diff --git a/.claude/settings.local.json b/.claude/settings.local.json
index 2c2a900..0b1b8a4 100644
--- a/.claude/settings.local.json
+++ b/.claude/settings.local.json
@@ -23,7 +23,8 @@
       "Bash(curl:*)",
       "Bash(npx lighthouse:*)",
       "Bash(echo \"exit:$?\")",
-      "Bash(python -c \"from config import get_settings; s = get_settings\\(\\); print\\('SA JSON set:', bool\\(s.firebase_service_account_json\\)\\)\")"
+      "Bash(python -c \"from config import get_settings; s = get_settings\\(\\); print\\('SA JSON set:', bool\\(s.firebase_service_account_json\\)\\)\")",
+      "Bash(python3 -c ' *)"
     ]
   }
 }
diff --git a/backend/auth.py b/backend/auth.py
new file mode 100644
index 0000000..eea6f46
--- /dev/null
+++ b/backend/auth.py
@@ -0,0 +1,47 @@
+"""Firebase token verification and ownership helpers."""
+import logging
+from fastapi import HTTPException, Header
+from firebase_admin import auth as firebase_auth
+import firebase_admin
+from bson import ObjectId
+from bson.errors import InvalidId
+
+log = logging.getLogger(__name__)
+
+
+async def get_current_user(authorization: str = Header(..., alias="Authorization")) -> dict:
+    """FastAPI dependency: verifies Firebase ID token and returns decoded payload."""
+    if not authorization.startswith("Bearer "):
+        raise HTTPException(status_code=401, detail="Invalid authorization header")
+
+    token = authorization[len("Bearer "):]
+
+    if not firebase_admin._apps:
+        raise HTTPException(status_code=503, detail="Authentication service unavailable")
+
+    try:
+        return firebase_auth.verify_id_token(token)
+    except firebase_auth.ExpiredIdTokenError:
+        raise HTTPException(status_code=401, detail="Token expired")
+    except Exception:
+        raise HTTPException(status_code=401, detail="Invalid token")
+
+
+def verify_user_access(user_id: str, db, token: dict) -> dict:
+    """
+    Fetch user by ObjectId and confirm the token owner matches.
+    Returns the user document. Raises 400/404/403 on failure.
+    """
+    try:
+        user_oid = ObjectId(user_id)
+    except InvalidId:
+        raise HTTPException(status_code=400, detail="Invalid user ID format")
+
+    user = db.users.find_one({"_id": user_oid})
+    if not user:
+        raise HTTPException(status_code=404, detail="User not found")
+
+    if user.get("email") != token.get("email"):
+        raise HTTPException(status_code=403, detail="Access denied")
+
+    return user
diff --git a/backend/main.py b/backend/main.py
index 40cef81..92c576f 100644
--- a/backend/main.py
+++ b/backend/main.py
@@ -52,7 +52,7 @@ app.add_middleware(
     allow_origins=cors_origins,
     allow_credentials=True,
     allow_methods=["GET", "POST", "PUT", "DELETE", "OPTIONS"],
-    allow_headers=["*"],
+    allow_headers=["Authorization", "Content-Type"],
 )
 
 # Include routers
diff --git a/backend/routers/entries.py b/backend/routers/entries.py
index 0e5978d..651794e 100644
--- a/backend/routers/entries.py
+++ b/backend/routers/entries.py
@@ -1,13 +1,13 @@
 """Journal entry routes"""
-from fastapi import APIRouter, HTTPException, Query
+import logging
+from fastapi import APIRouter, HTTPException, Query, Depends
 from db import get_database
-from models import JournalEntryCreate, JournalEntryUpdate, JournalEntry, EntriesListResponse, PaginationMeta
+from models import JournalEntryCreate, JournalEntryUpdate
 from datetime import datetime, timedelta
-from typing import List, Optional
-from bson import ObjectId
-from bson.errors import InvalidId
+from auth import get_current_user, verify_user_access
 from utils import format_ist_timestamp
 
+log = logging.getLogger(__name__)
 router = APIRouter()
 
