (template):

• Whether the system addresses it
• Which mitigation pattern
• Residual risk after mitigation

Section 3: Cross-references to mitigation skills

Section 4: Worked examples

Review B3 — evil-maid via unsigned bootstrap

This skill flags Threat 1 as in-scope. Mitigation per chain-of-trust-design (Pattern A + D). Cross-reference recorded in physical-threat-scenarios.md and chain-of-trust-design.md.

Review H4 — travel-host blast radius

This skill flags Threats 1, 2, 4, 6 as all in-scope for portable operator secrets system. Mitigations:

• Threat 1: signed bootstrap (chain-of-trust)
• Threat 2: BIOS DMA disable (documented in chain-of-trust-design.md open assumptions)
• Threat 4: signed live image (chain-of-trust Pattern B — single highest leverage)
• Threat 6: lock to S5 in hostile environments; document in operator runbook

Section 5: Output format

When invoked, produce a physical-threat-scenarios.md document (template) covering applicable threats, mitigations, and residual risk. When invoked as part of a review, produce findings in standard format identifying threats not yet addressed.

This skill is also referenced by sdlc-complete/skills/security-assessment when the threat-model template's "Physical Access Considerations" section is non-empty (see Tier 2 issue #1079).

Related

• Threat-model extension: existing sdlc-complete/templates/security/threat-model-template.md will be extended with a "Physical Access Considerations" section that triggers this skill (#1079)
• Template: physical-threat-scenarios.md
• Companion skills: chain-of-trust-design, auth-factor-design, secret-handling-runtime, supply-chain-trust

Standards referenced

• NIST SP 800-114 — User's Guide to Telework and BYOD Security (travel host scope)
• NIST SP 800-88 — Media Sanitization (lost device cleanup)
• "TEMPEST: A Signal Problem" — NSA declassified document (Threat 8 history)
• CIS Critical Security Controls v8 — physical security mappings
• "Lest We Remember: Cold-Boot Attacks on Encryption Keys" (Halderman et al., 2008)