diskless-workstation/docs/ssp-annex-a-ism-residual-risk.md

# Annex A — ISM Control Cross-Reference & Residual Risk Register

> **Protective marking:** `OFFICIAL: Sensitive` *(inherits the parent
> [SSP](ssp.md) marking; set per system classification).*
>
> **Companion to:** [System Security Plan](ssp.md). Read the SSP first for system
> description, boundary and roles.
>
> **A&A framework:** ASD *Information Security Manual* (ISM) — engagement target
> **May 2026 release**.
>
> **Annex version:** 0.2 (updated 2026-06-22). v0.2 reflects: STIG hardening
> applied + bench-verified, USBGuard device control enabled, admin `staff_u`
> confinement, audit offload verified, PXE/iPXE boot path added, and **UEFI Secure
> Boot signing verified enforcing on real hardware** (VM HTTP + 820 PXE) — R-13
> reduced from Medium to Low (residual is making signed + SB-on the standing
> default).

---

## How to read this annex

This annex does two things:

1. **§2 — ISM control cross-reference:** maps the ISM control set (by guideline
   chapter) to *how this system implements it*, with an implementation **status**
   and a pointer to any **residual risk**.
2. **§3 — Residual risk register:** the consolidated, rated list of residual
   risks (`R-NN`), each with ISM linkage, compensating controls, treatment and
   owner. This is the artifact the Authorising Officer reasons over.

### ⚠ Control-identifier currency caveat

The `ISM-NNNN` identifiers below are **representative** and were drafted against
an understanding of the ISM to a **January 2026 baseline**. The ISM is reissued
quarterly and **renumbers/adds/retires controls between releases.** Before this
annex is relied upon for assessment, the **IRAP assessor MUST reconcile every
identifier against the actual May 2026 ISM**, confirm no newly-introduced
controls are unmapped, and confirm none of the cited controls were retired. Where
an exact number is uncertain it is shown as `ISM-NNNN*` (asterisk = *verify
number*). The **control intent and the system's implementation** are the durable
content here; the numbers are a convenience for the assessor.

### Status legend

| Status | Meaning |
|---|---|
| **Implemented** | Control is in place and operating in the current build. |
| **Partial** | Implemented but incomplete, or scoped to lab config. |
| **Planned** | On the roadmap / POA&M; not yet implemented. |
| **N/A (justified)** | Structurally not applicable to a diskless/UKI/immutable design; compensating control noted. |
| **Waived** | Deliberately not met; documented in the [STIG deviation register](stig-deviations.md) and/or a residual risk here. |

### Risk rating scale

Likelihood × Consequence → **Low / Medium / High / Critical** (qualitative,
ISM/PSPF style). Ratings are **pre-treatment** unless stated; the AO accepts the
**post-treatment residual**.

---

## 2. ISM control cross-reference

### 2.1 Guidelines for Security Documentation

| Topic | ISM (rep.) | Requirement intent | System implementation | Status | Residual |
|---|---|---|---|---|---|
| System Security Plan | ISM-0041* | An SSP is developed and maintained. | This document set ([SSP](ssp.md) + this annex). | Partial | Formal sign-off, role appointment pending. |
| Incident Response Plan | ISM-0043* | An IRP is developed, maintained, exercised. | Not yet authored; inherent recovery via reboot/redeploy. | Planned | R-12 |
| Continuous monitoring plan | ISM-1163* | Continuous monitoring strategy documented. | auditd + log offload exist; plan not formalised. | Partial | R-07 |
| Strategies maintained & approved | ISM-0027* | Documentation approved by the authorising authority. | Pending AO appointment & approval. | Planned | — |

### 2.2 Guidelines for Cyber Security Roles

| Topic | ISM (rep.) | Requirement intent | System implementation | Status | Residual |
|---|---|---|---|---|---|
| CISO / system owner appointed | ISM-0714*, ISM-1071* | Defined accountable security roles. | Roles identified in [SSP §3]; appointments TBA. | Planned | — |
| System-specific security roles | ISM-1525* | Administrators with defined responsibilities. | Platform team operates build/release & runtime. | Partial | R-05 |