 
@@ -16,21 +16,20 @@ def _format_entry(entry: dict) -> dict:
     return {
         "id": str(entry["_id"]),
         "userId": str(entry["userId"]),
-        "title": entry.get("title"),  # None if encrypted
-        "content": entry.get("content"),  # None if encrypted
+        "title": entry.get("title"),
+        "content": entry.get("content"),
         "mood": entry.get("mood"),
         "tags": entry.get("tags", []),
         "isPublic": entry.get("isPublic", False),
         "entryDate": entry.get("entryDate", entry.get("createdAt")).isoformat() if entry.get("entryDate") or entry.get("createdAt") else None,
         "createdAt": entry["createdAt"].isoformat(),
         "updatedAt": entry["updatedAt"].isoformat(),
-        # Full encryption metadata including ciphertext and nonce
         "encryption": entry.get("encryption")
     }
 
 
 @router.post("/{user_id}", response_model=dict)
-async def create_entry(user_id: str, entry_data: JournalEntryCreate):
+async def create_entry(user_id: str, entry_data: JournalEntryCreate, token: dict = Depends(get_current_user)):
     """
     Create a new journal entry.
 
@@ -38,33 +37,18 @@ async def create_entry(user_id: str, entry_data: JournalEntryCreate):
     - Send encryption metadata with ciphertext and nonce
     - Omit title and content (they're encrypted in ciphertext)
 
-    For unencrypted entries (deprecated):
-    - Send title and content directly
-
     entryDate: The logical journal date for this entry (defaults to today UTC).
-    createdAt: Database write timestamp.
-
     Server stores only: encrypted ciphertext, nonce, and metadata.
     Server never sees plaintext.
     """
     db = get_database()
-
     try:
-        user_oid = ObjectId(user_id)
-    except InvalidId:
-        raise HTTPException(status_code=400, detail="Invalid user ID format")
-
-    try:
-        # Verify user exists
-        user = db.users.find_one({"_id": user_oid})
-        if not user:
-            raise HTTPException(status_code=404, detail="User not found")
+        user = verify_user_access(user_id, db, token)
+        user_oid = user["_id"]
 
         now = datetime.utcnow()
-        entry_date = entry_data.entryDate or now.replace(
-            hour=0, minute=0, second=0, microsecond=0)
+        entry_date = entry_data.entryDate or now.replace(hour=0, minute=0, second=0, microsecond=0)
 
-        # Validate encryption metadata if present
         if entry_data.encryption:
             if not entry_data.encryption.ciphertext or not entry_data.encryption.nonce:
                 raise HTTPException(
@@ -74,12 +58,12 @@ async def create_entry(user_id: str, entry_data: JournalEntryCreate):
 
         entry_doc = {
             "userId": user_oid,
-            "title": entry_data.title,  # None if encrypted
-            "content": entry_data.content,  # None if encrypted
+            "title": entry_data.title,
+            "content": entry_data.content,
             "mood": entry_data.mood,
             "tags": entry_data.tags or [],
             "isPublic": entry_data.isPublic or False,
-            "entryDate": entry_date,  # Logical journal date
+            "entryDate": entry_date,
             "createdAt": now,
             "updatedAt": now,
             "encryption": entry_data.encryption.model_dump() if entry_data.encryption else None
@@ -94,48 +78,29 @@ async def create_entry(user_id: str, entry_data: JournalEntryCreate):
         }
     except HTTPException:
         raise
-    except Exception as e:
-        raise HTTPException(
-            status_code=500, detail=f"Failed to create entry: {str(e)}")
+    except Exception:
+        log.exception("Failed to create entry")
+        raise HTTPException(status_code=500, detail="Internal server error")
 
