diff --git a/backend/app/routers/auth.py b/backend/app/routers/auth.py
index 053ed04..28a6deb 100644
--- a/backend/app/routers/auth.py
+++ b/backend/app/routers/auth.py
@@ -285,13 +285,19 @@ async def login(
     await check_account_lockout(user)
 
     if not valid:
-        await record_failed_login(db, user)
+        remaining = await record_failed_login(db, user)
         await log_audit_event(
             db, action="auth.login_failed", actor_id=user.id,
-            detail={"reason": "invalid_password"}, ip=client_ip,
+            detail={"reason": "invalid_password", "attempts_remaining": remaining}, ip=client_ip,
         )
         await db.commit()
-        raise HTTPException(status_code=401, detail="Invalid username or password")
+        if remaining == 0:
+            detail = "Account temporarily locked. Try again in 30 minutes."
+        elif remaining <= 3:
+            detail = f"Invalid username or password. {remaining} attempt(s) remaining before account locks."
+        else:
+            detail = "Invalid username or password"
+        raise HTTPException(status_code=401, detail=detail)
 
     # Block passwordless-only accounts from using the password login path.
     # Checked after password verification to avoid leaking account existence via timing.
diff --git a/backend/app/routers/passkeys.py b/backend/app/routers/passkeys.py
index 8266f6d..e026791 100644
--- a/backend/app/routers/passkeys.py
+++ b/backend/app/routers/passkeys.py
@@ -267,6 +267,16 @@ async def passkey_login_begin(
                     cid_bytes = base64url_to_bytes(row[0])
                     transports = json.loads(row[1]) if row[1] else None
                     credential_data.append((cid_bytes, transports))
+        else:
+            # F-01: User not found — run a no-op DB query to equalize timing with
+            # the credential fetch that executes for existing users.  Without this,
+            # the absence of the second query makes the "no user" path measurably
+            # faster, leaking whether the username exists.
+            await db.execute(
+                select(PasskeyCredential.credential_id).where(
+                    PasskeyCredential.user_id == 0
+                ).limit(1)
+            )
 
     # V-03: Generate options regardless of whether user exists or has passkeys.
     # Identical response shape prevents timing enumeration.
@@ -330,13 +340,15 @@ async def passkey_login_complete(
     except Exception as e:
         logger.warning("Passkey authentication verification failed for user %s: %s", user.id, e)
         # Increment failed login counter (shared with password auth)
-        await record_failed_login(db, user)
+        remaining = await record_failed_login(db, user)
         await log_audit_event(
             db, action="passkey.login_failed", actor_id=user.id,
-            detail={"reason": "verification_failed"},
+            detail={"reason": "verification_failed", "attempts_remaining": remaining},
             ip=get_client_ip(request),
         )
         await db.commit()
+        if remaining == 0:
+            raise HTTPException(status_code=401, detail="Account temporarily locked. Try again in 30 minutes.")
         raise HTTPException(status_code=401, detail="Authentication failed")
 
     # Update sign count (log anomaly but don't fail — S-05)
diff --git a/backend/app/routers/totp.py b/backend/app/routers/totp.py
index e2d8331..2dcb396 100644
--- a/backend/app/routers/totp.py
+++ b/backend/app/routers/totp.py
@@ -276,8 +276,10 @@ async def totp_verify(
         normalized = data.backup_code.strip().upper()
         valid = await _verify_backup_code(db, user.id, normalized)
         if not valid:
-            await record_failed_login(db, user)
+            remaining = await record_failed_login(db, user)
             await db.commit()
+            if remaining == 0:
+                raise HTTPException(status_code=401, detail="Account temporarily locked. Try again in 30 minutes.")
             raise HTTPException(status_code=401, detail="Invalid backup code")
 
         # Backup code accepted — reset lockout counter and issue session
@@ -293,8 +295,10 @@ async def totp_verify(
     # --- TOTP code path ---
     matched_window = verify_totp_code(user.totp_secret, data.code)
     if matched_window is None:
-        await record_failed_login(db, user)
+        remaining = await record_failed_login(db, user)
         await db.commit()
+        if remaining == 0:
+            raise HTTPException(status_code=401, detail="Account temporarily locked. Try again in 30 minutes.")
         raise HTTPException(status_code=401, detail="Invalid code")
 
     # Replay prevention — record (user_id, code, actual_matching_window)
diff --git a/backend/app/services/session.py b/backend/app/services/session.py
index ae33bda..3df0ca0 100644
--- a/backend/app/services/session.py
+++ b/backend/app/services/session.py
@@ -33,24 +33,31 @@ def set_session_cookie(response: Response, token: str) -> None:
 
 
 async def check_account_lockout(user: User) -> None:
-    """Raise HTTP 423 if the account is currently locked."""
+    """Raise HTTP 401 if the account is currently locked.
+
+    Uses 401 (same status as wrong-password) so that status-code analysis
+    cannot distinguish a locked account from an invalid credential (F-02).
+    """
     if user.locked_until and datetime.now() < user.locked_until:
         remaining = int((user.locked_until - datetime.now()).total_seconds() / 60) + 1
         raise HTTPException(
-            status_code=423,
-            detail=f"Account locked. Try again in {remaining} minutes.",
+            status_code=401,
+            detail=f"Account temporarily locked. Try again in {remaining} minutes.",
         )
 
 
-async def record_failed_login(db: AsyncSession, user: User) -> None:
+async def record_failed_login(db: AsyncSession, user: User) -> int:
     """Increment failure counter; lock account after 10 failures.
 
+    Returns the number of attempts remaining before lockout (0 = just locked).
     Does NOT commit — caller owns the transaction boundary.
     """
     user.failed_login_count += 1
+    remaining = max(0, 10 - user.failed_login_count)
     if user.failed_login_count >= 10:
         user.locked_until = datetime.now() + timedelta(minutes=30)
     await db.flush()
+    return remaining
 
 
 async def record_successful_login(db: AsyncSession, user: User) -> None:
diff --git a/frontend/nginx.conf b/frontend/nginx.conf
index 3b1ca90..97690b0 100644
--- a/frontend/nginx.conf
+++ b/frontend/nginx.conf
@@ -29,13 +29,14 @@ server {
     # Suppress nginx version in Server header
     server_tokens off;
 
-    # ── Real client IP restoration (PT-01) ────────────────────────────
+    # ── Real client IP restoration (PT-01 / F-03) ─────────────────────
     # Pangolin (TLS-terminating reverse proxy) connects via Docker bridge.
     # Restore the real client IP from X-Forwarded-For so that limit_req_zone
     # (which keys on $binary_remote_addr) throttles per-client, not per-proxy.
-    # Safe to trust all sources: nginx is only reachable via Docker networking,
-    # never directly internet-facing. Tighten if deployment model changes.
-    set_real_ip_from 0.0.0.0/0;
+    # Restricted to RFC 1918 ranges only — trusting 0.0.0.0/0 would allow an
+    # external client to spoof X-Forwarded-For and bypass rate limiting (F-03).
+    set_real_ip_from 172.16.0.0/12;
+    set_real_ip_from 10.0.0.0/8;
     real_ip_header X-Forwarded-For;
     real_ip_recursive on;