packetbeat/protos/http: don't panic when host is empty

Previously, extractHostHeader would panic if the host part of header was
empty. Avoid this by using standard library functions to do splits and
clean up IPv6 addresses.

Add tests to confirm old behaviour and test to cover panic case.

CHANGELOG.next.asciidoc

@@ -136,6 +136,7 @@ https://github.com/elastic/beats/compare/v8.8.1\...main[Check the HEAD diff]
 
 *Packetbeat*
 
+- Fix panic in HTTP protocol parsing when host header has empty host part. {issue}36497[36497] {issue}36518[36518]
 
 *Winlogbeat*
 

packetbeat/protos/http/http.go

@@ -20,6 +20,7 @@ package http
 import (
 	"bytes"
 	"encoding/base64"
+	"errors"
 	"fmt"
 	"net"
 	"net/url"
@@ -735,20 +736,29 @@ func parseCookieValue(raw string) string {
 }
 
 func extractHostHeader(header string) (host string, port int) {
-	if len(header) == 0 || net.ParseIP(header) != nil {
+	if header == "" || net.ParseIP(header) != nil {
 		return header, port
 	}
-	// Split :port trailer
-	if pos := strings.LastIndexByte(header, ':'); pos != -1 {
-		if num, err := strconv.Atoi(header[pos+1:]); err == nil && num > 0 && num < 65536 {
-			header, port = header[:pos], num
+	host, ps, err := net.SplitHostPort(header)
+	if err != nil {
+		var addrError *net.AddrError
+		if errors.As(err, &addrError) && addrError.Err == "missing port in address" {
+			return trimSquareBracket(header), port
 		}
 	}
-	// Remove square bracket boxing of IPv6 address.
-	if last := len(header) - 1; header[0] == '[' && header[last] == ']' && net.ParseIP(header[1:last]) != nil {
-		header = header[1:last]
+	pi, err := strconv.ParseInt(ps, 10, 16)
+	if err != nil || pi == 0 {
+		return header, port
+	}
+	return trimSquareBracket(host), int(pi)
+}
+
+func trimSquareBracket(s string) string {
+	s, ok := strings.CutPrefix(s, "[")
+	if !ok {
+		return s
 	}
-	return header, port
+	return strings.TrimSuffix(s, "]")
 }
 
 func (http *httpPlugin) hideHeaders(m *message) {

packetbeat/protos/http/http_test.go

@@ -1901,6 +1901,34 @@ func TestHttpParser_Extension(t *testing.T) {
 	}
 }
 
+func TestExtractHostHeader(t *testing.T) {
+	tests := []struct {
+		header   string
+		wantHost string
+		wantPort int
+	}{
+		{header: "", wantHost: "", wantPort: 0},
+		{header: "localhost:0", wantHost: "localhost:0", wantPort: 0},
+		{header: "127.0.0.1:0", wantHost: "127.0.0.1:0", wantPort: 0},
+		{header: "[::]:0", wantHost: "[::]:0", wantPort: 0},
+		{header: "localhost:9001", wantHost: "localhost", wantPort: 9001},
+		{header: "localhost:9000000", wantHost: "localhost:9000000", wantPort: 0},
+		{header: "127.0.0.1:9001", wantHost: "127.0.0.1", wantPort: 9001},
+		{header: "127.0.0.1", wantHost: "127.0.0.1", wantPort: 0},
+		{header: "[::]", wantHost: "::", wantPort: 0},
+		{header: ":9001", wantHost: "", wantPort: 9001},
+	}
+	for _, test := range tests {
+		host, port := extractHostHeader(test.header)
+		if host != test.wantHost {
+			t.Errorf("unexpected host for %q: got:%q want:%q", test.header, host, test.wantHost)
+		}
+		if port != test.wantPort {
+			t.Errorf("unexpected port for %q: got:%d want:%d", test.header, port, test.wantPort)
+		}
+	}
+}
+
 func benchmarkHTTPMessage(b *testing.B, data []byte) {
 	http := httpModForTests(nil)
 	parser := newParser(&http.parserConfig)