# UNDF: UNDF-2026-000000021
diff --git a/src/cargo/ops/tree/mod.rs b/src/cargo/ops/tree/mod.rs
index 4344daa..4c60bc9 100644
--- a/src/cargo/ops/tree/mod.rs
+++ b/src/cargo/ops/tree/mod.rs
@@ -277,7 +277,8 @@ fn print(
         let mut levels_continue = vec![];
         // The print stack is used to detect dependency cycles when
         // --no-dedupe is used. It contains a Node for each level.
-        let mut print_stack = vec![];
+        // CWE-407 fix: HashSet for O(1) contains() vs O(n) Vec::contains.
+        let mut print_stack = HashSet::new();
 
         print_node(
             ws,
@@ -311,7 +312,7 @@ fn print_node<'a>(
     display_depth: DisplayDepth,
     visited_deps: &mut HashSet<NodeId>,
     levels_continue: &mut Vec<(anstyle::Style, bool)>,
-    print_stack: &mut Vec<NodeId>,
+    print_stack: &mut HashSet<NodeId>,
 ) -> CargoResult<()> {
     let new = no_dedupe || visited_deps.insert(node_index);
 
@@ -355,7 +356,7 @@ fn print_node<'a>(
     if !new || in_cycle {
         return Ok(());
     }
-    print_stack.push(node_index);
+    print_stack.insert(node_index);
 
     for kind in &[
         EdgeKind::Dep(DepKind::Normal),
@@ -379,7 +380,7 @@ fn print_node<'a>(
             kind,
         )?;
     }
-    print_stack.pop();
+    print_stack.remove(&node_index);
 
     Ok(())
 }
@@ -397,7 +398,7 @@ fn print_dependencies<'a>(
     display_depth: DisplayDepth,
     visited_deps: &mut HashSet<NodeId>,
     levels_continue: &mut Vec<(anstyle::Style, bool)>,
-    print_stack: &mut Vec<NodeId>,
+    print_stack: &mut HashSet<NodeId>,
     kind: &EdgeKind,
 ) -> CargoResult<()> {
     let deps = graph.edges_of_kind(node_index, kind);