 
 @router.get("/{user_id}")
 async def get_user_entries(
     user_id: str,
     limit: int = Query(50, ge=1, le=100),
-    skip: int = Query(0, ge=0)
+    skip: int = Query(0, ge=0),
+    token: dict = Depends(get_current_user)
 ):
-    """
-    Get paginated entries for a user (most recent first).
-
-    Supports pagination via skip and limit.
-    """
+    """Get paginated entries for a user (most recent first)."""
     db = get_database()
-
     try:
-        user_oid = ObjectId(user_id)
-    except InvalidId:
-        raise HTTPException(status_code=400, detail="Invalid user ID format")
+        user = verify_user_access(user_id, db, token)
+        user_oid = user["_id"]
 
-    try:
-        # Verify user exists
-        user = db.users.find_one({"_id": user_oid})
-        if not user:
-            raise HTTPException(status_code=404, detail="User not found")
-
-        # Get entries
         entries = list(
-            db.entries.find(
-                {"userId": user_oid}
-            ).sort("createdAt", -1).skip(skip).limit(limit)
+            db.entries.find({"userId": user_oid}).sort("createdAt", -1).skip(skip).limit(limit)
         )
-
-        # Format entries
         formatted_entries = [_format_entry(entry) for entry in entries]
-
-        # Get total count
         total = db.entries.count_documents({"userId": user_oid})
-        has_more = (skip + limit) < total
 
         return {
             "entries": formatted_entries,
@@ -143,101 +108,95 @@ async def get_user_entries(
                 "total": total,
                 "limit": limit,
                 "skip": skip,
-                "hasMore": has_more
+                "hasMore": (skip + limit) < total
             }
         }
     except HTTPException:
         raise
-    except Exception as e:
-        raise HTTPException(
-            status_code=500, detail=f"Failed to fetch entries: {str(e)}")
+    except Exception:
+        log.exception("Failed to fetch entries")
+        raise HTTPException(status_code=500, detail="Internal server error")
 
 
 @router.get("/{user_id}/{entry_id}")
-async def get_entry(user_id: str, entry_id: str):
+async def get_entry(user_id: str, entry_id: str, token: dict = Depends(get_current_user)):
     """Get a specific entry by ID."""
+    from bson import ObjectId
+    from bson.errors import InvalidId
     db = get_database()
-
     try:
-        user_oid = ObjectId(user_id)
-        entry_oid = ObjectId(entry_id)
-    except InvalidId:
-        raise HTTPException(status_code=400, detail="Invalid ID format")
-
-    try:
-        entry = db.entries.find_one({
-            "_id": entry_oid,
-            "userId": user_oid
-        })
+        user = verify_user_access(user_id, db, token)
+        user_oid = user["_id"]
+        try:
+            entry_oid = ObjectId(entry_id)
+        except InvalidId:
+            raise HTTPException(status_code=400, detail="Invalid entry ID format")
 
+        entry = db.entries.find_one({"_id": entry_oid, "userId": user_oid})
         if not entry:
             raise HTTPException(status_code=404, detail="Entry not found")
 
         return _format_entry(entry)
     except HTTPException:
         raise
-    except Exception as e:
-        raise HTTPException(
-            status_code=500, detail=f"Failed to fetch entry: {str(e)}")
+    except Exception:
+        log.exception("Failed to fetch entry")
+        raise HTTPException(status_code=500, detail="Internal server error")
 
 
 @router.put("/{user_id}/{entry_id}")
-async def update_entry(user_id: str, entry_id: str, entry_data: JournalEntryUpdate):
+async def update_entry(user_id: str, entry_id: str, entry_data: JournalEntryUpdate, token: dict = Depends(get_current_user)):
     """Update a journal entry."""
+    from bson import ObjectId
+    from bson.errors import InvalidId
     db = get_database()
-
     try:
-        user_oid = ObjectId(user_id)
-        entry_oid = ObjectId(entry_id)
-    except InvalidId:
-        raise HTTPException(status_code=400, detail="Invalid ID format")
+        user = verify_user_access(user_id, db, token)
+        user_oid = user["_id"]
+        try:
+            entry_oid = ObjectId(entry_id)
+        except InvalidId:
+            raise HTTPException(status_code=400, detail="Invalid entry ID format")
 
