diff --git a/backend/src/auth/runtime-projection.ts b/backend/src/auth/runtime-projection.ts
index 782d9a32..851ad797 100644
--- a/backend/src/auth/runtime-projection.ts
+++ b/backend/src/auth/runtime-projection.ts
@@ -4,9 +4,9 @@ import {
   constants,
   fstatSync,
   lstatSync,
+  opendirSync,
   openSync,
   readSync,
-  readdirSync,
 } from "node:fs";
 import type { Stats } from "node:fs";
 import { isAbsolute, join, normalize } from "node:path";
@@ -23,6 +23,7 @@ const MAX_AUTH_BYTES = 1 << 20;
 const MAX_USERS_BYTES = 1 << 20;
 const MAX_SELECTOR_BYTES = 4096;
 const MAX_MANIFEST_BYTES = 4096;
+const MAX_DIRECTORY_ENTRIES = 16;
 const DIR_MODE = 0o700;
 const FILE_MODE = 0o600;
 const GENERATION = /^[0-9a-f]{64}$/;
@@ -69,6 +70,13 @@ function runtimeOwner(): number {
   if (!Number.isSafeInteger(uid) || uid < 0) throw invalid();
   return uid;
 }
+function runtimeGroup(): number {
+  if (process.platform === "win32" || typeof process.getegid !== "function")
+    throw invalid();
+  const gid = process.getegid();
+  if (!Number.isSafeInteger(gid) || gid < 0) throw invalid();
+  return gid;
+}
 function meta(info: Stats): Identity {
   return {
     dev: info.dev,
@@ -97,7 +105,12 @@ function same(a: Identity, b: Identity): boolean {
 }
 function directory(info: Stats, uid: number): Identity {
   const value = meta(info);
-  if (!info.isDirectory() || value.uid !== uid || value.mode !== DIR_MODE)
+  if (
+    !info.isDirectory() ||
+    value.uid !== uid ||
+    value.gid !== runtimeGroup() ||
+    value.mode !== DIR_MODE
+  )
     throw invalid();
   return value;
 }
@@ -106,6 +119,7 @@ function regular(info: Stats, uid: number, maximum: number): Identity {
   if (
     !info.isFile() ||
     value.uid !== uid ||
+    value.gid !== runtimeGroup() ||
     value.mode !== FILE_MODE ||
     value.nlink !== 1 ||
     value.size < 0 ||
@@ -120,7 +134,8 @@ function checkRoot(root: string): void {
     root.length === 0 ||
     root.includes("\0") ||
     !isAbsolute(root) ||
-    normalize(root) !== root
+    normalize(root) !== root ||
+    (root.length > 1 && root.endsWith("/"))
   )
     throw invalid();
 }
@@ -163,7 +178,22 @@ function stableDirectory(
 function entries(path: string, uid: number, expected: readonly string[]): void {
   const opened = openDirectory(path, uid);
   try {
-    const names = readdirSync(path);
+    const directory = opendirSync(`/proc/self/fd/${opened.fd}`, {
+      bufferSize: 1,
+    });
+    const names: string[] = [];
+    try {
+      for (;;) {
+        const entry = directory.readSync();
+        if (entry === null) break;
+        if (names.length === MAX_DIRECTORY_ENTRIES) throw invalid();
+        names.push(entry.name);
+      }
+    } finally {
+      try {
+        directory.closeSync();
+      } catch {}
+    }
     if (
       names.length !== expected.length ||
       new Set(names).size !== names.length ||
@@ -180,6 +210,55 @@ function entries(path: string, uid: number, expected: readonly string[]): void {
 function replaced(before: Identity, after: Identity): boolean {
   return before.dev !== after.dev || before.ino !== after.ino;
 }
+function rootIdentityAtPathAndDescriptor(
+  root: string,
+  openedRoot: { fd: number; identity: Identity },
+  uid: number,
+): Identity {
+  const openedAfter = directory(fstatSync(openedRoot.fd) as Stats, uid);
+  const pathAfter = directory(lstatSync(root) as Stats, uid);
+  if (!same(openedAfter, pathAfter)) throw invalid();
+  return openedAfter;
+}
+function validateRootAndCurrentAfterLoad(
+  root: string,
+  openedRoot: { fd: number; identity: Identity },
+  currentPath: string,
+  selectedCurrent: Identity,
+  uid: number,
+): void {
+  const rootBeforeCurrent = rootIdentityAtPathAndDescriptor(
+    root,
+    openedRoot,
+    uid,
+  );
+  const currentAfter = regular(
+    lstatSync(currentPath) as Stats,
+    uid,
+    MAX_SELECTOR_BYTES,
+  );
+  const rootAfterCurrent = rootIdentityAtPathAndDescriptor(
+    root,
+    openedRoot,
+    uid,
+  );
+  if (
+    !same(openedRoot.identity, rootBeforeCurrent) ||
+    !same(rootBeforeCurrent, rootAfterCurrent)
+  ) {
+    const latestCurrent = regular(
+      lstatSync(currentPath) as Stats,
+      uid,
+      MAX_SELECTOR_BYTES,
+    );
+    if (replaced(selectedCurrent, latestCurrent)) throw new CurrentReplaced();
+    throw invalid();
+  }
+  if (!same(selectedCurrent, currentAfter)) {
+    if (replaced(selectedCurrent, currentAfter)) throw new CurrentReplaced();
+    throw invalid();
+  }
+}
 function readRegular(
   path: string,
   parentPath: string,
@@ -484,17 +563,13 @@ function load(root: string): LoadedAuthConfig {
       for (const predecessor of selected.previousGenerations ?? [])
         validateGeneration(generationsPath, predecessor, uid);
       stableDirectory(generationsPath, openedGenerations, uid);
-      const afterCurrent = regular(
-        lstatSync(currentPath) as Stats,
+      validateRootAndCurrentAfterLoad(
+        root,
+        openedRoot,
+        currentPath,
+        selectedCurrent.identity,
         uid,
-        MAX_SELECTOR_BYTES,
       );
-      if (!same(selectedCurrent.identity, afterCurrent)) {
-        if (replaced(selectedCurrent.identity, afterCurrent))
-          throw new CurrentReplaced();
-        throw invalid();
-      }
-      stableDirectory(root, openedRoot, uid);
       return {
         value: selectedGeneration.value,
         revision: `sha256:${selected.generation}`,
diff --git a/backend/test/auth-runtime-projection.test.ts b/backend/test/auth-runtime-projection.test.ts
index 0fa6e38b..b76181e0 100644
--- a/backend/test/auth-runtime-projection.test.ts
+++ b/backend/test/auth-runtime-projection.test.ts
@@ -21,20 +21,40 @@ import { afterEach, expect, test, vi } from "vitest";
 import { createProjectedAuthenticationConfigProvider } from "../src/auth/runtime-projection.js";
 import { createCurrentLocalUserRegistryResolver } from "../src/auth/local-registry.js";
 
