diff --git a/frontend/src/hooks.server.test.ts b/frontend/src/hooks.server.test.ts
index 0892214..525e624 100644
--- a/frontend/src/hooks.server.test.ts
+++ b/frontend/src/hooks.server.test.ts
@@ -90,6 +90,114 @@ describe('hooks.server handle', () => {
 		}
 	});
 
+	it('should still return 502 for ordinary network errors (not 413)', async () => {
+		// Regression guard: error jaringan biasa (fetch failed) TIDAK Boleh
+		// diklasifikasikan 413 — harus tetap 502 "Backend tidak tersedia".
+		const arrayBufferMock = vi.fn().mockRejectedValue(new TypeError('fetch failed'));
+		const mockEvent: any = {
+			request: {
+				method: 'POST',
+				headers: new Headers({ 'content-type': 'application/json' }),
+				arrayBuffer: arrayBufferMock
+			},
+			url: new URL('https://sinau-c-dev.manakin-gentoo.ts.net/api/content/assets/upload')
+		};
+
+		const origFetch = global.fetch;
+		global.fetch = vi.fn() as any;
+
+		try {
+			const resolve = vi.fn();
+			const response = await handle({ event: mockEvent, resolve });
+
+			expect(response.status).toBe(502);
+			const body = await response.json();
+			expect(body.success).toBe(false);
+			expect(body.message).toBe('Backend tidak tersedia');
+			expect(global.fetch).not.toHaveBeenCalled();
+		} finally {
+			global.fetch = origFetch;
+		}
+	});
+
+	it('should return 413 when error has status 413 even without "exceeds limit" message', async () => {
+		// Deteksi via err.status === 413 harus bekerja tanpa regex pesan
+		// (pesan library bisa berubah; status code adalah kontrak).
+		const arrayBufferMock = vi.fn().mockRejectedValue(
+			Object.assign(new Error('request body too large'), { status: 413 })
+		);
+		const mockEvent: any = {
+			request: {
+				method: 'POST',
+				headers: new Headers({ 'content-type': 'application/json' }),
+				arrayBuffer: arrayBufferMock
+			},
+			url: new URL('https://sinau-c-dev.manakin-gentoo.ts.net/api/content/assets/upload')
+		};
+
+		const origFetch = global.fetch;
+		global.fetch = vi.fn() as any;
+
+		try {
+			const resolve = vi.fn();
+			const response = await handle({ event: mockEvent, resolve });
+
+			expect(response.status).toBe(413);
+			const body = await response.json();
+			expect(body.success).toBe(false);
+			expect(body.message).toMatch(/terlalu besar/i);
+		} finally {
+			global.fetch = origFetch;
+		}
+	});
+
+	it('should add restrictive CSP when proxying SVG assets (stored-XSS mitigasi)', async () => {
+		// SVG bisa memuat <script>. Saat SVG dibuka langsung di tab browser,
+		// script-nya jalan same-origin → bisa baca localStorage (token).
+		// Proxy HARUS memberi CSP ketat — backend juga mengirim header ini,
+		// tapi proxy sebelumnya hanya meneruskan content-type/set-cookie.
