diff --git a/cdk/po_stack.py b/cdk/po_stack.py
index 82a6bd6..2dadc6c 100644
--- a/cdk/po_stack.py
+++ b/cdk/po_stack.py
@@ -191,6 +191,19 @@ class PoIngestStack(Stack):
             log_retention=logs.RetentionDays.TWO_MONTHS,
             environment={
                 "PO_TABLE": "purchase-orders",
+                # Defense-in-depth shared secret for the web UI handler. The
+                # handler fails closed if this is unset, so any future invocation
+                # path cannot re-expose the PO DB unauthenticated. Sourced from the
+                # SSM String parameter /procurement-ingest/web-ui-auth-token; create
+                # it before deploy. NOTE: a plaintext String (not SecureString) is
+                # used deliberately — value_for_string_parameter resolves String
+                # params into Lambda env vars at deploy via {{resolve:ssm:...}};
+                # SecureString dynamic references are NOT resolved in Lambda env
+                # vars and would pass the token literally. This is a defense-in-depth
+                # floor for a currently-detached URL (INFRA-74), not a primary auth.
+                "WEB_UI_AUTH_TOKEN": ssm.StringParameter.value_for_string_parameter(
+                    self, "/procurement-ingest/web-ui-auth-token"
+                ),
             },
         )
 
diff --git a/cdk/wo_stack.py b/cdk/wo_stack.py
index acedb1a..ef1335e 100644
--- a/cdk/wo_stack.py
+++ b/cdk/wo_stack.py
@@ -201,6 +201,19 @@ class WorkorderIngestStack(Stack):
             environment={
                 "WORK_ORDERS_TABLE": work_orders_table.table_name,
                 "COMMENTS_TABLE": comments_table.table_name,
+                # Defense-in-depth shared secret for the web UI handler. The
+                # handler fails closed if this is unset, so any future invocation
+                # path cannot re-expose the WO DB unauthenticated. Sourced from the
+                # SSM String parameter /procurement-ingest/web-ui-auth-token; create
+                # it before deploy. NOTE: a plaintext String (not SecureString) is
+                # used deliberately — value_for_string_parameter resolves String
+                # params into Lambda env vars at deploy via {{resolve:ssm:...}};
+                # SecureString dynamic references are NOT resolved in Lambda env
+                # vars and would pass the token literally. This is a defense-in-depth
+                # floor for a currently-detached URL (INFRA-74), not a primary auth.
+                "WEB_UI_AUTH_TOKEN": ssm.StringParameter.value_for_string_parameter(
+                    self, "/procurement-ingest/web-ui-auth-token"
+                ),
             },
         )
 
diff --git a/lambdas/po/web_ui/handler.py b/lambdas/po/web_ui/handler.py
index beaa174..dc2cb72 100644
--- a/lambdas/po/web_ui/handler.py
+++ b/lambdas/po/web_ui/handler.py
@@ -5,6 +5,7 @@ Serves a simple HTML dashboard for viewing purchase orders.
 Accessed via Lambda Function URL.
 """
 
+import hmac
 import json
 import os
 from decimal import Decimal
@@ -16,6 +17,40 @@ dynamodb = boto3.resource("dynamodb")
 
 PO_TABLE = os.environ.get("PO_TABLE", "purchase-orders")
 
+# Defense-in-depth auth gate. The public Function URL was removed (INFRA-74), but
+# the handler must still refuse unauthenticated requests so any future invocation
+# path (re-attached Function URL, API Gateway, etc.) does not re-expose the whole
+# PO DB. Callers must present the shared secret in the X-Auth-Token header (or
+# Authorization: Bearer <token>). The secret is supplied via the WEB_UI_AUTH_TOKEN
+# env var (wire to Secrets Manager / SSM at deploy time). If it is unset, the
+# handler fails closed and denies every request.
+WEB_UI_AUTH_TOKEN = os.environ.get("WEB_UI_AUTH_TOKEN")
+
+
+def _header(event: dict, name: str) -> str:
+    """Case-insensitive header lookup from a Lambda Function URL / APIGW event."""
+    headers = event.get("headers") or {}
+    name_lower = name.lower()
+    for key, value in headers.items():
+        if key.lower() == name_lower:
+            return value or ""
+    return ""
+
+
+def is_authenticated(event: dict) -> bool:
+    """Constant-time check of the request's shared secret against the configured
+    token. Fails closed when no token is configured."""
+    if not WEB_UI_AUTH_TOKEN:
+        return False
+    presented = _header(event, "x-auth-token")
+    if not presented:
+        auth = _header(event, "authorization")
+        if auth.lower().startswith("bearer "):
+            presented = auth[7:].strip()
+    if not presented:
+        return False
+    return hmac.compare_digest(presented, WEB_UI_AUTH_TOKEN)
+
 