-const lstatHook = vi.hoisted(() => ({
+const fsHook = vi.hoisted(() => ({
   path: undefined as string | undefined,
   callback: undefined as (() => void) | undefined,
+  rejectPathReaddir: false,
+  foreignGid: undefined as number | undefined,
 }));
 
 vi.mock("node:fs", async (importOriginal) => {
   const actual = await importOriginal<typeof import("node:fs")>();
+  const observed = <T extends import("node:fs").Stats>(value: T): T =>
+    fsHook.foreignGid === undefined
+      ? value
+      : new Proxy(value, {
+          get(target, property) {
+            if (property === "gid") return fsHook.foreignGid;
+            const member = Reflect.get(target, property, target);
+            return typeof member === "function" ? member.bind(target) : member;
+          },
+        });
   return {
     ...actual,
     lstatSync(path: import("node:fs").PathLike) {
-      const result = actual.lstatSync(path);
-      if (lstatHook.path === String(path)) lstatHook.callback?.();
+      const result = observed(actual.lstatSync(path));
+      if (fsHook.path === String(path)) fsHook.callback?.();
       return result;
     },
+    fstatSync(fd: number) {
+      return observed(actual.fstatSync(fd));
+    },
+    readdirSync(path: import("node:fs").PathLike) {
+      if (fsHook.rejectPathReaddir)
+        throw new Error("path readdir is forbidden");
+      return actual.readdirSync(path);
+    },
   };
 });
 