+		const mockEvent: any = {
+			request: { method: 'GET', headers: new Headers() },
+			url: new URL('https://sinau-c-dev.manakin-gentoo.ts.net/assets/malware.svg')
+		};
+		const origFetch = global.fetch;
+		global.fetch = vi.fn().mockResolvedValue(
+			new Response('<svg><script>alert(1)</script></svg>', {
+				status: 200,
+				headers: { 'content-type': 'image/svg+xml' }
+			})
+		) as any;
+
+		try {
+			const resolve = vi.fn();
+			const response = await handle({ event: mockEvent, resolve });
+			expect(response.status).toBe(200);
+			expect(response.headers.get('content-security-policy')).toBe("default-src 'none'; sandbox");
+		} finally {
+			global.fetch = origFetch;
+		}
+	});
+
+	it('should NOT add CSP for non-SVG assets', async () => {
+		const mockEvent: any = {
+			request: { method: 'GET', headers: new Headers() },
+			url: new URL('https://sinau-c-dev.manakin-gentoo.ts/assets/diagram.png')
+		};
+		const origFetch = global.fetch;
+		global.fetch = vi.fn().mockResolvedValue(
+			new Response('binary', { status: 200, headers: { 'content-type': 'image/png' } })
+		) as any;
+
+		try {
+			const resolve = vi.fn();
+			const response = await handle({ event: mockEvent, resolve });
+			expect(response.status).toBe(200);
+			expect(response.headers.get('content-security-policy')).toBeNull();
+		} finally {
+			global.fetch = origFetch;
+		}
+	});
+
 	it('should read body as arrayBuffer when proxying POST requests', async () => {
 		const arrayBufferMock = vi.fn().mockResolvedValue(new Uint8Array([1, 2, 3]).buffer);
 		const mockEvent: any = {
diff --git a/frontend/src/hooks.server.ts b/frontend/src/hooks.server.ts
index 31ebdf4..4c9eeca 100644
--- a/frontend/src/hooks.server.ts
+++ b/frontend/src/hooks.server.ts
@@ -73,6 +73,16 @@ export const handle: Handle = async ({ event, resolve }) => {
 			const setCookie = res.headers.get('set-cookie');
 			if (setCookie) resHeaders['set-cookie'] = setCookie;
 
+			// Mitigasi stored-XSS: SVG asset bisa memuat <script>. Route
+			// /assets/* publik (same-origin) — kalau SVG dibuka langsung di
+			// tab browser, script-nya bisa membaca localStorage (token
+			// siswa/guru). Backend juga mengirim CSP ini (routes/lessons.py
+			// send_assets), tapi proxy hanya meneruskan content-type/
+			// set-cookie — jadi beri ulang di sini.
+			if (isAsset && event.url.pathname.toLowerCase().endsWith('.svg')) {
+				resHeaders['content-security-policy'] = "default-src 'none'; sandbox";
+			}
+
 			return new Response(body, {
 				status: res.status,
 				headers: resHeaders,
@@ -89,6 +99,7 @@ export const handle: Handle = async ({ event, resolve }) => {
 					JSON.stringify({
 						success: false,
 						error: 'Payload too large',
+						// Nilai sumber: _MAX_ASSET_BYTES di routes/content_editor.py (backend).
 						message: 'Ukuran file terlalu besar (maksimum 5 MiB per asset)'
 					}),
 					{
diff --git a/podman-compose.yml b/podman-compose.yml
index 50a5290..763cfb2 100644
--- a/podman-compose.yml
+++ b/podman-compose.yml
@@ -85,6 +85,7 @@ services:
       - API_BACKEND=http://127.0.0.1:5000
       # Backend menerima asset ≤ 5 MiB (JSON base64 ≈ 7 MiB di wire).
       # Default adapter-node 512K menolak upload gambar sebelum sampai backend.
+      # Nilai sumber: _MAX_ASSET_BYTES di routes/content_editor.py — sinkronkan.
       - BODY_SIZE_LIMIT=8M
       - PUBLIC_APP_BAR_TITLE=${APP_BAR_TITLE}
       - PUBLIC_COPYRIGHT_TEXT=${COPYRIGHT_TEXT}
diff --git a/routes/content_editor.py b/routes/content_editor.py
index bddff71..62c0598 100644
--- a/routes/content_editor.py
+++ b/routes/content_editor.py
@@ -40,7 +40,11 @@ content_editor_bp = Blueprint("content_editor", __name__, url_prefix="/content")
 _CONTENT_DIR_ABS = os.path.abspath(CONTENT_DIR)
 _ASSETS_DIR_ABS = os.path.abspath(ASSETS_DIR)
 _ALLOWED_IMAGE_EXT = {".png", ".jpg", ".jpeg", ".gif", ".webp", ".svg"}
-_MAX_ASSET_BYTES = 5 * 1024 * 1024
+_MAX_ASSET_BYTES = 5 * 1024 * 1024  # 5 MiB — batas upload per asset.
