net: fail closed when BlockList peer address is unavailable

lib/net.js

@@ -2826,7 +2826,10 @@ function onconnection(err, clientHandle) {
     const remoteInfo = { __proto__: null };
     clientHandle.getpeername(remoteInfo);
     const addressType = isIP(remoteInfo.address);
-    if (addressType && self.blockList.check(remoteInfo.address, `ipv${addressType}`)) {
+    // Fail closed: if a BlockList is configured but the peer address cannot
+    // be obtained (e.g. the connection was reset before getpeername()),
+    // refuse the connection instead of silently bypassing the access control.
+    if (!addressType || self.blockList.check(remoteInfo.address, `ipv${addressType}`)) {
       clientHandle.close();
       return;
     }

test/parallel/test-net-server-blocklist-fail-closed.js

@@ -0,0 +1,33 @@
+'use strict';
+const common = require('../common');
+const net = require('net');
+
+const blockList = new net.BlockList();
+blockList.addCIDR('0.0.0.0/0');
+blockList.addCIDR('::/0');
+
+// A connection whose peer address cannot be determined (e.g. it was reset
+// before getpeername()) must be rejected when a BlockList is configured,
+// rather than being delivered to the application (fail closed).
+const server = net.createServer({ blockList }, common.mustNotCall());
+
+server.listen(0, common.mustCall(() => {
+  const socket = net.connect(server.address().port);
+  socket.on('error', () => {});
+}));
+
+const onconnection = server._handle.onconnection;
+server._handle.onconnection = common.mustCall((err, clientHandle) => {
+  const close = clientHandle.close;
+  // Simulate the reset-before-getpeername() condition: onconnection() is
+  // unable to obtain a valid peer address from the accepted connection.
+  clientHandle.getpeername = function(remoteInfo) {
+    remoteInfo.address = undefined;
+  };
+  clientHandle.close = common.mustCall(() => {
+    clientHandle.close = close;
+    close.call(clientHandle);
+    server.close();
+  });
+  onconnection.call(server._handle, err, clientHandle);
+});