diff --git a/context.go b/context.go
index ca6dd87..903d3a9 100644
--- a/context.go
+++ b/context.go
@@ -308,67 +308,79 @@ func (s *srtpSSRCState) updateRolloverCount(sequenceNumber uint16, difference in
 	}
 }
 
-func (c *Context) getSRTPSSRCState(ssrc uint32) *srtpSSRCState {
-	s, ok := c.srtpSSRCStates[ssrc]
+func (c *Context) getSRTPSSRCState(ssrc uint32, keepNew bool) (*srtpSSRCState, bool) {
+	state, ok := c.srtpSSRCStates[ssrc]
 	if ok {
-		return s
+		return state, true
 	}
 
-	s = &srtpSSRCState{
+	state = &srtpSSRCState{
 		ssrc:           ssrc,
 		replayDetector: c.newSRTPReplayDetector(),
 	}
-	c.srtpSSRCStates[ssrc] = s
+	if keepNew {
+		c.srtpSSRCStates[ssrc] = state
+	}
 
-	return s
+	return state, false
 }
 
-func (c *Context) getSRTCPSSRCState(ssrc uint32) *srtcpSSRCState {
-	s, ok := c.srtcpSSRCStates[ssrc]
+func (c *Context) getSRTCPSSRCState(ssrc uint32, keepNew bool) (*srtcpSSRCState, bool) {
+	state, ok := c.srtcpSSRCStates[ssrc]
 	if ok {
-		return s
+		return state, true
 	}
 
-	s = &srtcpSSRCState{
+	state = &srtcpSSRCState{
 		ssrc:           ssrc,
 		replayDetector: c.newSRTCPReplayDetector(),
 	}
-	c.srtcpSSRCStates[ssrc] = s
+	if keepNew {
+		c.srtcpSSRCStates[ssrc] = state
+	}
+
+	return state, false
+}
+
+func (c *Context) setSRTPSSRCState(state *srtpSSRCState) {
+	c.srtpSSRCStates[state.ssrc] = state
+}
 
-	return s
+func (c *Context) setSRTCPSSRCState(state *srtcpSSRCState) {
+	c.srtcpSSRCStates[state.ssrc] = state
 }
 
 // ROC returns SRTP rollover counter value of specified SSRC.
 func (c *Context) ROC(ssrc uint32) (uint32, bool) {
-	s, ok := c.srtpSSRCStates[ssrc]
+	state, ok := c.srtpSSRCStates[ssrc]
 	if !ok {
 		return 0, false
 	}
 
-	return uint32(s.index >> 16), true //nolint:gosec // G115
+	return uint32(state.index >> 16), true //nolint:gosec // G115
 }
 
 // SetROC sets SRTP rollover counter value of specified SSRC.
 func (c *Context) SetROC(ssrc uint32, roc uint32) {
-	s := c.getSRTPSSRCState(ssrc)
-	s.index = uint64(roc) << 16
-	s.rolloverHasProcessed = false
+	state, _ := c.getSRTPSSRCState(ssrc, true)
+	state.index = uint64(roc) << 16
+	state.rolloverHasProcessed = false
 }
 
 // Index returns SRTCP index value of specified SSRC.
 func (c *Context) Index(ssrc uint32) (uint32, bool) {
-	s, ok := c.srtcpSSRCStates[ssrc]
+	state, ok := c.srtcpSSRCStates[ssrc]
 	if !ok {
 		return 0, false
 	}
 
-	return s.srtcpIndex, true
+	return state.srtcpIndex, true
 }
 
 // SetIndex sets SRTCP index value of specified SSRC.
 func (c *Context) SetIndex(ssrc uint32, index uint32) {
-	s := c.getSRTCPSSRCState(ssrc)
-	s.srtcpIndex = index % (maxSRTCPIndex + 1)
+	state, _ := c.getSRTCPSSRCState(ssrc, true)
+	state.srtcpIndex = index % (maxSRTCPIndex + 1)
 }
 