+# SINKRONISASI: frontend hook (frontend/src/hooks.server.ts) mengembalikan
+# pesan 413 "maksimum 5 MiB per asset", dan podman-compose.yml menetapkan
+# BODY_SIZE_LIMIT=8M (5 MiB × 4/3 base64 + envelope). Ubah nilai di sini →
+# perbarui keduanya.
 _PROTECTED_BASENAMES = {"home.md", "sub-home.md"}
 
 
diff --git a/routes/lessons.py b/routes/lessons.py
index b098fe3..b49ad21 100644
--- a/routes/lessons.py
+++ b/routes/lessons.py
@@ -294,4 +294,11 @@ def get_key_text(filename):
 @lessons_bp.route('/assets/<path:path>')
 def send_assets(path):
     """Serve asset files (images, etc.) from ASSETS_DIR."""
-    return send_from_directory(ASSETS_DIR, path)
+    response = send_from_directory(ASSETS_DIR, path)
+    # Mitigasi stored-XSS: SVG bisa memuat <script>. Route ini publik
+    # (same-origin) — kalau SVG dibuka langsung di tab browser, script-nya
+    # bisa membaca localStorage (token siswa/guru). CSP ketat mematikan
+    # semua eksekusi di dokumen tersebut; sandbox melarang navigasi/form.
+    if path.lower().endswith(".svg"):
+        response.headers["Content-Security-Policy"] = "default-src 'none'; sandbox"
+    return response
diff --git a/scripts/verify_asset_upload.py b/scripts/verify_asset_upload.py
new file mode 100644
index 0000000..facf69a
--- /dev/null
+++ b/scripts/verify_asset_upload.py
@@ -0,0 +1,219 @@
+#!/usr/bin/env python3
+"""E2E: upload asset asli (../mr-edu/assets/ilustrasi_beban.jpeg) via UI.
+
+Verifikasi fix "Backend tidak tersedia" (502) saat upload:
+ 1. Upload file yang ASLINYA gagal (710 KB, body JSON-base64 ~947 KB) via UI.
+ 2. POST /api/content/assets/upload → 200, entri di tree, download bytes-cocok.
+ 3. Jalur 413: body > 8M → 413 + pesan ukuran (bukan 502).
+ 4. Cleanup probe (entri asli ilustrasi_beban.jpeg DIPERTAHANKAN).
+
+Keamanan data user:
+ - Bila file dengan nama sama SUDAH ada di server:
+     • identik dengan source  → delete via API lalu re-upload via UI (bukti fix,
+       nama persis sama; file identik jadi tak ada data yang hilang);
+     • berbeda  → upload dengan nama probe (timestamp), entri asli tidak disentuh.
+
+Run: /home/a2nr/lms-dev/venv/bin/python scripts/verify_asset_upload.py
+"""
+import base64
+import json
+import os
+import random
+import shutil
+import sys
+import time
+import urllib.request
+
+BASE = "https://sinau-c-dev.manakin-gentoo.ts.net"
+TOKEN = "141214"
+SRC = "/home/a2nr/mr-edu/assets/ilustrasi_beban.jpeg"
+ORIGINAL_NAME = "ilustrasi_beban.jpeg"
+
+from playwright.sync_api import sync_playwright
+
+
+def login(page) -> None:
+    """Login flow via POST /api/login (paling andal, tak kena race auth.init)."""
+    page.goto(BASE + "/login", wait_until="domcontentloaded", timeout=60000)
+    page.wait_for_timeout(2000)
+    st = page.evaluate("""(token) => fetch('/api/login', {
+        method: 'POST', headers: {'Content-Type': 'application/json'},
+        body: JSON.stringify({token})
+    }).then(x => x.status)""", TOKEN)
+    assert st == 200, f"login gagal (status {st}) — cek token di DB"
+
+
+def wait_editor_ready(page) -> None:
+    """Tunggu tree sidebar ter-render (tab Materi/Assets) — bukan networkidle
+    (sering tak tercapai: long-poll/keep-alive, container baru warm-up)."""