-    try:
         update_data = entry_data.model_dump(exclude_unset=True)
         update_data["updatedAt"] = datetime.utcnow()
 
-        # If entryDate provided in update data, ensure it's a datetime
         if "entryDate" in update_data and isinstance(update_data["entryDate"], str):
             update_data["entryDate"] = datetime.fromisoformat(
                 update_data["entryDate"].replace("Z", "+00:00"))
 
         result = db.entries.update_one(
-            {
-                "_id": entry_oid,
-                "userId": user_oid
-            },
+            {"_id": entry_oid, "userId": user_oid},
             {"$set": update_data}
         )
 
         if result.matched_count == 0:
             raise HTTPException(status_code=404, detail="Entry not found")
 
-        # Fetch and return updated entry
         entry = db.entries.find_one({"_id": entry_oid})
         return _format_entry(entry)
     except HTTPException:
         raise
-    except Exception as e:
-        raise HTTPException(
-            status_code=500, detail=f"Failed to update entry: {str(e)}")
+    except Exception:
+        log.exception("Failed to update entry")
+        raise HTTPException(status_code=500, detail="Internal server error")
 
 
 @router.delete("/{user_id}/{entry_id}")
-async def delete_entry(user_id: str, entry_id: str):
+async def delete_entry(user_id: str, entry_id: str, token: dict = Depends(get_current_user)):
     """Delete a journal entry."""
+    from bson import ObjectId
+    from bson.errors import InvalidId
     db = get_database()
-
     try:
-        user_oid = ObjectId(user_id)
-        entry_oid = ObjectId(entry_id)
-    except InvalidId:
-        raise HTTPException(status_code=400, detail="Invalid ID format")
+        user = verify_user_access(user_id, db, token)
+        user_oid = user["_id"]
+        try:
+            entry_oid = ObjectId(entry_id)
+        except InvalidId:
+            raise HTTPException(status_code=400, detail="Invalid entry ID format")
 
-    try:
-        result = db.entries.delete_one({
-            "_id": entry_oid,
-            "userId": user_oid
-        })
+        result = db.entries.delete_one({"_id": entry_oid, "userId": user_oid})
 
         if result.deleted_count == 0:
             raise HTTPException(status_code=404, detail="Entry not found")
@@ -245,108 +204,83 @@ async def delete_entry(user_id: str, entry_id: str):
         return {"message": "Entry deleted successfully"}
     except HTTPException:
         raise
-    except Exception as e:
-        raise HTTPException(
-            status_code=500, detail=f"Failed to delete entry: {str(e)}")
+    except Exception:
+        log.exception("Failed to delete entry")
+        raise HTTPException(status_code=500, detail="Internal server error")
 
 
 @router.get("/{user_id}/by-date/{date_str}")
-async def get_entries_by_date(user_id: str, date_str: str):
-    """
-    Get entries for a specific date (format: YYYY-MM-DD).
-
-    Matches entries by entryDate field.
-    """
+async def get_entries_by_date(user_id: str, date_str: str, token: dict = Depends(get_current_user)):
+    """Get entries for a specific date (format: YYYY-MM-DD)."""
     db = get_database()
-
     try:
-        user_oid = ObjectId(user_id)
-    except InvalidId:
-        raise HTTPException(status_code=400, detail="Invalid user ID format")
+        user = verify_user_access(user_id, db, token)
+        user_oid = user["_id"]
+
+        try:
+            target_date = datetime.strptime(date_str, "%Y-%m-%d")
+        except ValueError:
+            raise HTTPException(status_code=400, detail="Invalid date format. Use YYYY-MM-DD")
 
-    try:
-        # Parse date
-        target_date = datetime.strptime(date_str, "%Y-%m-%d")
         next_date = target_date + timedelta(days=1)
 
         entries = list(
             db.entries.find({
                 "userId": user_oid,
-                "entryDate": {
-                    "$gte": target_date,
-                    "$lt": next_date
-                }
+                "entryDate": {"$gte": target_date, "$lt": next_date}
             }).sort("createdAt", -1)
         )
 