### 2.3 Guidelines for System Hardening — Operating systems & applications

| Topic | ISM (rep.) | Requirement intent | System implementation | Status | Residual |
|---|---|---|---|---|---|
| Hardened OS to ASD/vendor guidance | ISM-1407*, ISM-1408* | Apply a recognised hardening baseline. | **DISA STIG** (SSG `profile_stig`, RHEL 9) via `oscap --remediate` + documented tailoring + per-build deviations report ([stig-deviations.md](stig-deviations.md)); **applied + bench-verified 2026-06-20**, all REVIEW rows triaged. | Partial | R-03 (CI must ship `enable_stig=true` + signed/SB-on) |
| Application control / allow-listing | ISM-0843*, ISM-1490*, ISM-1657* | Only approved executables run. | **fapolicyd** (RPM-trust + two trusted local binaries: kiosk-session, fleet-sb-enroll) + immutable dm-verity root. | Implemented | — |
| Device / removable-media control | ISM-0341*, ISM-1551* | Control USB / peripheral devices. | **USBGuard enforcing** (`ImplicitPolicyTarget=block` + hardware-independent HID/hub allowlist + LinuxAudit). Verified: USB mass storage blocked, console input preserved. | Implemented | — |
| Software / OS integrity | ISM-1493* | Detect unauthorised modification. | **dm-verity** continuous block-level verification of the read-only root against a signed hash; **reproducible** build. | Implemented | — |
| Disable unneeded functionality | ISM-0380*, ISM-1416* | Minimise installed software/services. | `@^minimal` install; scanner tooling removed post-scan; FROM-scratch artifact image; kiosk = single app. | Implemented | — |
| Hardened user applications (browser) | ISM-1486*, ISM-1412* | Lock down high-risk apps. | Chromium enterprise lockdown policy; weston kiosk; no shell/nav/plugins. | Implemented | — |
| Memory/exploit mitigations | ISM-1341*, ISM-1342* | Enable platform exploit mitigations. | UKI cmdline: `slub_debug=P`, `page_poison=1`, `init_on_alloc/free=1`, `vsyscall=none`. | Implemented | — |
| Mandatory access control | ISM-1416* | MAC enforced. | **SELinux enforcing**; labels preserved into erofs (asserted at build). | Implemented | — |
| Host-based intrusion prevention / file integrity (AIDE) | ISM-1288* | File integrity monitoring. | **N/A** — superseded by dm-verity (read-only, signed, continuous). | N/A (justified) | — |
| Antivirus / endpoint scanning | ISM-1417* | Malware protection on endpoints. | **N/A/compensated** — immutable verified root + allow-listing means no persistent writable executable surface; overlay is RAM-only, discarded each boot. | N/A (justified) | — |

### 2.4 Guidelines for System Hardening — Authentication

| Topic | ISM (rep.) | Requirement intent | System implementation | Status | Residual |
|---|---|---|---|---|---|
| Strong machine authentication | ISM-1546*, ISM-0974* | Mutual, certificate-based auth. | **802.1X EAP-TLS**, client key non-exportable in **TPM** (`0x81010001`), cert in TPM NV. | Partial | R-01 (not bench-verified; disabled in test builds), R-08 (placeholder CA) |
| Privileged access restricted | ISM-1507*, ISM-0445* | Limit & control admin access. | Single key-only `wheel` admin; root login disabled; admin **confined to `staff_u`** with a sudo `sysadm_r`/`sysadm_t` role/type transition (SELinux context elevates on privilege use). | Partial | R-05 |
| Secure remote administration | ISM-1506*, ISM-1232* | Hardened admin channel. | SSH **key-only**, password & keyboard-interactive off, hardened ciphers/MACs/KEX (STIG crypto policy; ECDSA/RSA keys — ed25519 excluded by policy). | Implemented | — |
| No default/shared credentials | ISM-1304* | No baked credentials. | No per-host secrets in any artifact; identity from TPM at runtime; `machine-id` empty; host keys regenerated at first boot. | Implemented | — |
| MFA for privileged users | ISM-1173*, ISM-1504* | MFA on privileged access. | Machine auth is cert+TPM (strong). **Human admin MFA on the management plane not yet enforced.** | Partial | R-05 |
| Passwordless appliance accounts | — (STIG #10) | Account lockout/aging. | `kiosk`/`fleetadmin` are key-only; password aging/faillock **exempted for these two only**, full policy system-wide otherwise. | Implemented | R-05 (folds into rotatable-access roadmap) |

