Fix out of bounds access to slice in MongoDB parser (#6256)

* Fix out of bounds access to slice in MongoDB parser

Ignore MongoDB message and drop the TCP stream if a malformed
query / response is received, instead of logging a panic.

Closes #5188

* Update CHANGELOG

CHANGELOG.asciidoc

@@ -140,6 +140,7 @@ https://github.com/elastic/beats/compare/v6.0.0-beta2...master[Check the HEAD di
 - Fix mysql SQL parser to trim `\r` from Windows Server `SELECT\r\n\t1`. {pull}5572[5572]
 - Fix corruption when parsing repeated headers in an HTTP request or response. {pull}6325[6325]
 - Fix panic when parsing partial AMQP messages. {pull}6384[6384]
+- Fix out of bounds access to slice in MongoDB parser. {pull}6256[6256]
 
 *Winlogbeat*
 

packetbeat/protos/mongodb/mongodb_parser.go

@@ -341,6 +341,9 @@ func (d *decoder) readDocument() (bson.M, error) {
 	start := d.i
 	documentLength, err := d.readInt32()
 	d.i = start + documentLength
+	if len(d.in) < d.i {
+		return nil, errors.New("document out of bounds")
+	}
 
 	documentMap := bson.M{}
 

packetbeat/protos/mongodb/mongodb_test.go

@@ -345,3 +345,29 @@ func TestOpCodeNames(t *testing.T) {
 		assert.Equal(t, testData.expected, opCode(testData.code).String())
 	}
 }
+
+// Test for a (recovered) panic parsing document length in request/response messages
+func TestDocumentLengthBoundsChecked(t *testing.T) {
+	logp.TestingSetup(logp.WithSelectors("mongodb", "mongodbdetailed"))
+
+	_, mongodb := mongodbModForTests()
+
+	// request and response from tests/pcaps/mongo_one_row.pcap
+	reqData, err := hex.DecodeString(
+		// Request message with out of bounds document
+		"320000000a000000ffffffffd4070000" +
+			"00000000746573742e72667374617572" +
+			"616e7473000000000001000000" +
+			// Document length (including itself)
+			"06000000" +
+			// Document (1 byte instead of 2)
+			"00")
+	assert.Nil(t, err)
+
+	tcptuple := testTCPTuple()
+	req := protos.Packet{Payload: reqData}
+	private := protos.ProtocolData(new(mongodbConnectionData))
+
+	private = mongodb.Parse(&req, tcptuple, 0, private)
+	assert.NotNil(t, private, "mongodb parser recovered from a panic")
+}