Fix http parsing of repeated headers (#6325)

Packetbeat HTTP protocol parser was not concatenating properly repeated
headers in a request or response. This caused corruption in the header
in the form of null bytes (\u0000).

CHANGELOG.asciidoc

@@ -128,6 +128,7 @@ https://github.com/elastic/beats/compare/v6.0.0-beta2...master[Check the HEAD di
 - Fix http status phrase parsing not allow spaces. {pull}5312[5312]
 - Fix http parse to allow to parse get request with space in the URI. {pull}5495[5495]
 - 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]
 
 *Winlogbeat*
 

packetbeat/protos/http/http_parser.go

@@ -356,8 +356,8 @@ func (parser *parser) parseHeader(m *message, data []byte) (bool, bool, int) {
 				if val, ok := m.headers[string(headerName)]; ok {
 					composed := make([]byte, len(val)+len(headerVal)+2)
 					off := copy(composed, val)
-					off = copy(composed[off:], []byte(", "))
-					copy(composed[off:], headerVal)
+					copy(composed[off:], []byte(", "))
+					copy(composed[off+2:], headerVal)
 
 					m.headers[string(headerName)] = composed
 				} else {

packetbeat/protos/http/http_test.go

@@ -1168,6 +1168,28 @@ func Test_gap_in_body_http1dot0(t *testing.T) {
 	assert.Equal(t, false, complete)
 }
 
+func TestHttpParser_composedHeaders(t *testing.T) {
+	data := "HTTP/1.1 200 OK\r\n" +
+		"Content-Length: 0\r\n" +
+		"Date: Tue, 14 Aug 2012 22:31:45 GMT\r\n" +
+		"Set-Cookie: aCookie=yummy\r\n" +
+		"Set-Cookie: anotherCookie=why%20not\r\n" +
+		"\r\n"
+	http := httpModForTests(nil)
+	http.parserConfig.sendHeaders = true
+	http.parserConfig.sendAllHeaders = true
+	message, ok, complete := testParse(http, data)
+
+	assert.True(t, ok)
+	assert.True(t, complete)
+	assert.False(t, message.isRequest)
+	assert.Equal(t, 200, int(message.statusCode))
+	assert.Equal(t, "OK", string(message.statusPhrase))
+	header, ok := message.headers["set-cookie"]
+	assert.True(t, ok)
+	assert.Equal(t, "aCookie=yummy, anotherCookie=why%20not", string(header))
+}
+
 func testCreateTCPTuple() *common.TCPTuple {
 	t := &common.TCPTuple{
 		IPLength: 4,