-        formatted_entries = [_format_entry(entry) for entry in entries]
-
         return {
-            "entries": formatted_entries,
+            "entries": [_format_entry(e) for e in entries],
             "date": date_str,
-            "count": len(formatted_entries)
+            "count": len(entries)
         }
-    except ValueError:
-        raise HTTPException(
-            status_code=400, detail="Invalid date format. Use YYYY-MM-DD")
     except HTTPException:
         raise
-    except Exception as e:
-        raise HTTPException(
-            status_code=500, detail=f"Failed to fetch entries: {str(e)}")
+    except Exception:
+        log.exception("Failed to fetch entries by date")
+        raise HTTPException(status_code=500, detail="Internal server error")
 
 
 @router.get("/{user_id}/by-month/{year}/{month}")
-async def get_entries_by_month(user_id: str, year: int, month: int, limit: int = Query(100, ge=1, le=500)):
-    """
-    Get entries for a specific month (for calendar view).
-
-    Query format: GET /api/entries/{user_id}/by-month/{year}/{month}?limit=100
-    """
+async def get_entries_by_month(
+    user_id: str,
+    year: int,
+    month: int,
+    limit: int = Query(100, ge=1, le=500),
+    token: dict = Depends(get_current_user)
+):
+    """Get entries for a specific month (for calendar view)."""
     db = get_database()
-
     try:
-        user_oid = ObjectId(user_id)
-    except InvalidId:
-        raise HTTPException(status_code=400, detail="Invalid user ID format")
+        user = verify_user_access(user_id, db, token)
+        user_oid = user["_id"]
 
-    try:
         if not (1 <= month <= 12):
-            raise HTTPException(
-                status_code=400, detail="Month must be between 1 and 12")
+            raise HTTPException(status_code=400, detail="Month must be between 1 and 12")
 
-        # Calculate date range
         start_date = datetime(year, month, 1)
-        if month == 12:
-            end_date = datetime(year + 1, 1, 1)
-        else:
-            end_date = datetime(year, month + 1, 1)
+        end_date = datetime(year + 1, 1, 1) if month == 12 else datetime(year, month + 1, 1)
 
         entries = list(
             db.entries.find({
                 "userId": user_oid,
-                "entryDate": {
-                    "$gte": start_date,
-                    "$lt": end_date
-                }
+                "entryDate": {"$gte": start_date, "$lt": end_date}
             }).sort("entryDate", -1).limit(limit)
         )
 
-        formatted_entries = [_format_entry(entry) for entry in entries]
-
         return {
-            "entries": formatted_entries,
+            "entries": [_format_entry(e) for e in entries],
             "year": year,
             "month": month,
-            "count": len(formatted_entries)
+            "count": len(entries)
         }
-    except ValueError:
-        raise HTTPException(status_code=400, detail="Invalid year or month")
     except HTTPException:
         raise
-    except Exception as e:
-        raise HTTPException(
-            status_code=500, detail=f"Failed to fetch entries: {str(e)}")
+    except Exception:
+        log.exception("Failed to fetch entries by month")
+        raise HTTPException(status_code=500, detail="Internal server error")
 
 
 @router.post("/convert-timestamp/utc-to-ist")
@@ -355,18 +289,14 @@ async def convert_utc_to_ist(data: dict):
     try:
         utc_timestamp = data.get("timestamp")
         if not utc_timestamp:
-            raise HTTPException(
-                status_code=400, detail="Missing 'timestamp' field")
+            raise HTTPException(status_code=400, detail="Missing 'timestamp' field")
 