### 2.5 Guidelines for System Management

| Topic | ISM (rep.) | Requirement intent | System implementation | Status | Residual |
|---|---|---|---|---|---|
| Patching / vulnerability remediation | ISM-1493*, ISM-1144*, ISM-1472* | Timely patching. | Patch = **rebuild + re-sign + redeploy a new digest**; atomic, uniform fleet update; cadence/SLA to define. | Partial | R-10 (patch SLA undefined) |
| Secure build & change control | ISM-1483* | Controlled, repeatable builds. | Reproducible Packer build in CI; digest-pinned deployment; one commit/release; rollback = revert commit. | Implemented | — |
| Protect build/signing key material | ISM-0501*, ISM-1233* | Protect keys. | Signing keys CI/HSM-held; **never** in repo or build VM; never logged. | Implemented | — |
| Privileged actions logged | ISM-1509* | Admin actions auditable. | auditd rules capture privileged/admin events. | Implemented | R-07 (offload protection) |
| Disable debug/back-door access | ISM-1491* | No undocumented access paths. | `debug_shell=true` currently bakes a **passwordless root tty** (lab debugging). | Partial | **R-02 (must remove before prod)** |

### 2.6 Guidelines for System Monitoring (event logging)

| Topic | ISM (rep.) | Requirement intent | System implementation | Status | Residual |
|---|---|---|---|---|---|
| Centralised event logging | ISM-0988*, ISM-1405* | Logs centrally collected. | `audisp-syslog`→rsyslog `omfwd` (disk-assisted queue) → dedicated collector (`serving/audit-collector/`); **bench-verified end-to-end 2026-06-20**. auditd `space_left_action`→syslog (no MTA). | Partial | R-07 |
| Required events logged | ISM-0582*, ISM-1537* | Log security-relevant events. | STIG auditd rule set; kernel `audit=1`, `audit_backlog_limit=8192`. | Implemented | — |
| Protect logs in transit | ISM-1432*, ISM-0407* | Encrypt/authenticate log transport. | **Plaintext** offload on isolated LAN (rsyslog TLS rules waived — see [STIG #6](stig-deviations.md)). | Waived (lab) | **R-07** |
| Log retention & integrity | ISM-0859*, ISM-1815* | Retain & protect logs. | Per-source-host files on collector; retention policy & tamper-protection not yet defined. | Planned | R-07 |
| Event log analysis / alerting | ISM-0109*, ISM-1228* | Review logs; alert on incidents. | No SIEM correlation/alerting defined. | Planned | R-07, R-12 |

### 2.7 Guidelines for Networking

| Topic | ISM (rep.) | Requirement intent | System implementation | Status | Residual |
|---|---|---|---|---|---|
| Network access control (802.1X) | ISM-0520*, ISM-1182* | Authenticate devices to the network. | **802.1X** port-based control; unauthenticated port = no access. | Partial | R-01 |
| Network segmentation | ISM-1181*, ISM-1532* | Segregate & control traffic. | VLAN assignment post-auth; isolated boot LAN; serving stack separated. | Partial | R-09 |
| Host-based firewall | ISM-1416*, ISM-1416a* | Restrict host network exposure. | **firewalld disabled** (risk to NVMe/TCP & NM during bring-up). | Waived (lab) | **R-09** |
| Disable unused network functionality | ISM-1296* | Minimise network services. | Minimal services; no extraneous listeners; kiosk has no inbound. | Implemented | — |

### 2.8 Guidelines for Communications Infrastructure / Systems

| Topic | ISM (rep.) | Requirement intent | System implementation | Status | Residual |
|---|---|---|---|---|---|
| Link-layer encryption (cable/fabric) | ISM-1187*, ISM-0181* | Protect data on the link. | **MACsec** on the access link (encryption + integrity), keyed via MKA after EAP. | Partial | R-01 (MKA survival across switch-root not yet bench-verified) |