 //nolint:cyclop
diff --git a/session_srtcp_test.go b/session_srtcp_test.go
index 820e220..178fd6e 100644
--- a/session_srtcp_test.go
+++ b/session_srtcp_test.go
@@ -306,6 +306,56 @@ func getSenderSSRC(t *testing.T, stream *ReadStreamSRTCP) (ssrc uint32, err erro
 	return pli.SenderSSRC, nil
 }
 
+func TestSessionSRTCPFailedAuthDoesNotGrowStreams(t *testing.T) {
+	lim := test.TimeOut(time.Second * 10)
+	defer lim.Stop()
+
+	report := test.CheckRoutines(t)
+	defer report()
+
+	const (
+		badMediaSSRC  = uint32(0xDEADBEEF)
+		goodMediaSSRC = uint32(0x12345678)
+	)
+
+	aSession, bSession := buildSessionSRTCPPair(t)
+
+	// Encrypt a PLI for badMediaSSRC with the correct key, then corrupt its auth tag
+	// so that bSession's remote context rejects it without creating a stream entry.
+	badEncrypted, err := encryptSRTCP(aSession.session.localContext,
+		&rtcp.PictureLossIndication{MediaSSRC: badMediaSSRC})
+	assert.NoError(t, err)
+	badEncrypted[len(badEncrypted)-1] ^= 0xFF // corrupt last byte of auth tag
+
+	_, err = aSession.session.nextConn.Write(badEncrypted)
+	assert.NoError(t, err)
+
+	// Write a valid PLI for goodMediaSSRC so we can synchronize via AcceptStream.
+	// The read goroutine processes packets sequentially, so when AcceptStream
+	// returns the bad packet has already been handled.
+	goodEncrypted, err := encryptSRTCP(aSession.session.localContext,
+		&rtcp.PictureLossIndication{MediaSSRC: goodMediaSSRC})
+	assert.NoError(t, err)
+
+	_, err = aSession.session.nextConn.Write(goodEncrypted)
+	assert.NoError(t, err)
+
+	_, receivedSSRC, err := bSession.AcceptStream()
+	assert.NoError(t, err)
+	assert.Equal(t, goodMediaSSRC, receivedSSRC, "AcceptStream must return the valid SSRC")
+
+	bSession.session.readStreamsLock.Lock()
+	streamCount := len(bSession.session.readStreams)
+	_, hasBadMediaSSRC := bSession.session.readStreams[badMediaSSRC]
+	bSession.session.readStreamsLock.Unlock()
+
+	assert.Equal(t, 1, streamCount, "readStreams must not grow after failed authentication")
+	assert.False(t, hasBadMediaSSRC, "readStreams must not contain the SSRC from the rejected packet")
+
+	assert.NoError(t, aSession.Close())
+	assert.NoError(t, bSession.Close())
+}
+
 func encryptSRTCP(context *Context, pkt rtcp.Packet) ([]byte, error) {
 	decryptedRaw, err := pkt.Marshal()
 	if err != nil {
diff --git a/session_srtp.go b/session_srtp.go
index 203852d..7b35b69 100644
--- a/session_srtp.go
+++ b/session_srtp.go
@@ -189,6 +189,16 @@ func (s *SessionSRTP) decrypt(buf []byte) error {
 		return err
 	}
 
+	// Decrypt and authenticate the packet before using the SSRC from the header.
+	// The SSRC field is part of the unauthenticated RTP header, so it must not be
+	// used to allocate per-SSRC state (stream, replay detector, etc.) until after
+	// the auth tag has been verified. Doing so before authentication would allow an
+	// unauthenticated peer to exhaust memory by spoofing arbitrary SSRCs.
+	decrypted, err := s.remoteContext.decryptRTP(buf, buf, header, headerLen)
+	if err != nil {
+		return err
+	}
+
 	r, isNew := s.session.getOrCreateReadStream(header.SSRC, s, newReadStreamSRTP)
 	if r == nil {
 		return nil // Session has been closed
@@ -204,11 +214,6 @@ func (s *SessionSRTP) decrypt(buf []byte) error {
 		return errFailedTypeAssertion
 	}
 
