diff --git a/api/custom_fields.php b/api/custom_fields.php
index 689170e..65b4c42 100644
--- a/api/custom_fields.php
+++ b/api/custom_fields.php
@@ -66,23 +66,35 @@ try {
 
         case 'POST':
             $data = json_decode(file_get_contents('php://input'), true);
+            if (!is_array($data)) {
+                http_response_code(400);
+                echo json_encode(['success' => false, 'error' => 'Invalid JSON']);
+                exit;
+            }
             $result = $model->createDefinition($data);
             echo json_encode($result);
             break;
 
         case 'PUT':
             if (!$id) {
+                http_response_code(400);
                 echo json_encode(['success' => false, 'error' => 'ID required']);
                 exit;
             }
 
             $data = json_decode(file_get_contents('php://input'), true);
+            if (!is_array($data)) {
+                http_response_code(400);
+                echo json_encode(['success' => false, 'error' => 'Invalid JSON']);
+                exit;
+            }
             $result = $model->updateDefinition($id, $data);
             echo json_encode($result);
             break;
 
         case 'DELETE':
             if (!$id) {
+                http_response_code(400);
                 echo json_encode(['success' => false, 'error' => 'ID required']);
                 exit;
             }
diff --git a/api/ticket_dependencies.php b/api/ticket_dependencies.php
index b3c781c..17ecc96 100644
--- a/api/ticket_dependencies.php
+++ b/api/ticket_dependencies.php
@@ -143,6 +143,10 @@ switch ($method) {
         // Add a new dependency
         $data = json_decode(file_get_contents('php://input'), true);
 
+        if (!is_array($data)) {
+            ResponseHelper::error('Invalid JSON');
+        }
+
         $ticketId = $data['ticket_id'] ?? null;
         $dependsOnId = $data['depends_on_id'] ?? null;
         $type = $data['dependency_type'] ?? 'blocks';
@@ -151,6 +155,16 @@ switch ($method) {
             ResponseHelper::error('Both ticket_id and depends_on_id are required');
         }
 
+        // Verify user can access both tickets before creating dependency
+        $srcTicket = $ticketModel->getTicketById((int)$ticketId);
+        if (!$srcTicket || !$ticketModel->canUserAccessTicket($srcTicket, $currentUser)) {
+            ResponseHelper::notFound('Ticket not found');
+        }
+        $tgtTicket = $ticketModel->getTicketById((int)$dependsOnId);
+        if (!$tgtTicket || !$ticketModel->canUserAccessTicket($tgtTicket, $currentUser)) {
+            ResponseHelper::notFound('Target ticket not found');
+        }
+
         $result = $dependencyModel->addDependency($ticketId, $dependsOnId, $type, $userId);
 
         if ($result['success']) {
@@ -171,6 +185,10 @@ switch ($method) {
         // Remove a dependency
         $data = json_decode(file_get_contents('php://input'), true);
 
+        if (!is_array($data)) {
+            ResponseHelper::error('Invalid JSON');
+        }
+
         $dependencyId = $data['dependency_id'] ?? null;
 
         // Alternative: delete by ticket IDs
@@ -179,6 +197,18 @@ switch ($method) {
             $dependsOnId = $data['depends_on_id'];
             $type = $data['dependency_type'] ?? 'blocks';
 
+            // Validate dependency type
+            $validTypes = ['blocks', 'blocked_by', 'relates_to', 'duplicates'];
+            if (!in_array($type, $validTypes, true)) {
+                ResponseHelper::error('Invalid dependency type');
+            }
+
+            // Verify user can access the source ticket
+            $srcTicket = $ticketModel->getTicketById((int)$ticketId);
+            if (!$srcTicket || !$ticketModel->canUserAccessTicket($srcTicket, $currentUser)) {
+                ResponseHelper::notFound('Ticket not found');
+            }
+
             $result = $dependencyModel->removeDependencyByTickets($ticketId, $dependsOnId, $type);
 
             if ($result) {
@@ -192,6 +222,23 @@ switch ($method) {
                 ResponseHelper::error('Failed to remove dependency');
             }
         } elseif ($dependencyId) {
+            // Look up dependency to verify ticket access before deletion
+            $depLookupSql = "SELECT ticket_id FROM ticket_dependencies WHERE dependency_id = ?";
+            $depLookupStmt = $conn->prepare($depLookupSql);
+            $depLookupStmt->bind_param("i", $dependencyId);
+            $depLookupStmt->execute();
+            $depRow = $depLookupStmt->get_result()->fetch_assoc();
+            $depLookupStmt->close();
+
+            if (!$depRow) {
+                ResponseHelper::notFound('Dependency not found');
+            }
+
+            $depTicket = $ticketModel->getTicketById((int)$depRow['ticket_id']);
+            if (!$depTicket || !$ticketModel->canUserAccessTicket($depTicket, $currentUser)) {
+                ResponseHelper::forbidden('Access denied');
+            }
+
             $result = $dependencyModel->removeDependency($dependencyId);
 
             if ($result) {