@@ -46,8 +66,10 @@ const userId = "6ba7b810-9dad-4ed1-80b4-00c04fd430c8";
 const roots: string[] = [];
 
 afterEach(() => {
-  lstatHook.path = undefined;
-  lstatHook.callback = undefined;
+  fsHook.path = undefined;
+  fsHook.callback = undefined;
+  fsHook.rejectPathReaddir = false;
+  fsHook.foreignGid = undefined;
   for (const root of roots.splice(0))
     rmSync(root, { recursive: true, force: true });
 });
@@ -175,6 +197,66 @@ function writeReadyProjection(
   return generation;
 }
 
+function writeReadyOidcProjection(root: string): string {
+  mkdirSync(join(root, "generations"), { mode: 0o700 });
+  chmodSync(join(root, "generations"), 0o700);
+  const auth = stringify({
+    version: 1,
+    mode: "oidc",
+    publicUrl: "https://thothii.example.org",
+    oidc: {
+      issuer: "https://authentik.example.org/application/o/thothii/",
+      clientId: "thothii",
+      clientSecretRef: "THT_OIDC_CLIENT_SECRET",
+      scopes: ["openid", "profile", "email"],
+      groupsClaim: "groups",
+    },
+    groupCatalog: {
+      driver: "authentik",
+      baseUrl: "https://authentik.example.org",
+      apiTokenRef: "THT_AUTHENTIK_API_TOKEN",
+    },
+    authorization: {
+      groupRoles: { "TOT Users": ["user"], "TOT Admin": ["admin"] },
+    },
+  });
+  const generation = sha256(
+    `thothii-auth-projection-v1\nmode=oidc\nauth=${sha256(auth)}\nusers=-\n`,
+  );
+  const directory = join(root, "generations", generation);
+  mkdirSync(directory, { mode: 0o700 });
+  chmodSync(directory, 0o700);
+  const manifest = `${JSON.stringify({
+    version: 1,
+    generation,
+    mode: "oidc",
+    canonicalRevision: `sha256:${generation}`,
+    files: [
+      {
+        name: "auth.yaml",
+        size: Buffer.byteLength(auth),
+        sha256: sha256(auth),
+      },
+    ],
+  })}\n`;
+  writeFileSync(join(directory, "manifest.json"), manifest, {
+    encoding: "utf8",
+    mode: 0o600,
+  });
+  writeFileSync(join(directory, "auth.yaml"), auth, {
+    encoding: "utf8",
+    mode: 0o600,
+  });
+  chmodSync(join(directory, "manifest.json"), 0o600);
+  chmodSync(join(directory, "auth.yaml"), 0o600);
+  writeFileSync(join(root, "CURRENT"), currentDocument(generation), {
+    encoding: "utf8",
+    mode: 0o600,
+  });
+  chmodSync(join(root, "CURRENT"), 0o600);
+  return generation;
+}
+
 function expectDenied(operation: () => unknown): void {
   try {
     operation();
@@ -209,6 +291,28 @@ test("loads ready projection as one immutable auth and local-users snapshot", ()
   ).toBe(true);
 });
 
+test("loads a complete OIDC projection without a users snapshot", () => {
+  const root = projectionRoot();
+  const generation = writeReadyOidcProjection(root);
+  const loaded = createProjectedAuthenticationConfigProvider(root).current();
+  expect(loaded.value.mode).toBe("oidc");
+  expect(loaded.runtimeProjection).toEqual({
+    generation,
+    canonicalRevision: `sha256:${generation}`,
+  });
+});
+
+test("rejects a trailing-slash runtime root", () => {
+  const root = projectionRoot();
+  writeReadyProjection(
+    root,
+    localProjectionFixture("synthetic-user", passwordHash),
+  );
+  expectDenied(() =>
+    createProjectedAuthenticationConfigProvider(`${root}/`).current(),
+  );
+});
+
 test.each([
   ["missing", undefined],
   [
@@ -296,6 +400,32 @@ test.runIf(process.geteuid?.() === 0)(
   },
 );
 
