diff --git a/sdk/src/runner.ts b/sdk/src/runner.ts
index fb58c965..02317038 100644
--- a/sdk/src/runner.ts
+++ b/sdk/src/runner.ts
@@ -13,13 +13,15 @@
 //   background work is invisible to supervision; export a plugin to be supervised).
 //
 // Trust model (RFC §9.4): a unit is code the operator chose to run — no sandbox is pretended.
-// The same sshd rule as hooks applies: a world-writable unit file is refused loudly.
+// The same sshd rule as hooks applies on BOTH platforms: a unit file a non-privileged principal
+// could have written is refused loudly (mode bits on Unix, owner + DACL on Windows).
 import {
 	Cause,
 	Duration,
 	Effect,
 	Schedule,
 } from "effect";
+import { spawnSync } from "node:child_process";
 import * as fs from "node:fs";
 import * as path from "node:path";
 import { pathToFileURL } from "node:url";
@@ -53,9 +55,217 @@ export interface Unit {
 const defaultLog = (line: string) =>
 	console.log(`${new Date().toISOString()} ${line}`);
 
-/** The sshd rule (RFC §9.1/§9.4): refuse group/world-writable unit files, loudly. */
+// ---- unit-file trust (the sshd rule, both halves) ---------------------------------------------
+
+// SDDL access-mask bits that let a principal change the file's content or its protection:
+// write/append data + EAs, DELETE (delete + recreate), WRITE_DAC / WRITE_OWNER (rewrite the
+// protection itself), and the generic write/all bits. FILE_WRITE_ATTRIBUTES (0x100) is
+// deliberately NOT here: it only toggles timestamps/readonly/hidden — never content — and the
+// runner's own service principal legitimately holds it (bun's module loader opens unit files
+// requesting RX+WA; `plugins enable` grants exactly that on the plugins/scripts dirs — counting
+// WA as tampering would make the runner refuse every unit it is supposed to run).
+const SDDL_WRITE_BITS =
+	0x2 | 0x4 | 0x10 | 0x10000 | 0x40000 | 0x80000 | 0x10000000 | 0x40000000;
+
+// SDDL two-letter rights tokens → access-mask bits (generic, standard, file-specific, and the
+// low object-rights aliases hex masks sometimes render as). Anything unrecognized is treated as
+// write-capable — fail closed.
+const SDDL_RIGHT_TOKENS: Record<string, number> = {
+	GA: 0x10000000,
+	GX: 0x20000000,
+	GW: 0x40000000,
+	GR: 0x80000000,
+	RC: 0x20000,
+	SD: 0x10000,
+	WD: 0x40000,
+	WO: 0x80000,
+	FA: 0x1f01ff,
+	FR: 0x120089,
+	FW: 0x120116,
+	FX: 0x1200a0,
+	CC: 0x1,
+	DC: 0x2,
+	LC: 0x4,
+	SW: 0x8,
+	RP: 0x10,
+	WP: 0x20,
+	DT: 0x40,
+	LO: 0x80,
+	CR: 0x100,
+};
+
+// SDDL two-letter account abbreviations we may meet on a unit file, → full SIDs.
+const SDDL_SID_ABBREV: Record<string, string> = {
+	SY: "S-1-5-18", // NT AUTHORITY\SYSTEM
+	BA: "S-1-5-32-544", // BUILTIN\Administrators
+	OW: "S-1-3-4", // OWNER RIGHTS
+	CO: "S-1-3-0", // CREATOR OWNER
+	LS: "S-1-5-19", // NT AUTHORITY\LOCAL SERVICE
+	NS: "S-1-5-20", // NT AUTHORITY\NETWORK SERVICE
+	BU: "S-1-5-32-545", // BUILTIN\Users
+	AU: "S-1-5-11", // Authenticated Users
+	IU: "S-1-5-4", // INTERACTIVE
+	WD: "S-1-1-0", // Everyone
+};
+
+const TRUSTED_OWNER_SIDS = new Set([
+	"S-1-5-18", // SYSTEM
+	"S-1-5-32-544", // Administrators
+	"S-1-5-80-956008885-3418522649-1831038044-1853292631-2271478464", // TrustedInstaller
+]);
+
+/** An SDDL rights field as an access mask; -1 when it can't be fully understood (fail closed). */
+const sddlRightsMask = (rights: string): number => {
+	if (rights.startsWith("0x") || rights.startsWith("0X")) {
+		const n = Number.parseInt(rights, 16);
+		return Number.isNaN(n) ? -1 : n;
+	}
+	if (rights.length % 2 !== 0) return -1;
+	let mask = 0;
+	for (let i = 0; i < rights.length; i += 2) {
+		const bits = SDDL_RIGHT_TOKENS[rights.slice(i, i + 2)];
+		if (bits === undefined) return -1;
+		mask = (mask | bits) >>> 0;
+	}
+	return mask;
+};
+
+/**
+ * The Windows half of the sshd rule, as a pure function over the file's SDDL (exported for
+ * tests). Returns `null` when the descriptor is trustworthy — owner is
+ * SYSTEM/Administrators/TrustedInstaller (or `extraTrustedSid`, the account running the runner:
+ * the Unix rule's "your own file is fine") and no other principal holds a write-capable allow
+ * ACE — else a human-readable refusal reason. Unknown ACE shapes count as write-capable.