### 2.9 Guidelines for Cryptography

| Topic | ISM (rep.) | Requirement intent | System implementation | Status | Residual |
|---|---|---|---|---|---|
| ASD-Approved Cryptographic Algorithms (AACA) | ISM-0471*, ISM-1446* | Use approved algorithms. | STIG crypto policy (modern ciphers/MACs/KEX); EAP-TLS, MACsec-GCM. **Conformance to ISM AACA not formally asserted; FIPS off.** | Partial | **R-06** |
| ASD-Approved Cryptographic Protocols (AACP) | ISM-1139*, ISM-1453* | Use approved protocols (TLS 1.2+/1.3). | EAP-TLS, NVMe/TCP & log transport currently plaintext on LAN. | Partial | R-04, R-06, R-07 |
| Key protection / non-exportability | ISM-0501*, ISM-1091* | Protect keys in hardware where feasible. | EAP-TLS private key **non-exportable in TPM**; UKI signing keys in HSM/CI. | Implemented | — |
| Cryptographic equipment / module evaluation | ISM-0467*, ISM-1277* | Use evaluated crypto modules. | TPM 2.0 hardware; module-evaluation status to confirm; FIPS module not enabled. | Partial | R-06 |

### 2.10 Guidelines for ICT Equipment & Media

| Topic | ISM (rep.) | Requirement intent | System implementation | Status | Residual |
|---|---|---|---|---|---|
| Media sanitisation / no data remanence | ISM-0347*, ISM-1218* | Prevent data remanence. | **Diskless** — no persistent local media; writable layer is **RAM tmpfs**, gone at power-off. | Implemented | — |
| Tamper-evidence / boot integrity | ISM-1542*, ISM-0312* | Detect firmware/boot tampering. | Design: **UEFI Secure Boot** + signed UKI (+ signed iPXE on the PXE path) + immutable signed cmdline + dm-verity; TPM measurements available. **Verified enforcing on real hardware (2026-06-22):** CI-signed UKI under SB on a VM (HTTP) and signed iPXE→UKI under SB on an 820 (PXE), `db` enrolled via `fleet-sb-enroll`. Residual is making signed + SB-on the standing default for all builds/hosts. | Implemented (default-posture residual) | **R-13** (not yet default), R-11 (no remote attestation) |
| Device identification | ISM-1195* | Uniquely identify devices. | Identity derived from TPM cert (hostnqn from cert serial); no spoofable per-host config. | Implemented | — |

### 2.11 Guidelines for Procurement, Outsourcing & Evaluated Products

| Topic | ISM (rep.) | Requirement intent | System implementation | Status | Residual |
|---|---|---|---|---|---|
| Supply-chain / artifact integrity | ISM-1567*, ISM-1568* | Trustworthy software supply chain. | FROM-scratch OCI artifact; SHA256SUMS; digest-pinned manifest; reproducible build. | Implemented | — |
| Evaluated/ supported products | ISM-0280*, ISM-1467* | Prefer evaluated/vendor-supported. | RHEL/Rocky 9 (vendor-supported); TPM 2.0; evaluation status of crypto to confirm. | Partial | R-06 |

---

## 3. Residual risk register

Ratings are **pre-treatment** → with the **post-treatment residual** the AO is
asked to accept. Treatments map to [docs/TODO.md](TODO.md) (working POA&M).