+    page.goto(BASE + "/teacher/content", wait_until="domcontentloaded", timeout=90000)
+    page.locator('button.tree-tab', has_text='Assets').first.wait_for(state="visible", timeout=45000)
+
+
+def fetch_tree(page):
+    return page.evaluate("""() => fetch('/api/content/tree?root=assets',
+        {credentials:'include'}).then(x => x.json())""")
+
+
+def find_asset(nodes: list, name: str):
+    """Cari node file bernama `name` di list tree (rekursif masuk children)."""
+    for n in nodes:
+        is_file = (n.get("type") == "file") or (n.get("is_dir") is False)
+        if is_file and n.get("name") == name:
+            return n
+        if n.get("children"):
+            r = find_asset(n["children"], name)
+            if r:
+                return r
+    return None
+
+
+def list_files(nodes: list, prefix: str = "") -> list[str]:
+    """Semua path file di tree (rekursif)."""
+    out = []
+    for n in nodes:
+        p = f"{prefix}/{n['name']}" if prefix else n["name"]
+        if (n.get("type") == "file") or (n.get("is_dir") is False):
+            out.append(p)
+        if n.get("children"):
+            out.extend(list_files(n["children"], p))
+    return out
+
+
+def delete_entry(page, path: str) -> None:
+    page.evaluate("""(a) => fetch(`/api/content/tree/entry?root=assets&path=${encodeURIComponent(a)}&force=true`,
+        {method:'DELETE', credentials:'include'})""", path)
+
+
+def upload_via_ui(page, local_file: str) -> None:
+    """Upload via UI: tab Assets → ⬆️ → file chooser → prompt folder ''.
+    Tombol ⬆️ hanya render saat tab Assets aktif ({#if} di +page.svelte)."""
+    page.locator('button.tree-tab', has_text='Assets').first.click()
+    page.locator('button[title="Upload gambar"]').first.wait_for(state="visible", timeout=15000)
+    # Dialog handler HARUS didaftarkan sebelum click (skill pitfall):
+    #   prompt "Folder tujuan" → accept("") = root assets
+    #   confirm "Salin path ke clipboard?" → dismiss
+    page.on("dialog", lambda d: d.accept("") if "Folder tujuan" in (d.message or "") else d.dismiss())
+    with page.expect_file_chooser(timeout=10000) as fc:
+        page.locator('button[title="Upload gambar"]').first.click()
+    fc.value.set_files(local_file)
+    page.wait_for_timeout(8000)  # upload base64 ~950 KB + refresh tree
+
+
+def test_413_path(page) -> None:
+    """Body > 8M (file 8 MiB) harus 413 + pesan ukuran, bukan 502."""
+    b64 = base64.b64encode(random.Random(1).randbytes(8 * 1024 * 1024)).decode()
+    res = page.evaluate("""(b64) => fetch('/api/content/assets/upload', {
+        method:'POST', headers:{'Content-Type':'application/json'},
+        body: JSON.stringify({filename:'huge.png', folder:'', content_base64:b64})
+    }).then(async x => ({status: x.status, body: await x.json()}))""", b64)
+    assert res["status"] == 413, f"413 path: dapat {res['status']}: {res['body']}"
+    assert "terlalu besar" in res["body"].get("message", "").lower(), \
+        f"413 path: pesan salah: {res['body']}"
+    print(f"413 path: status={res['status']}, message='{res['body']['message']}' ✓")
+
+
+def main() -> None:
+    if not os.path.isfile(SRC):
+        sys.exit(f"File sumber tidak ada: {SRC}")
+    with open(SRC, "rb") as f:
+        src_raw = f.read()
+    print(f"source: {SRC} ({len(src_raw)} bytes, body ~{len(src_raw) * 4 // 3 // 1024} KB)")
+
+    errors: list[str] = []
+    api_events: list[tuple[str, int, str]] = []
+    upload_name = ORIGINAL_NAME
+    local_file = SRC
+    deleted_existing = False
+
+    with sync_playwright() as p:
+        browser = p.chromium.launch(headless=True)
+        ctx = browser.new_context(
