# UNDF: UNDF-2026-000000079
diff --git a/compiler/GHC/Data/Graph/Ops.hs b/compiler/GHC/Data/Graph/Ops.hs
index dc90b9e..16e097a 100644
--- a/compiler/GHC/Data/Graph/Ops.hs
+++ b/compiler/GHC/Data/Graph/Ops.hs
@@ -571,7 +571,7 @@ scanGraph match graph
 --      If they don't then throw an error
 --
 validateGraph
-        :: (Uniquable k, Outputable k, Eq color)
+        :: (Uniquable k, Outputable k, Uniquable color)
         => SDoc                         -- ^ extra debugging info to display on error
         -> Bool                         -- ^ whether this graph is supposed to be colored.
         -> Graph k cls color            -- ^ graph to validate
@@ -622,7 +622,7 @@ validateGraph doc isColored graph
 -- | If this node is colored, check that all the nodes which
 --      conflict with it have different colors.
 checkNode
-        :: (Uniquable k, Eq color)
+        :: (Uniquable k, Uniquable color)
         => Graph k cls color
         -> Node  k cls color
         -> Bool                 -- ^ True if this node is ok
@@ -633,8 +633,11 @@ checkNode graph node
                                 $  nonDetEltsUniqSet $ nodeConflicts node
             -- See Note [Unique Determinism and code generation]
 
+        -- CWE-407 fix: build a UniqSet of neighbour colors for O(1) lookup.
+        -- `elem color neighbourColors` is O(deg); UniqSet is O(1) amortized.
         , neighbourColors       <- mapMaybe nodeColor neighbors
-        , elem color neighbourColors
+        , let neighbourColorSet = mkUniqSet neighbourColors
+        , elementOfUniqSet color neighbourColorSet
         = False
 
         | otherwise