| ID | Residual risk | ISM linkage | Likelihood × Consequence → **Rating** | Existing compensating controls | Treatment (POA&M) | Post-treat. residual | Owner / target |
|---|---|---|---|---|---|---|---|
| **R-01** | Secure path (802.1X EAP-TLS + MACsec) **implemented but not bench-verified end-to-end**; disabled in test builds (`disable_dot1x=true`). Until verified, devices may attach to an unauthenticated/unencrypted link. | 2.4, 2.7, 2.8 (ISM-1546*, 0520*, 1187*) | Likely × Major → **High** | Test path on an isolated lab LAN; full auth stack present (dormant); design follows ASD intent. | Full boot-chain bench test (swtpm + hostapd + FreeRADIUS): port `Authorized`, macsec0 addressed, MKA survives switch-root, NVMe root mounts. Re-enable in CI. | **Low** (once verified) | Platform team — *before production ATO* |
| **R-02** | `debug_shell=true` bakes a **passwordless root shell** on a tty — an undocumented privileged access path. | 2.5 (ISM-1491*) | Possible × Severe → **High** | Lab-only; physical access required; isolated environment. | Set `debug_shell=false` for all non-lab builds; assert absent in CI. | **Low** | Platform team — *before production* |
| **R-03** | STIG hardening **applied + bench-verified** (2026-06-20, all REVIEW rows triaged); residual is ensuring **CI ships `enable_stig=true`** for production builds (it was toggled off during bring-up). | 2.3 (ISM-1407*/1408*) | Possible × Moderate → **Medium** | Tailoring + register + per-build report; SELinux/fapolicyd/USBGuard on; verified on a real hardened build. | Confirm `ENABLE_STIG="true"` in CI; keep the 0-eval guard; re-run the deviations report on the assessed build. | **Low** | Platform team — *before assessment* |
| **R-04** | NVMe/TCP **root traffic plaintext** on the LAN — exposure of OS content / injection if the segment is compromised. | 2.9 (ISM-1139*/1453*) | Unlikely × Major → **Medium** | Isolated boot LAN; MACsec on the access link (once R-01 done) protects the first hop; dm-verity makes injected root blocks fail verification (integrity preserved even if confidentiality isn't). | Production: TLS/IPsec for the storage transport, or rely on MACsec end-to-end across the fabric; document the decision. | **Low–Medium** | Platform/network — *production* |
| **R-05** | **NOPASSWD key-only admin**; no per-host or rotatable access model; human admin MFA on the mgmt plane not enforced. | 2.4 (ISM-1507*/1173*) | Possible × Moderate → **Medium** | Single fleet-generic admin, key-only, root disabled, ECDSA/RSA key, **now SELinux-confined to `staff_u`** with sudo role/type elevation, fully audited; no interactive password to attack. | Per-host/rotatable access (SSH CA / config-agent) + admin MFA on the management plane. | **Low–Medium** | Platform team — *roadmap* |
| **R-06** | **FIPS off**; ISM **AACA/AACP conformance not formally asserted**; crypto-module evaluation status unconfirmed. | 2.9 (ISM-0471*/1139*/1277*) | Possible × Major → **Medium** | Modern STIG crypto policy; non-exportable TPM key; no deliberately weak algorithms. Acceptable at OFFICIAL; **mandatory to resolve for PROTECTED+.** | Assert AACA/AACP conformance; decide on FIPS module / evaluated crypto alongside Secure Boot signing. | **Low** (at appropriate classification) | Security + Platform — *before PROTECTED+* |
| **R-07** | Audit-log **offload plaintext**, **single-site**, **no SIEM alerting**; logs interceptable/repudiable and not actively reviewed. (Transport **bench-verified working** 2026-06-20.) | 2.6 (ISM-1432*/0109*/0859*) | Possible × Moderate → **Medium** | Isolated LAN; disk-assisted queue (no loss on collector blip); auditd local copy; per-source files; LinuxAudit captures USBGuard events. | rsyslog **TLS** (gtls/6514) + bundle CA in image (re-select the 3 waived `rsyslog_encrypt_offload_*` rules); define retention + integrity; forward to SIEM with alerting. | **Low** | Platform/SecOps — *production* |
| **R-13** | **Boot-chain signing not yet the standing default** — signing + Secure Boot enforcement are **verified end-to-end on real hardware** (VM HTTP UKI 2026-06-20; 820 PXE signed iPXE→UKI 2026-06-22, SB enforcing, `db` enrolled), but unsigned/SB-OFF lab builds remain possible (CI signs only when `SB_KEY`/`SB_CERT` are set) and not every host has `db` enrolled, so a substituted artifact could still boot on an un-enrolled/unsigned host. | 2.10 (ISM-1542*/0312*) | Unlikely × Major → **Low** | Capability **proven on hardware**; dm-verity rejects a tampered *root* regardless; isolated lab LAN. | Make signed + Secure-Boot-on the **default for all production builds/hosts**: keep `SB_KEY`/`SB_CERT` CI secrets set; enrol `db` fleet-wide; treat an unsigned build as lab-only. | **Low** | Platform team — *before production* |
| **R-08** | **Placeholder RADIUS CA** (`files/etc/dot1x/radius-ca.pem`) — real EAP-TLS trust anchor not provisioned; auth cannot succeed and trust is unestablished. | 2.4 (ISM-1546*) | Certain (lab) × Major → **Medium** | Documented as a placeholder; environment-specific provisioning is the intended path; no real CA committed. | Provision the environment's real RADIUS CA out-of-band (not committed unless classification permits). | **Low** | Environment owner — *before production* |
| **R-09** | **Host firewall disabled**; host exposes more network surface than necessary. | 2.7 (ISM-1416*) | Possible × Moderate → **Low–Medium** | Isolated LAN; minimal services; egress controlled at the network/serving layer; no kiosk inbound. | Re-enable firewalld with explicit allow rules (NVMe/TCP `:4420`, DHCP, log offload, SSH) once the secure path is verified. | **Low** | Platform/network — *after R-01* |
| **R-10** | **Patch SLA undefined** — no documented cadence for rebuilding/redeploying against new CVEs. | 2.5 (ISM-1144*/1472*) | Likely × Moderate → **Medium** | Atomic rebuild/redeploy model makes patching fast & uniform; reproducible builds. | Define patch cadence & critical-CVE SLA; automate rebuild triggers. | **Low** | Platform team — *roadmap* |
| **R-11** | **No remote attestation** of TPM boot measurements — a compromised boot can't be detected centrally. | 2.10 (ISM-1542*) | Unlikely × Major → **Low–Medium** | Secure Boot + signed UKI + dm-verity already prevent unsigned/modified boot; measurements exist locally. | Implement remote attestation (TPM quote → verifier) gating network/storage access. | **Low** | Platform team — *roadmap* |
| **R-12** | **No formal Incident Response Plan**; volatile RAM overlay complicates live forensics. | 2.1, 2.6 (ISM-0043*/0109*) | Possible × Moderate → **Medium** | Inherent recovery (reboot → verified state; revert digest); centralised logs survive host wipe. | Author IRP + log-retention + break-glass/forensics procedure suited to stateless hosts. | **Low** | SecOps — *before production* |

