diff --git a/analyzer/udp/internal/quic/header.go b/analyzer/udp/internal/quic/header.go
index 791f023..4d46e6e 100644
--- a/analyzer/udp/internal/quic/header.go
+++ b/analyzer/udp/internal/quic/header.go
@@ -9,6 +9,8 @@ import (
 	"github.com/quic-go/quic-go/quicvarint"
 )
 
+var ErrNotInitialPacket = errors.New("not initial packet")
+
 // The Header represents a QUIC header.
 type Header struct {
 	Type             uint8
@@ -36,6 +38,9 @@ func parseLongHeader(b *bytes.Reader) (*Header, error) {
 	if err != nil {
 		return nil, err
 	}
+	if !isLongHeader(typeByte) {
+		return nil, ErrNotInitialPacket
+	}
 	h := &Header{}
 	ver, err := beUint32(b)
 	if err != nil {
@@ -66,18 +71,19 @@ func parseLongHeader(b *bytes.Reader) (*Header, error) {
 	if h.Version == V2 {
 		initialPacketType = 0b01
 	}
-	if (typeByte >> 4 & 0b11) == initialPacketType {
-		tokenLen, err := quicvarint.Read(b)
-		if err != nil {
-			return nil, err
-		}
-		if tokenLen > uint64(b.Len()) {
-			return nil, io.EOF
-		}
-		h.Token = make([]byte, tokenLen)
-		if _, err := io.ReadFull(b, h.Token); err != nil {
-			return nil, err
-		}
+	if (typeByte>>4)&0b11 != initialPacketType {
+		return nil, ErrNotInitialPacket
+	}
+	tokenLen, err := quicvarint.Read(b)
+	if err != nil {
+		return nil, err
+	}
+	if tokenLen > uint64(b.Len()) {
+		return nil, io.EOF
+	}
+	h.Token = make([]byte, tokenLen)
+	if _, err := io.ReadFull(b, h.Token); err != nil {
+		return nil, err
 	}
 
 	pl, err := quicvarint.Read(b)
diff --git a/analyzer/udp/internal/quic/payload.go b/analyzer/udp/internal/quic/payload.go
index 9a2fa26..75d9da8 100644
--- a/analyzer/udp/internal/quic/payload.go
+++ b/analyzer/udp/internal/quic/payload.go
@@ -51,15 +51,27 @@ func ReadCryptoFrames(packet []byte) ([]CryptoFrame, error) {
 	if int64(len(packet)) < offset+hdr.Length {
 		return nil, fmt.Errorf("packet is too short: %d < %d", len(packet), offset+hdr.Length)
 	}
-	unProtectedPayload, err := pp.UnProtect(packet[:offset+hdr.Length], offset, 2)
-	if err != nil {
-		return nil, err
+	packetView := packet[:offset+hdr.Length]
+	pnMaxGuesses := []int64{0, 1, 2, 3, 4, 8, 16}
+	var lastErr error
+	for _, pnMax := range pnMaxGuesses {
+		packetCopy := append([]byte(nil), packetView...)
+		unProtectedPayload, err := pp.UnProtect(packetCopy, offset, pnMax)
+		if err != nil {
+			lastErr = err
+			continue
+		}
+		frs, err := extractCryptoFrames(bytes.NewReader(unProtectedPayload))
+		if err != nil {
+			lastErr = err
+			continue
+		}
+		return frs, nil
 	}
-	frs, err := extractCryptoFrames(bytes.NewReader(unProtectedPayload))
-	if err != nil {
-		return nil, err
+	if lastErr != nil {
+		return nil, lastErr
 	}
-	return frs, nil
+	return nil, errors.New("unable to decrypt initial packet")
 }
 
 const (
diff --git a/analyzer/udp/internal/quic/payload_test.go b/analyzer/udp/internal/quic/payload_test.go
index d44fb4f..4fe7e21 100644
--- a/analyzer/udp/internal/quic/payload_test.go
+++ b/analyzer/udp/internal/quic/payload_test.go
@@ -55,3 +55,14 @@ func TestExtractCryptoFrames_UnknownAfterCrypto(t *testing.T) {
 		t.Fatalf("frame0 = %+v, want offset=0 data=abc", frames[0])
 	}
 }
+
+func TestReadCryptoFrames_NonInitialHeader(t *testing.T) {
+	// Short header packet marker should be rejected as non-initial.
+	_, err := ReadCryptoFrames([]byte{0x40, 0x01, 0x02, 0x03, 0x04})
+	if err == nil {
+		t.Fatal("ReadCryptoFrames() error = nil, want non-nil")
+	}
+	if err.Error() != ErrNotInitialPacket.Error() {
+		t.Fatalf("ReadCryptoFrames() error = %v, want %v", err, ErrNotInitialPacket)
+	}
+}
diff --git a/analyzer/udp/quic.go b/analyzer/udp/quic.go
index dae90e5..98e3abf 100644
--- a/analyzer/udp/quic.go
+++ b/analyzer/udp/quic.go
@@ -1,6 +1,7 @@
 package udp
 
 import (
+	"errors"
 	"sort"
 
 	"git.difuse.io/Difuse/Mellaris/analyzer"
@@ -50,7 +51,14 @@ func (s *quicStream) Feed(rev bool, data []byte) (u *analyzer.PropUpdate, done b
 	const minDataSize = 41
 
 	frs, err := quic.ReadCryptoFrames(data)
-	if err != nil || len(frs) == 0 {
+	if err != nil {
+		if errors.Is(err, quic.ErrNotInitialPacket) {
+			return nil, false
+		}
+		s.invalidCount++
+		return nil, s.invalidCount >= quicInvalidCountThreshold
+	}
+	if len(frs) == 0 {
 		s.invalidCount++
 		return nil, s.invalidCount >= quicInvalidCountThreshold
 	}
@@ -64,8 +72,8 @@ func (s *quicStream) Feed(rev bool, data []byte) (u *analyzer.PropUpdate, done b
 	}
 
 	if pl[0] != internal.TypeClientHello {
-		s.invalidCount++
-		return nil, s.invalidCount >= quicInvalidCountThreshold
+		// Not a ClientHello (e.g. server-direction CRYPTO); ignore.
+		return nil, false
 	}
 
 	chLen := int(pl[1])<<16 | int(pl[2])<<8 | int(pl[3])