+            ignore_https_errors=True,
+            service_workers="block",
+            viewport={"width": 1280, "height": 800},
+        )
+        ctx.add_cookies([
+            {"name": "student_token", "value": TOKEN,
+             "domain": "sinau-c-dev.manakin-gentoo.ts.net", "path": "/",
+             "http_only": True, "same_site": "Lax"}
+        ])
+        ctx.add_init_script(
+            "localStorage.setItem('student_token','141214');"
+            "localStorage.setItem('student_last_active','" + str(int(time.time() * 1000)) + "');"
+        )
+        page = ctx.new_page()
+        page.on("pageerror", lambda e: errors.append(f"pageerror: {e}"))
+        page.on("response", lambda r: api_events.append(
+            (r.request.method, r.status, r.url)) if "/api/" in r.url else None)
+
+        login(page)
+        wait_editor_ready(page)
+
+        # ── 0) Cek state awal: entri dengan nama sama sudah ada? ──
+        existing = find_asset(fetch_tree(page).get("tree", []), ORIGINAL_NAME)
+        if existing:
+            req = urllib.request.Request(f"{BASE}/assets/{existing['path']}")
+            with urllib.request.urlopen(req) as resp:
+                existing_raw = resp.read()
+            if existing_raw == src_raw:
+                # Identik → delete + re-upload (bukti fix, nama persis sama)
+                print(f"entri sama (identik) sudah ada di server — delete + re-upload {ORIGINAL_NAME}")
+                delete_entry(page, existing["path"])
+                page.wait_for_timeout(1000)
+                deleted_existing = True
+            else:
+                # Berbeda → jangan timpa; pakai nama probe
+                upload_name = f"beban_probe_{int(time.time())}.jpeg"
+                local_file = f"/tmp/{upload_name}"
+                shutil.copy(SRC, local_file)
+                print(f"entri berbeda sudah ada di server — upload sebagai {upload_name} (asli tak disentuh)")
+
+        # ── 1) Upload via UI ──
+        upload_via_ui(page, local_file)
+        upload_evt = [(m, s, u) for m, s, u in api_events if "assets/upload" in u]
+        print("upload events:", upload_evt)
+        assert upload_evt and upload_evt[-1][1] == 200, \
+            f"upload tidak sukses: {upload_evt or 'tidak ada POST assets/upload'}"
+
+        # ── 2) Tree memuat entri; download bytes-cocok ──
+        node = find_asset(fetch_tree(page).get("tree", []), upload_name)
+        assert node, f"entri {upload_name} tidak ada di tree assets"
+        print(f"tree: entri {upload_name} ada ✓")
+        with urllib.request.urlopen(f"{BASE}/assets/{node['path']}") as resp:
+            data = resp.read()
+            assert resp.status == 200 and data == src_raw, \
+                f"download mismatch: {resp.status}, {len(data)} vs {len(src_raw)} bytes"
+        print(f"download /assets/{node['path']}: 200, {len(data)} bytes cocok ✓")
+
+        # ── 3) Jalur 413 (body > 8M) ──
+        test_413_path(page)
+
+        # ── 4) Cleanup: semua probe beban_probe_* (artefak kita), entri asli tetap ──
+        stale = [p_ for p_ in list_files(fetch_tree(page).get("tree", [])) if p_.startswith("beban_probe")]
+        for s in stale:
+            delete_entry(page, s)
+        page.wait_for_timeout(1000)
+        final_files = list_files(fetch_tree(page).get("tree", []))
+        left = [f for f in final_files if f.startswith("beban_probe")]
+        assert not left, f"cleanup gagal — probe tersisa: {left}"
+        print(f"cleanup {len(stale)} probe ✓; entri akhir: {len(final_files)} file")
+        if upload_name == ORIGINAL_NAME:
+            assert ORIGINAL_NAME in final_files, "entri asli hilang setelah cleanup!"