+ */
+export const windowsSddlUnsafeReason = (
+	sddl: string,
+	extraTrustedSid?: string,
+): string | null => {
+	const trusted = new Set(TRUSTED_OWNER_SIDS);
+	trusted.add("S-1-3-4"); // OWNER RIGHTS — constrained by the owner check below
+	if (extraTrustedSid) trusted.add(extraTrustedSid);
+
+	const owner = /^O:(S-[0-9-]+|[A-Z]{2})/.exec(sddl)?.[1];
+	const ownerSid =
+		owner === undefined
+			? undefined
+			: owner.startsWith("S-")
+				? owner
+				: SDDL_SID_ABBREV[owner];
+	if (ownerSid === undefined || !trusted.has(ownerSid)) {
+		return `owner ${owner ?? "unknown"} is not SYSTEM/Administrators/TrustedInstaller`;
+	}
+
+	const daclAt = sddl.indexOf("D:");
+	if (daclAt < 0) return "no DACL in the security descriptor";
+	// ACE format: (type;flags;rights;objectGuid;inheritGuid;sid[;condition]). SACL ACEs after
+	// "S:" match the regex too but are audit types, filtered by the allow-type check.
+	for (const [, ace] of sddl.slice(daclAt + 2).matchAll(/\(([^)]*)\)/g)) {
+		const [type, flags = "", rights = "", , , sid = ""] = ace.split(";");
+		if (type !== "A" && type !== "XA") continue; // deny/audit ACEs only ever tighten
+		// Inherit-only ACEs are templates for children; they grant nothing on this file. Flags
+		// come in two-letter tokens — compare exactly, not by substring.
+		const flagTokens: string[] = flags.match(/.{2}/g) ?? [];
+		if (flagTokens.includes("IO")) continue;
+		const mask = sddlRightsMask(rights);
+		if (mask !== -1 && (mask & SDDL_WRITE_BITS) === 0) continue; // read-only ACE
+		const resolved = sid.startsWith("S-") ? sid : (SDDL_SID_ABBREV[sid] ?? sid);
+		if (!trusted.has(resolved)) {
+			return `${sid} can write it (only SYSTEM/Administrators may)`;
+		}
+	}
+	return null;
+};
+
+/** The SID this process runs as, fetched once (`undefined` when it can't be determined). */
+let processSidCache: string | undefined | false;
+const processSid = (): string | undefined => {
+	if (processSidCache === undefined) {
+		const res = spawnSync(
+			windowsPowershell(),
+			[
+				"-NoProfile",
+				"-NonInteractive",
+				"-Command",
+				"[System.Security.Principal.WindowsIdentity]::GetCurrent().User.Value",
+			],
+			{
+				encoding: "utf8",
+				windowsHide: true,
+				timeout: 15_000,
+				env: windowsPowershellEnv(),
+			},
+		);
+		const sid = res.status === 0 ? (res.stdout ?? "").trim() : "";
+		processSidCache = /^S-[0-9-]+$/.test(sid) ? sid : false;
+	}
+	return processSidCache === false ? undefined : processSidCache;
+};
+
+// Full System32 path, never PATH — a planted powershell.exe must not run with our privileges
+// (mirrors the host CLI's powershell_path, security-review 2026-07-17).