         ist_timestamp = format_ist_timestamp(utc_timestamp)
-        return {
-            "utc": utc_timestamp,
-            "ist": ist_timestamp
-        }
+        return {"utc": utc_timestamp, "ist": ist_timestamp}
     except HTTPException:
         raise
     except ValueError as e:
         raise HTTPException(status_code=400, detail=str(e))
-    except Exception as e:
-        raise HTTPException(
-            status_code=500, detail=f"Conversion failed: {str(e)}")
+    except Exception:
+        log.exception("Timestamp conversion failed")
+        raise HTTPException(status_code=500, detail="Internal server error")
diff --git a/backend/routers/notifications.py b/backend/routers/notifications.py
index 9b2a2c3..1b26168 100644
--- a/backend/routers/notifications.py
+++ b/backend/routers/notifications.py
@@ -1,12 +1,13 @@
 """Notification routes — FCM token registration and reminder settings."""
-from fastapi import APIRouter, HTTPException
+import logging
+from fastapi import APIRouter, HTTPException, Depends
 from db import get_database
 from pydantic import BaseModel
 from typing import Optional
-from bson import ObjectId
-from bson.errors import InvalidId
 from datetime import datetime
+from auth import get_current_user, verify_user_access
 
+log = logging.getLogger(__name__)
 router = APIRouter()
 
 
@@ -22,57 +23,52 @@ class ReminderSettingsRequest(BaseModel):
 
 
 @router.post("/fcm-token", response_model=dict)
-async def register_fcm_token(body: FcmTokenRequest):
+async def register_fcm_token(body: FcmTokenRequest, token: dict = Depends(get_current_user)):
     """
     Register (or refresh) an FCM device token for a user.
     Stores unique tokens per user — duplicate tokens are ignored.
     """
     db = get_database()
-
     try:
-        user_oid = ObjectId(body.userId)
-    except InvalidId:
-        raise HTTPException(status_code=400, detail="Invalid user ID")
+        user = verify_user_access(body.userId, db, token)
+        user_oid = user["_id"]
 
-    user = db.users.find_one({"_id": user_oid})
-    if not user:
-        raise HTTPException(status_code=404, detail="User not found")
-
-    # Add token to set (avoid duplicates)
-    db.users.update_one(
-        {"_id": user_oid},
-        {
-            "$addToSet": {"fcmTokens": body.fcmToken},
-            "$set": {"updatedAt": datetime.utcnow()},
-        }
-    )
-    return {"message": "FCM token registered"}
+        db.users.update_one(
+            {"_id": user_oid},
+            {
+                "$addToSet": {"fcmTokens": body.fcmToken},
+                "$set": {"updatedAt": datetime.utcnow()},
+            }
+        )
+        return {"message": "FCM token registered"}
+    except HTTPException:
+        raise
+    except Exception:
+        log.exception("Failed to register FCM token")
+        raise HTTPException(status_code=500, detail="Internal server error")
 
 
 @router.put("/reminder/{user_id}", response_model=dict)
-async def update_reminder(user_id: str, settings: ReminderSettingsRequest):
-    """
-    Save or update daily reminder settings for a user.
-    """
+async def update_reminder(user_id: str, settings: ReminderSettingsRequest, token: dict = Depends(get_current_user)):
+    """Save or update daily reminder settings for a user."""
     db = get_database()
-
     try:
-        user_oid = ObjectId(user_id)
-    except InvalidId:
-        raise HTTPException(status_code=400, detail="Invalid user ID")
+        user = verify_user_access(user_id, db, token)
+        user_oid = user["_id"]
 
-    user = db.users.find_one({"_id": user_oid})
-    if not user:
-        raise HTTPException(status_code=404, detail="User not found")
+        reminder_update: dict = {"reminder.enabled": settings.enabled}
+        if settings.time is not None:
+            reminder_update["reminder.time"] = settings.time
+        if settings.timezone is not None:
+            reminder_update["reminder.timezone"] = settings.timezone
 
