# UNDF: UNDF-2026-000000284
--- a/src/Simplex/Chat/Library/Commands.hs
+++ b/src/Simplex/Chat/Library/Commands.hs
@@ -2378,8 +2378,9 @@ processChatCommand vr nm = \case
   APIBlockMembersForAll groupId memberIds blockFlag -> withUser $ \user ->
     withGroupLock "blockForAll" groupId $ do
       Group gInfo members <- withFastStore $ \db -> getGroup db vr user groupId
-      when (selfSelected gInfo) $ throwCmdError "can't block/unblock self"
-      let (blockMems, remainingMems, maxRole, anyAdmin, anyPending) = selectMembers members
+      let gmIdSet = S.fromList (L.toList memberIds)
+      when (selfSelected gmIdSet gInfo) $ throwCmdError "can't block/unblock self"
+      let (blockMems, remainingMems, maxRole, anyAdmin, anyPending) = selectMembers gmIdSet members
       when (length blockMems /= length memberIds) $ throwChatError CEGroupMemberNotFound
       when (length memberIds > 1 && anyAdmin) $ throwCmdError "can't block/unblock multiple members when admins selected"
       when anyPending $ throwCmdError "can't block/unblock members pending approval"
@@ -2388,11 +2389,11 @@ processChatCommand vr nm = \case
       blockMembers user gInfo blockMems remainingMems
     where
-      selfSelected GroupInfo {membership} = elem (groupMemberId' membership) memberIds
-      selectMembers :: [GroupMember] -> ([GroupMember], [GroupMember], GroupMemberRole, Bool, Bool)
-      selectMembers = foldr' addMember ([], [], GRObserver, False, False)
+      selfSelected gmIdSet GroupInfo {membership} = S.member (groupMemberId' membership) gmIdSet
+      selectMembers :: S.Set GroupMemberId -> [GroupMember] -> ([GroupMember], [GroupMember], GroupMemberRole, Bool, Bool)
+      selectMembers gmIdSet = foldr' addMember ([], [], GRObserver, False, False)
         where
           addMember m@GroupMember {groupMemberId, memberRole} (block, remaining, maxRole, anyAdmin, anyPending)
-            | groupMemberId `elem` memberIds =
+            | groupMemberId `S.member` gmIdSet =
                 let maxRole' = max maxRole memberRole
                     anyAdmin' = anyAdmin || memberRole >= GRAdmin
                     anyPending' = anyPending || memberPending m