+const windowsPowershell = (): string =>
+	path.join(
+		process.env.SystemRoot ?? "C:\\Windows",
+		"System32",
+		"WindowsPowerShell",
+		"v1.0",
+		"powershell.exe",
+	);
+
+// Spawn env for Windows PowerShell 5.1 with PSModulePath STRIPPED (5.1 rebuilds its default).
+// An inherited PSModulePath that includes a PowerShell 7 module dir (pwsh adds a machine-scope
+// entry) makes 5.1 fail to autoload Microsoft.PowerShell.Security — type-data conflict
+// ("AuditToString is already present") — so Get-Acl dies and every unit is refused. Seen live
+// on the .173 host box; intermittent per spawn, deterministic once stripped.
+const windowsPowershellEnv = (): Record<string, string | undefined> => {
+	const env: Record<string, string | undefined> = { ...process.env };
+	delete env.PSModulePath;
+	return env;
+};
+
+/** Read a file's SDDL and apply [`windowsSddlUnsafeReason`]. Unreadable ACL ⇒ refuse. */
+const windowsFileIsSafe = (file: string, log: (l: string) => void): boolean => {
+	const escaped = file.replace(/'/g, "''");
+	const res = spawnSync(
+		windowsPowershell(),
+		[
+			"-NoProfile",
+			"-NonInteractive",
+			"-Command",
+			`(Get-Acl -LiteralPath '${escaped}').Sddl`,
+		],
+		{
+			encoding: "utf8",
+			windowsHide: true,
+			timeout: 15_000,
+			env: windowsPowershellEnv(),
+		},
+	);
+	const sddl = res.status === 0 ? (res.stdout ?? "").trim() : "";
+	if (!sddl) {
+		log(`[runner] REFUSING ${file} — could not read its ACL`);
+		return false;
+	}
+	const reason = windowsSddlUnsafeReason(sddl, processSid());
+	if (reason !== null) {
+		log(
+			`[runner] REFUSING ${file} — ${reason}. Reinstall the plugin with ` +
+				`\`punktfunk-host plugins add\`, or re-own the file to Administrators and strip ` +
+				`non-admin write ACEs (icacls).`,
+		);
+		return false;
+	}
+	return true;
+};
+
+/**
+ * The sshd rule (RFC §9.1/§9.4): refuse a unit file anyone less privileged than the operator
+ * could have written — group/world-writable mode on Unix; on Windows, an owner outside
+ * SYSTEM/Administrators/TrustedInstaller or a write-capable ACE for a non-admin principal.
+ */
 const fileIsSafe = (file: string, log: (l: string) => void): boolean => {
-	if (process.platform === "win32") return true; // config dir is DACL'd; ACL check is a follow-up
+	if (process.platform === "win32") return windowsFileIsSafe(file, log);
 	try {
 		const mode = fs.statSync(file).mode & 0o022;
 		if (mode !== 0) {
diff --git a/sdk/test/runner.test.ts b/sdk/test/runner.test.ts
index 77a8d55c..ed529775 100644
--- a/sdk/test/runner.test.ts
+++ b/sdk/test/runner.test.ts
@@ -5,7 +5,12 @@ import { afterAll, describe, expect, test } from "bun:test";
 import { Effect, Fiber } from "effect";
 import * as fs from "node:fs";
 import * as path from "node:path";
-import { discoverUnits, runner, superviseUnit } from "../src/runner.js";
+import {
+	discoverUnits,
+	runner,
+	superviseUnit,
+	windowsSddlUnsafeReason,
+} from "../src/runner.js";
 
 const TOKEN = "runner-token";
 // Fixtures live under sdk/ so the generated plugin files can resolve "effect" and the SDK.
@@ -104,6 +109,86 @@ describe("discovery", () => {
 	});
 });
 
+describe("windowsSddlUnsafeReason (the sshd rule's Windows half, pure)", () => {
+	// What a file under the host's ACL'd %ProgramData%\punktfunk actually looks like: owned by
+	// Administrators, protected DACL, admin/SYSTEM/OWNER-RIGHTS full + Users read-execute.
