# UNDF: UNDF-2026-000000239
--- a/deps/rabbit/src/rabbit_classic_queue.erl
+++ b/deps/rabbit/src/rabbit_classic_queue.erl
@@ -9,7 +9,7 @@
 %% TODO possible to use sets / maps instead of lists?
 %% Check performance with QoS 1 and 1 million target queues.
--record(msg_status, {pending :: [pid()],
-                     confirmed = [] :: [pid()]}).
+-record(msg_status, {pending :: #{pid() => true},      %% CWE-407 fix: O(1) membership via map
+                     confirmed = [] :: [pid()]}).

 -define(STATE, ?MODULE).
 -record(?STATE, {
@@ -406,8 +406,8 @@ handle_event(QName, {down, Pid, Info}, #?STATE{monitored = Monitored,
         false ->
             MsgSeqNos = maps:keys(
                           maps:filter(fun (_, #msg_status{pending = Pids}) ->
-                                              lists:member(Pid, Pids)
+                                              maps:is_key(Pid, Pids)  %% CWE-407 fix: O(1) map lookup
                                       end, U0)),
             {Unconfirmed, Settled, Rejected} = settle_seq_nos(MsgSeqNos, Pid, U0, down),
@@ -428,8 +428,8 @@ handle_event(QName, {down, Pid, Info}, #?STATE{monitored = Monitored,
         true ->
             MsgIds = maps:fold(
                           fun (SeqNo, Status, Acc) ->
-                                  case lists:member(Pid, Status#msg_status.pending) of
+                                  case maps:is_key(Pid, Status#msg_status.pending) of  %% CWE-407 fix: O(1) map lookup
                                       true ->
                                           [SeqNo | Acc];
                                       false ->
@@ -591,7 +591,7 @@ qpids(Qs, Confirm, MsgNo) ->
                       #?STATE{unconfirmed = U0} ->
-                          Rec = [QPid],
+                          Rec = #{QPid => true},  %% CWE-407 fix: initialise pending as map/set
                           U = case Confirm of
                                   false ->
                                       U0;
@@ -683,10 +683,10 @@ settle_seq_nos(MsgSeqNos, Pid, U0, Reason) ->
                   #{SeqNo := Status0} ->
                       case update_msg_status(Reason, Pid, Status0) of
-                          #msg_status{pending = [],
-                                      confirmed = []} ->
+                          #msg_status{pending = P, confirmed = []} when map_size(P) =:= 0 ->  %% CWE-407 fix: empty-map guard
                               %% no pending left and nothing confirmed
                               %% then we reject it
                               {maps:remove(SeqNo, U), C0, [SeqNo | R0]};
-                          #msg_status{pending = [],
-                                      confirmed = _} ->
+                          #msg_status{pending = P2, confirmed = _} when map_size(P2) =:= 0 ->  %% CWE-407 fix: empty-map guard
                               %% this can be confirmed as there are no pending
                               %% and confirmed isn't empty
                               {maps:remove(SeqNo, U), [SeqNo | C0], R0};
@@ -704,9 +704,9 @@ settle_seq_nos(MsgSeqNos, Pid, U0, Reason) ->
 update_msg_status(confirm, Pid, #msg_status{pending = P,
                                             confirmed = C} = S) ->
-    Rem = lists:delete(Pid, P),
+    Rem = maps:remove(Pid, P),  %% CWE-407 fix: O(log N) map remove vs O(P) list scan
     S#msg_status{pending = Rem, confirmed = [Pid | C]};
 update_msg_status(down, Pid, #msg_status{pending = P} = S) ->
-    S#msg_status{pending = lists:delete(Pid, P)}.
+    S#msg_status{pending = maps:remove(Pid, P)}.  %% CWE-407 fix: O(log N) map remove vs O(P) list scan