+test("rejects a foreign group with the correct owner", () => {
+  const root = projectionRoot();
+  writeReadyProjection(
+    root,
+    localProjectionFixture("synthetic-user", passwordHash),
+  );
+  const gid = process.getegid?.() ?? 0;
+  fsHook.foreignGid = gid === 1 ? 2 : 1;
+  expectDenied(() =>
+    createProjectedAuthenticationConfigProvider(root).current(),
+  );
+});
+
+test("enumerates closed namespaces without path-based readdirSync", () => {
+  const root = projectionRoot();
+  const generation = writeReadyProjection(
+    root,
+    localProjectionFixture("synthetic-user", passwordHash),
+  );
+  fsHook.rejectPathReaddir = true;
+  expect(
+    createProjectedAuthenticationConfigProvider(root).current()
+      .runtimeProjection?.generation,
+  ).toBe(generation);
+});
+
 test("rejects a symlinked runtime root", () => {
   const root = projectionRoot();
   writeReadyProjection(
@@ -409,9 +539,9 @@ test("retries once when CURRENT is atomically replaced between lstat and open",
     localProjectionFixture("second", passwordHash),
   );
   const temporary = join(root, ".current-replacement.tmp");
-  lstatHook.path = join(root, "CURRENT");
-  lstatHook.callback = () => {
-    lstatHook.callback = undefined;
+  fsHook.path = join(root, "CURRENT");
+  fsHook.callback = () => {
+    fsHook.callback = undefined;
     writeFileSync(temporary, currentDocument(second, [first]), {
       encoding: "utf8",
       mode: 0o600,
@@ -425,6 +555,43 @@ test("retries once when CURRENT is atomically replaced between lstat and open",
   ).toBe(second);
 });
 
+test("retries once when CURRENT is replaced after the final identity read", () => {
+  const root = projectionRoot();
+  const first = writeReadyProjection(
+    root,
+    localProjectionFixture("first", passwordHash),
+  );
+  const second = writeGeneration(
+    root,
+    localProjectionFixture("second", passwordHash),
+  );
+  const stagedParent = mkdtempSync(
+    join(tmpdir(), "tht-auth-projection-late-generation-"),
+  );
+  roots.push(stagedParent);
+  renameSync(join(root, "generations", second), join(stagedParent, second));
+  let observations = 0;
+  fsHook.path = join(root, "CURRENT");
+  fsHook.callback = () => {
+    observations += 1;
+    if (observations !== 3) return;
+    fsHook.callback = undefined;
+    renameSync(join(stagedParent, second), join(root, "generations", second));
+    const temporary = join(root, ".current-late-replacement.tmp");
+    writeFileSync(temporary, currentDocument(second, [first]), {
+      encoding: "utf8",
+      mode: 0o600,
+    });
+    chmodSync(temporary, 0o600);
+    renameSync(temporary, join(root, "CURRENT"));
+  };
+  expect(
+    createProjectedAuthenticationConfigProvider(root).current()
+      .runtimeProjection?.generation,
+  ).toBe(second);
+  expect(observations).toBe(3);
+});
+
 test("rejects a second CURRENT replacement after the one permitted retry", () => {
   const root = projectionRoot();
   const first = writeReadyProjection(
@@ -443,8 +610,8 @@ test("rejects a second CURRENT replacement after the one permitted retry", () =>
     { generation: second, previous: [first] },
     { generation: third, previous: [second, first] },
   ];
-  lstatHook.path = join(root, "CURRENT");
-  lstatHook.callback = () => {
+  fsHook.path = join(root, "CURRENT");
+  fsHook.callback = () => {
     const replacement = replacements.shift();
     if (!replacement) return;
     const temporary = join(
@@ -484,9 +651,9 @@ test("fails deterministically when generations is replaced during a load", () =>
     });
     chmodSync(join(replacement, generation, name), 0o600);
   }
-  lstatHook.path = join(root, "generations");
-  lstatHook.callback = () => {
-    lstatHook.callback = undefined;
+  fsHook.path = join(root, "generations");
+  fsHook.callback = () => {
+    fsHook.callback = undefined;
     renameSync(join(root, "generations"), join(root, "generations-retired"));
     renameSync(replacement, join(root, "generations"));
   };
@@ -513,9 +680,9 @@ test("fails deterministically when the selected generation directory is replaced
     });
     chmodSync(join(replacement, name), 0o600);
   }
-  lstatHook.path = join(root, "generations", generation);
-  lstatHook.callback = () => {
-    lstatHook.callback = undefined;
+  fsHook.path = join(root, "generations", generation);
+  fsHook.callback = () => {
+    fsHook.callback = undefined;
     renameSync(
       join(root, "generations", generation),
       join(root, "generation-retired"),