diff --git a/content/browser/interest_group/interest_group_auction.cc b/content/browser/interest_group/interest_group_auction.cc
index 04f467fa75449..51eb0ede0d074 100644
--- a/content/browser/interest_group/interest_group_auction.cc
+++ b/content/browser/interest_group/interest_group_auction.cc
@@ -1628,6 +1628,17 @@ class InterestGroupAuction::BuyerHelper
       base::TimeDelta trusted_signals_fetch_latency,
       std::optional<base::TimeDelta> update_if_older_than,
       base::OnceClosure resume_generate_bid_callback) override {
+    // Reject non-finite values. The worklet code should prevent this, but the
+    // process may be compromised. A NaN that reaches `calculated_priority`
+    // breaks the strict-weak-ordering invariant of the comparators used in
+    // ApplySizeLimitAndSort(), which can cause out-of-bounds iterator walks.
+    if (std::ranges::any_of(priority_vector, [](const auto& pair) {
+          return !std::isfinite(pair.second);
+        })) {
+      generate_bid_client_receiver_set_.ReportBadMessage(
+          "Invalid priority vector");
+      return;
+    }
     BidState* state = generate_bid_client_receiver_set_.current_context();
     const blink::InterestGroup& interest_group = state->bidder->interest_group;
     state->pa_timings(PrivateAggregationPhase::kBidder).signals_fetch_time =