-	decrypted, err := s.remoteContext.decryptRTP(buf, buf, header, headerLen)
-	if err != nil {
-		return err
-	}
-
 	_, err = readStream.write(decrypted)
 	if err != nil {
 		return err
diff --git a/session_srtp_test.go b/session_srtp_test.go
index dbca950..cdeb29b 100644
--- a/session_srtp_test.go
+++ b/session_srtp_test.go
@@ -349,6 +349,62 @@ func assertPayloadSRTP(
 	return hdr.SequenceNumber, nil
 }
 
+func TestSessionSRTPFailedAuthDoesNotGrowStreams(t *testing.T) {
+	lim := test.TimeOut(time.Second * 5)
+	defer lim.Stop()
+
+	report := test.CheckRoutines(t)
+	defer report()
+
+	const (
+		badSSRC  = uint32(0xDEADBEEF)
+		goodSSRC = uint32(0x12345678)
+	)
+
+	aSession, bSession := buildSessionSRTPPair(t)
+
+	// Encrypt a packet for badSSRC with the correct key, then corrupt its auth tag
+	// so that bSession's remote context rejects it without creating a stream entry.
+	badPkt := &rtp.Packet{
+		Payload: []byte{0x00, 0x01, 0x02, 0x03},
+		Header:  rtp.Header{SSRC: badSSRC, SequenceNumber: 1},
+	}
+	badEncrypted, err := encryptSRTP(aSession.session.localContext, badPkt)
+	assert.NoError(t, err)
+	badEncrypted[len(badEncrypted)-1] ^= 0xFF // corrupt last byte of auth tag
+
+	_, err = aSession.session.nextConn.Write(badEncrypted)
+	assert.NoError(t, err)
+
+	// Write a valid packet for goodSSRC so we can synchronize via AcceptStream.
+	// The read goroutine processes packets sequentially, so when AcceptStream
+	// returns the bad packet has already been handled.
+	goodPkt := &rtp.Packet{
+		Payload: []byte{0x00, 0x01, 0x02, 0x03},
+		Header:  rtp.Header{SSRC: goodSSRC, SequenceNumber: 1},
+	}
+	goodEncrypted, err := encryptSRTP(aSession.session.localContext, goodPkt)
+	assert.NoError(t, err)
+
+	_, err = aSession.session.nextConn.Write(goodEncrypted)
+	assert.NoError(t, err)
+
+	_, receivedSSRC, err := bSession.AcceptStream()
+	assert.NoError(t, err)
+	assert.Equal(t, goodSSRC, receivedSSRC, "AcceptStream must return the valid SSRC")
+
+	bSession.session.readStreamsLock.Lock()
+	streamCount := len(bSession.session.readStreams)
+	_, hasBadSSRC := bSession.session.readStreams[badSSRC]
+	bSession.session.readStreamsLock.Unlock()
+
+	assert.Equal(t, 1, streamCount, "readStreams must not grow after failed authentication")
+	assert.False(t, hasBadSSRC, "readStreams must not contain the SSRC from the rejected packet")
+
+	assert.NoError(t, aSession.Close())
+	assert.NoError(t, bSession.Close())
+}
+
 func encryptSRTP(context *Context, pkt *rtp.Packet) ([]byte, error) {
 	decryptedRaw, err := pkt.Marshal()
 	if err != nil {
diff --git a/srtcp.go b/srtcp.go
index 5874f0d..496fb19 100644
--- a/srtcp.go
+++ b/srtcp.go
@@ -47,8 +47,18 @@ func (c *Context) decryptRTCP(dst, encrypted []byte) ([]byte, error) {
 	index := c.cipher.getRTCPIndex(encrypted)
 	ssrc := binary.BigEndian.Uint32(encrypted[4:])
 
