From 791d4e2c6335e80caebef5cecbb91a9809a933b8 Mon Sep 17 00:00:00 2001
From: zzz <zzz@i2pmail.org>
Date: Sat, 1 Aug 2026 15:24:01 -0400
Subject: [PATCH] NetDB: Prevent ClassCastException in HFDSMJ on hash collision

---
 .../HandleFloodfillDatabaseStoreMessageJob.java        | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/router/java/src/net/i2p/router/networkdb/kademlia/HandleFloodfillDatabaseStoreMessageJob.java b/router/java/src/net/i2p/router/networkdb/kademlia/HandleFloodfillDatabaseStoreMessageJob.java
index 021532746..6e65afc30 100644
--- a/router/java/src/net/i2p/router/networkdb/kademlia/HandleFloodfillDatabaseStoreMessageJob.java
+++ b/router/java/src/net/i2p/router/networkdb/kademlia/HandleFloodfillDatabaseStoreMessageJob.java
@@ -249,7 +249,15 @@ class HandleFloodfillDatabaseStoreMessageJob extends JobImpl {
                 boolean shouldStore = true;
                 if (ri.getReceivedAsPublished()) {
                     // these are often just dup stores from concurrent lookups
-                    prevNetDb = (RouterInfo) _facade.lookupLocallyWithoutValidation(key);
+                    DatabaseEntry dbe = _facade.lookupLocallyWithoutValidation(key);
+                    if (dbe != null) {
+                        if (dbe.getType() != DatabaseEntry.KEY_TYPE_ROUTERINFO) {
+                            // hash collision
+                            // prevent ClassCastException here
+                            throw new IllegalArgumentException("Attempt to replace LS with " + ri);
+                        }
+                        prevNetDb = (RouterInfo) dbe;
+                    }
                     if (prevNetDb == null) {
                         // actually new
                         int count = _facade.getDataStore().size();