### 3.1 Risk heatmap (post-treatment targets)

```
Consequence →     Minor    Moderate   Major    Severe
Likelihood ↓
  Certain                                       
  Likely                    R-10        R-01*   
  Possible                  R-05 R-07   R-03     R-02*
                            R-12        R-06 R-08
  Unlikely                              R-04     
                                        R-11 R-09
  Rare
( * = currently High pre-treatment; all targeted to Low after treatment )
```

*The heatmap shows pre-treatment positions; every item has a treatment to Low /
Low–Medium. The AO is asked to authorise on the basis that **R-01, R-02, R-08,
R-12 and R-13** (and confirming R-03's `enable_stig=true`) are **closed before
production**, and R-04/R-05/R-06/R-07/R-09/R-10/R-11 are either accepted with
compensating controls (isolated-LAN lab) or carried on the POA&M with target
dates. Newly strengthened in v0.2: device control (USBGuard, now Implemented),
admin SELinux confinement (R-05), STIG applied+verified (R-03 reduced), and
**Secure Boot signing verified enforcing on real hardware (R-13 reduced
Medium→Low)**.*

---

## 4. Assessor notes / open items for the IRAP assessment

1. **Reconcile all `ISM-NNNN*` identifiers** against the May 2026 ISM (see
   currency caveat) and confirm no new controls are unmapped.
2. **Confirm the system data classification** with the System Owner — it sets the
   required control baseline and may reclassify several "N/A/accepted" items
   (notably R-06 cryptography) as mandatory.
3. **Witness the secure-path bench test** (R-01) — this is the central
   effectiveness check; the rest of the architecture's value is contingent on it.
4. **Verify reproducibility** — two builds → identical `verity.json` root hash
   (build-integrity control evidence).
5. **Review the STIG generated report** for the build under assessment — every
   non-pass rule should map to the [deviation register](stig-deviations.md);
   anything flagged **REVIEW** is an unjustified finding.
6. **Confirm lab-only flags are off** in the assessed build: `debug_shell=false`,
   `enable_stig=true`, `disable_dot1x=false`, real RADIUS CA in place.
7. **Confirm boot-chain signing is the default** (R-13): signing + Secure Boot
   enforcement are **verified on hardware** (VM HTTP + 820 PXE, 2026-06-22) —
   confirm the *assessed* build is signed and the *assessed* hosts have `db`
   enrolled with Secure Boot ON (`mokutil --sb-state`, `sbverify` UKI + iPXE). The
   PXE/iPXE SB-OFF cmdline workaround is a no-op once SB is on.
8. **Witness device control** (USBGuard): a USB mass-storage device is blocked and
   logged, console HID still works.

---

*End of Annex A.*