+	const LOCKED =
+		"O:BAG:SYD:PAI(A;;FA;;;SY)(A;;FA;;;BA)(A;;FA;;;OW)(A;;0x1200a9;;;BU)";
+
+	test("accepts the host's locked-down layout", () => {
+		expect(windowsSddlUnsafeReason(LOCKED)).toBeNull();
+	});
+
+	test("accepts inherited allow ACEs spelled with token runs", () => {
+		expect(
+			windowsSddlUnsafeReason("O:SYG:SYD:(A;ID;FA;;;SY)(A;ID;FA;;;BA)(A;ID;FR;;;BU)"),
+		).toBeNull();
+	});
+
+	test("refuses an untrusted owner", () => {
+		expect(
+			windowsSddlUnsafeReason("O:BUG:SYD:(A;;FA;;;SY)(A;;FA;;;BA)"),
+		).toContain("owner");
+		expect(
+			windowsSddlUnsafeReason(
+				"O:S-1-5-21-1111111111-2222222222-3333333333-1001G:SYD:(A;;FA;;;SY)",
+			),
+		).toContain("owner");
+	});
+
+	test("accepts the running account as owner and writer (the Unix 'your own file' rule)", () => {
+		const me = "S-1-5-21-1111111111-2222222222-3333333333-1001";
+		const sddl = `O:${me}G:SYD:(A;;FA;;;SY)(A;;FA;;;BA)(A;;FA;;;${me})`;
+		expect(windowsSddlUnsafeReason(sddl)).toContain("owner");
+		expect(windowsSddlUnsafeReason(sddl, me)).toBeNull();
+	});
+
+	test("refuses write-capable ACEs for non-admin principals, by token and by hex", () => {
+		// BUILTIN\Users with modify (hex, as Windows renders 0x1301bf)
+		expect(
+			windowsSddlUnsafeReason(`${LOCKED}(A;;0x1301bf;;;BU)`),
+		).toContain("BU");
+		// Everyone with generic write
+		expect(windowsSddlUnsafeReason(`${LOCKED}(A;;GW;;;WD)`)).toContain("WD");
+		// Authenticated Users with file-write
+		expect(windowsSddlUnsafeReason(`${LOCKED}(A;;FW;;;AU)`)).toContain("AU");
+		// DELETE alone is enough (delete + recreate)
+		expect(windowsSddlUnsafeReason(`${LOCKED}(A;;SD;;;BU)`)).toContain("BU");
+		// WRITE_DAC alone is enough (rewrite the protection)
+		expect(windowsSddlUnsafeReason(`${LOCKED}(A;;WD;;;BU)`)).toContain("BU");
+		// an explicit user SID
+		expect(
+			windowsSddlUnsafeReason(
+				`${LOCKED}(A;;FA;;;S-1-5-21-1111111111-2222222222-3333333333-1001)`,
+			),
+		).toContain("S-1-5-21");
+	});
+
+	test("accepts the runner principal's RX+WA grant, refuses real writes for it", () => {
+		// `plugins enable` grants LocalService (RX,WA) on the unit dirs — bun's module loader
+		// opens files requesting FILE_WRITE_ATTRIBUTES on top of read+execute (0x1201a9).
+		// WA can't alter content, so it must not read as tamper-capable…
+		expect(
+			windowsSddlUnsafeReason(`${LOCKED}(A;OICIID;0x1201a9;;;LS)`),
+		).toBeNull();
+		// …but actual write-data for the service principal is still refused.
+		expect(windowsSddlUnsafeReason(`${LOCKED}(A;;FW;;;LS)`)).toContain("LS");
+	});
+
+	test("inherit-only ACEs and deny ACEs don't trip it; unknown shapes fail closed", () => {
+		// inherit-only: a template for children, grants nothing on this file
+		expect(
+			windowsSddlUnsafeReason(`${LOCKED}(A;OICIIO;FA;;;BU)`),
+		).toBeNull();
+		// deny ACEs only tighten
+		expect(windowsSddlUnsafeReason(`${LOCKED}(D;;FA;;;BU)`)).toBeNull();
+		// unknown rights token → treated as write-capable
+		expect(windowsSddlUnsafeReason(`${LOCKED}(A;;ZZ;;;BU)`)).toContain("BU");
+		// no DACL at all → refused
+		expect(windowsSddlUnsafeReason("O:BAG:SY")).toContain("DACL");
+	});
+});
+
 describe("supervision", () => {
 	test("async-fn plugin runs with a facade client; clean return completes", async () => {
 		const server = mockHost();