+            print(f"entri asli {ORIGINAL_NAME} tersimpan di server ✓")
+
+        errs = [e for e in errors if "favicon" not in e.lower()]
+        if errs:
+            print("PAGE ERRORS:")
+            for e in errs[:10]:
+                print("  ", e)
+            sys.exit(1)
+        print("pageerrors: bersih ✓")
+        browser.close()
+
+    print(f"\nSUCCESS: upload {upload_name} ({len(src_raw)} B, body ~{len(src_raw) * 4 // 3 // 1024} KB) "
+          f"lolos proxy tanpa 502; 413 path jelas; data user utuh.")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/services/tests/test_content_editor_routes.py b/services/tests/test_content_editor_routes.py
index 89fec25..1265d54 100644
--- a/services/tests/test_content_editor_routes.py
+++ b/services/tests/test_content_editor_routes.py
@@ -670,3 +670,67 @@ def test_export_scope_content_excludes_assets(client):
     names = zipfile.ZipFile(io.BytesIO(resp.data)).namelist()
     assert any(n.startswith("content/") for n in names)
     assert not any(n.startswith("assets/") for n in names)
+# ── Boundary 5 MiB (cabang len(raw) — bukan pre-check len(b64)) ──────
+# SVG dipakai karena dikecualikan dari validasi gambar PIL (Pillow tidak
+# parse SVG), sehingga ukuran murni yang diuji, bukan validasi isi.
+# Blok ini self-contained (helper sendiri) agar tidak bergantung helper
+# lain di file ini.
+
+import base64 as _b64_mod  # noqa: E402  (import lokal — blok ini standalone)
+
+_SVG_HDR = b'<svg xmlns="http://www.w3.org/2000/svg" width="1" height="1"/>'
+
+
+def _svg_of(raw_size: int) -> bytes:
+    return _SVG_HDR + b" " * (raw_size - len(_SVG_HDR))
+
+
+def _post_upload_plain(client, filename, raw):
+    """POST upload tanpa flag overwrite (blok ini standalone — tidak
+    memakai helper _upload di atas agar commit-nya bisa berdiri sendiri)."""
+    return client.post("/content/assets/upload", json={
+        "filename": filename,
+        "folder": "",
+        "content_base64": _b64_mod.b64encode(raw).decode(),
+    })
+
+
+def test_upload_exactly_5mib_accepted(client):
+    """File TEPAT _MAX_ASSET_BYTES (5 MiB) harus lolos — bukan ditolak."""
+    _seed_and_setup()
+    _login_teacher(client)
+    resp = _post_upload_plain(client, "exact5mib.svg", _svg_of(5 * 1024 * 1024))
+    assert resp.status_code == 200, resp.get_json()
+    assert resp.get_json()["success"] is True
+
+
+def test_upload_5mib_plus_1_rejected(client):
+    """5 MiB + 1 byte harus ditolak cabang len(raw) dengan pesan ukuran."""
+    _seed_and_setup()
+    _login_teacher(client)
+    resp = _post_upload_plain(client, "over5mib.svg", _svg_of(5 * 1024 * 1024 + 1))
+    assert resp.status_code == 400
+    assert "mib" in resp.get_json()["message"].lower()
+
+
+# ── CSP saat serve SVG (mitigasi stored-XSS via SVG ber-script) ──────
+
+
+def test_serve_svg_has_restrictive_csp(client):
+    """SVG diserve dengan CSP ketat — <script> di dalamnya tak bisa jalan
+    saat SVG dibuka langsung di tab browser (same-origin)."""
+    (TEST_ASSETS / "malware.svg").write_bytes(
+        b'<svg xmlns="http://www.w3.org/2000/svg"><script>fetch("/api/login")</script></svg>'
+    )
+    resp = client.get("/assets/malware.svg")
+    assert resp.status_code == 200
+    assert resp.headers.get("Content-Security-Policy") == "default-src 'none'; sandbox"
+
+
+def test_serve_raster_image_no_csp(client):
+    """Gambar raster (PNG) tidak diberi CSP — hanya SVG yang berisiko."""
+    (TEST_ASSETS / "normal.png").write_bytes(b"\x89PNG\r\n\x1a\n")
+    resp = client.get("/assets/normal.png")
+    assert resp.status_code == 200
+    assert "Content-Security-Policy" not in resp.headers
+