-    reminder_update: dict = {"reminder.enabled": settings.enabled}
-    if settings.time is not None:
-        reminder_update["reminder.time"] = settings.time
-    if settings.timezone is not None:
-        reminder_update["reminder.timezone"] = settings.timezone
-
-    db.users.update_one(
-        {"_id": user_oid},
-        {"$set": {**reminder_update, "updatedAt": datetime.utcnow()}}
-    )
-    return {"message": "Reminder settings updated"}
+        db.users.update_one(
+            {"_id": user_oid},
+            {"$set": {**reminder_update, "updatedAt": datetime.utcnow()}}
+        )
+        return {"message": "Reminder settings updated"}
+    except HTTPException:
+        raise
+    except Exception:
+        log.exception("Failed to update reminder settings")
+        raise HTTPException(status_code=500, detail="Internal server error")
diff --git a/backend/routers/users.py b/backend/routers/users.py
index c4a77ab..f432684 100644
--- a/backend/routers/users.py
+++ b/backend/routers/users.py
@@ -1,17 +1,17 @@
 """User management routes"""
-from fastapi import APIRouter, HTTPException
+import logging
+from fastapi import APIRouter, HTTPException, Depends
 from db import get_database
-from models import UserCreate, UserUpdate, User
+from models import UserCreate, UserUpdate
 from datetime import datetime
-from typing import Optional
-from bson import ObjectId
-from bson.errors import InvalidId
+from auth import get_current_user, verify_user_access
 
+log = logging.getLogger(__name__)
 router = APIRouter()
 
 
 @router.post("/register", response_model=dict)
-async def register_user(user_data: UserCreate):
+async def register_user(user_data: UserCreate, token: dict = Depends(get_current_user)):
     """
     Register or get user (idempotent).
 
@@ -19,10 +19,11 @@ async def register_user(user_data: UserCreate):
     If user already exists, returns existing user.
     Called after Firebase Google Auth on frontend.
     """
-    db = get_database()
+    if user_data.email != token.get("email"):
+        raise HTTPException(status_code=403, detail="Access denied")
 
+    db = get_database()
     try:
-        # Upsert: Update if exists, insert if not
         result = db.users.update_one(
             {"email": user_data.email},
             {
@@ -40,11 +41,9 @@ async def register_user(user_data: UserCreate):
             upsert=True
         )
 
-        # Fetch the user (either newly created or existing)
         user = db.users.find_one({"email": user_data.email})
         if not user:
-            raise HTTPException(
-                status_code=500, detail="Failed to retrieve user after upsert")
+            raise HTTPException(status_code=500, detail="Failed to retrieve user after upsert")
 
         return {
             "id": str(user["_id"]),
@@ -62,15 +61,17 @@ async def register_user(user_data: UserCreate):
     except HTTPException:
         raise
     except Exception as e:
-        raise HTTPException(
-            status_code=500, detail=f"Registration failed: {str(e)}")
+        log.exception("Registration failed")
+        raise HTTPException(status_code=500, detail="Internal server error")
 
 
 @router.get("/by-email/{email}", response_model=dict)
-async def get_user_by_email(email: str):
+async def get_user_by_email(email: str, token: dict = Depends(get_current_user)):
     """Get user profile by email (called after Firebase Auth)."""
-    db = get_database()
+    if email != token.get("email"):
+        raise HTTPException(status_code=403, detail="Access denied")
 
+    db = get_database()
     try:
         user = db.users.find_one({"email": email})
         if not user:
@@ -91,26 +92,17 @@ async def get_user_by_email(email: str):
         }
     except HTTPException:
         raise
-    except Exception as e:
-        raise HTTPException(
-            status_code=500, detail=f"Failed to fetch user: {str(e)}")
+    except Exception:
+        log.exception("Failed to fetch user by email")
+        raise HTTPException(status_code=500, detail="Internal server error")
 
 
 @router.get("/{user_id}", response_model=dict)
-async def get_user_by_id(user_id: str):
+async def get_user_by_id(user_id: str, token: dict = Depends(get_current_user)):
     """Get user profile by ID."""
     db = get_database()
-
     try:
-        user_oid = ObjectId(user_id)
-    except InvalidId:
-        raise HTTPException(status_code=400, detail="Invalid user ID format")
-
-    try:
-        user = db.users.find_one({"_id": user_oid})
-        if not user:
-            raise HTTPException(status_code=404, detail="User not found")
-
+        user = verify_user_access(user_id, db, token)
         return {
             "id": str(user["_id"]),
             "email": user["email"],
@@ -124,72 +116,54 @@ async def get_user_by_id(user_id: str):
         }
     except HTTPException:
         raise
-    except Exception as e:
-        raise HTTPException(
-            status_code=500, detail=f"Failed to fetch user: {str(e)}")
+    except Exception:
+        log.exception("Failed to fetch user by ID")
+        raise HTTPException(status_code=500, detail="Internal server error")
 