 def get_purchase_orders(limit=500):
     table = dynamodb.Table(PO_TABLE)
@@ -60,7 +95,10 @@ def fmt_currency(val):
         val = float(val)
     if isinstance(val, (int, float)):
         return f"${val:,.2f}"
-    return str(val)
+    # Non-numeric fallback: a prompt-injected email can make Claude return
+    # total_amount/amount as an arbitrary string. Escape it before it is
+    # interpolated raw into the HTML to prevent stored XSS.
+    return esc(str(val))
 
 
 def render_po_detail(po):
@@ -250,6 +288,13 @@ def render_po_list(purchase_orders):
 
 
 def handler(event, context):
+    if not is_authenticated(event):
+        return {
+            "statusCode": 401,
+            "headers": {"Content-Type": "text/html"},
+            "body": "<h1>401 Unauthorized</h1>",
+        }
+
     path = event.get("rawPath", "/")
     qs = event.get("queryStringParameters") or {}
 
diff --git a/lambdas/wo/web_ui/handler.py b/lambdas/wo/web_ui/handler.py
index 4af9120..5f80abc 100644
--- a/lambdas/wo/web_ui/handler.py
+++ b/lambdas/wo/web_ui/handler.py
@@ -5,6 +5,7 @@ Serves a simple HTML dashboard for viewing work orders and comments.
 Accessed via Lambda Function URL.
 """
 
+import hmac
 import json
 import os
 from html import escape as esc
@@ -16,6 +17,39 @@ dynamodb = boto3.resource("dynamodb")
 WORK_ORDERS_TABLE = os.environ.get("WORK_ORDERS_TABLE", "WorkOrders")
 COMMENTS_TABLE = os.environ.get("COMMENTS_TABLE", "WorkOrderComments")
 
+# Defense-in-depth auth gate. The public Function URL was removed (INFRA-74), but
+# the handler must still refuse unauthenticated requests so any future invocation
+# path does not re-expose the whole WO DB. Callers must present the shared secret
+# in the X-Auth-Token header (or Authorization: Bearer <token>). The secret is
+# supplied via the WEB_UI_AUTH_TOKEN env var (wire to Secrets Manager / SSM at
+# deploy time). If it is unset, the handler fails closed and denies every request.
+WEB_UI_AUTH_TOKEN = os.environ.get("WEB_UI_AUTH_TOKEN")
+
+
+def _header(event: dict, name: str) -> str:
+    """Case-insensitive header lookup from a Lambda Function URL / APIGW event."""
+    headers = event.get("headers") or {}
+    name_lower = name.lower()
+    for key, value in headers.items():
+        if key.lower() == name_lower:
+            return value or ""
+    return ""
+
+
+def is_authenticated(event: dict) -> bool:
+    """Constant-time check of the request's shared secret against the configured
+    token. Fails closed when no token is configured."""
+    if not WEB_UI_AUTH_TOKEN:
+        return False
+    presented = _header(event, "x-auth-token")
+    if not presented:
+        auth = _header(event, "authorization")
+        if auth.lower().startswith("bearer "):
+            presented = auth[7:].strip()
+    if not presented:
+        return False
+    return hmac.compare_digest(presented, WEB_UI_AUTH_TOKEN)
+
 
 def get_work_orders(limit=500):
     table = dynamodb.Table(WORK_ORDERS_TABLE)
@@ -228,6 +262,13 @@ def render_work_orders_list(work_orders):
 
 
 def handler(event, context):
+    if not is_authenticated(event):
+        return {
+            "statusCode": 401,
+            "headers": {"Content-Type": "text/html"},
+            "body": "<h1>401 Unauthorized</h1>",
+        }
+
     path = event.get("rawPath", "/")
     qs = event.get("queryStringParameters") or {}