 
 @router.put("/{user_id}", response_model=dict)
-async def update_user(user_id: str, user_data: UserUpdate):
+async def update_user(user_id: str, user_data: UserUpdate, token: dict = Depends(get_current_user)):
     """Update user profile."""
     db = get_database()
-
     try:
-        user_oid = ObjectId(user_id)
-    except InvalidId:
-        raise HTTPException(status_code=400, detail="Invalid user ID format")
+        user = verify_user_access(user_id, db, token)
+        user_oid = user["_id"]
 
-    try:
-        # Prepare update data (exclude None values)
         update_data = user_data.model_dump(exclude_unset=True)
         update_data["updatedAt"] = datetime.utcnow()
 
-        result = db.users.update_one(
-            {"_id": user_oid},
-            {"$set": update_data}
-        )
+        db.users.update_one({"_id": user_oid}, {"$set": update_data})
 
-        if result.matched_count == 0:
-            raise HTTPException(status_code=404, detail="User not found")
-
-        # Fetch and return updated user
-        user = db.users.find_one({"_id": user_oid})
+        updated = db.users.find_one({"_id": user_oid})
         return {
-            "id": str(user["_id"]),
-            "email": user["email"],
-            "displayName": user.get("displayName"),
-            "photoURL": user.get("photoURL"),
-            "theme": user.get("theme", "light"),
-            "backgroundImage": user.get("backgroundImage"),
-            "backgroundImages": user.get("backgroundImages", []),
-            "tutorial": user.get("tutorial"),
-            "createdAt": user["createdAt"].isoformat(),
-            "updatedAt": user["updatedAt"].isoformat(),
+            "id": str(updated["_id"]),
+            "email": updated["email"],
+            "displayName": updated.get("displayName"),
+            "photoURL": updated.get("photoURL"),
+            "theme": updated.get("theme", "light"),
+            "backgroundImage": updated.get("backgroundImage"),
+            "backgroundImages": updated.get("backgroundImages", []),
+            "tutorial": updated.get("tutorial"),
+            "createdAt": updated["createdAt"].isoformat(),
+            "updatedAt": updated["updatedAt"].isoformat(),
             "message": "User updated successfully"
         }
     except HTTPException:
         raise
-    except Exception as e:
-        raise HTTPException(status_code=500, detail=f"Update failed: {str(e)}")
+    except Exception:
+        log.exception("User update failed")
+        raise HTTPException(status_code=500, detail="Internal server error")
 
 
 @router.delete("/{user_id}")
-async def delete_user(user_id: str):
+async def delete_user(user_id: str, token: dict = Depends(get_current_user)):
     """Delete user account and all associated data."""
     db = get_database()
-
     try:
-        user_oid = ObjectId(user_id)
-    except InvalidId:
-        raise HTTPException(status_code=400, detail="Invalid user ID format")
+        user = verify_user_access(user_id, db, token)
+        user_oid = user["_id"]
 
-    try:
-        # Delete user
         user_result = db.users.delete_one({"_id": user_oid})
-        if user_result.deleted_count == 0:
-            raise HTTPException(status_code=404, detail="User not found")
-
-        # Delete all user's entries
         entry_result = db.entries.delete_many({"userId": user_oid})
 
         return {
@@ -199,6 +173,6 @@ async def delete_user(user_id: str):
         }
     except HTTPException:
         raise
-    except Exception as e:
-        raise HTTPException(
-            status_code=500, detail=f"Deletion failed: {str(e)}")
+    except Exception:
+        log.exception("User deletion failed")
+        raise HTTPException(status_code=500, detail="Internal server error")