-	s := c.getSRTCPSSRCState(ssrc)
-	markAsValid, ok := s.replayDetector.Check(uint64(index))
+	// The SSRC is read from the unauthenticated RTCP header at this point.
+	// getSRTCPSSRCState is called in read-only mode so that no new map entry is
+	// inserted until after the auth tag has been verified. The state is committed
+	// to the map by setSRTCPSSRCState only after markAsValid() succeeds below.
+	ssrcState, existingState := c.getSRTCPSSRCState(ssrc, false)
+
+	// The replay check is intentionally performed before authentication.
+	// Rejecting already-seen sequence numbers here avoids the CPU cost of
+	// AES decryption and HMAC/GCM verification on flooded duplicate packets.
+	// Safety relies on the replay detector only committing the index as "seen"
+	// when markAsValid() is explicitly called after successful authentication.
+	markAsValid, ok := ssrcState.replayDetector.Check(uint64(index))
 	if !ok {
 		return nil, &duplicatedError{Proto: "srtcp", SSRC: ssrc, Index: index}
 	}
@@ -70,6 +80,10 @@ func (c *Context) decryptRTCP(dst, encrypted []byte) ([]byte, error) {
 
 	markAsValid()
 
+	if !existingState {
+		c.setSRTCPSSRCState(ssrcState)
+	}
+
 	return out, nil
 }
 
@@ -92,7 +106,7 @@ func (c *Context) encryptRTCP(dst, decrypted []byte) ([]byte, error) {
 	}
 
 	ssrc := binary.BigEndian.Uint32(decrypted[4:])
-	ssrcState := c.getSRTCPSSRCState(ssrc)
+	ssrcState, _ := c.getSRTCPSSRCState(ssrc, true)
 
 	if ssrcState.srtcpIndex >= maxSRTCPIndex {
 		// ... when 2^48 SRTP packets or 2^31 SRTCP packets have been secured with the same key
diff --git a/srtcp_test.go b/srtcp_test.go
index 1fa92e2..cae6626 100644
--- a/srtcp_test.go
+++ b/srtcp_test.go
@@ -703,3 +703,37 @@ func TestRTCPSwitchMKI(t *testing.T) { //nolint:cyclop
 		})
 	}
 }
+
+func TestSRTCPFailedAuthDoesNotGrowSSRCMap(t *testing.T) {
+	testSRTCPFailedAuthDoesNotGrowSSRCMap := func(t *testing.T, profile ProtectionProfile) {
+		t.Helper()
+
+		encryptCtx, err := buildTestContext(profile)
+		assert.NoError(t, err)
+
+		// Use all-zero key and salt so the authentication tag will never match.
+		keyLen, err := profile.KeyLen()
+		assert.NoError(t, err)
+
+		saltLen, err := profile.SaltLen()
+		assert.NoError(t, err)
+
+		decryptCtx, err := CreateContext(make([]byte, keyLen), make([]byte, saltLen), profile)
+		assert.NoError(t, err)
+
+		pktRaw, err := rtcp.Marshal([]rtcp.Packet{&rtcp.PictureLossIndication{MediaSSRC: 0xDEADBEEF}})
+		assert.NoError(t, err)
+
+		encrypted, err := encryptCtx.EncryptRTCP(nil, pktRaw, nil)
+		assert.NoError(t, err)
+
+		_, err = decryptCtx.DecryptRTCP(nil, encrypted, nil)
+		assert.Error(t, err, "DecryptRTCP must fail when key does not match")
+
+		assert.Empty(t, decryptCtx.srtcpSSRCStates,
+			"SRTCP SSRC state map must not grow after failed authentication")
+	}
+
+	t.Run("CTR", func(t *testing.T) { testSRTCPFailedAuthDoesNotGrowSSRCMap(t, profileCTR) })
+	t.Run("GCM", func(t *testing.T) { testSRTCPFailedAuthDoesNotGrowSSRCMap(t, profileGCM) })
+}
diff --git a/srtp.go b/srtp.go
index 9c611ab..e5591e9 100644
--- a/srtp.go
+++ b/srtp.go
@@ -20,6 +20,7 @@ Simplified structure of SRTP Packets:
 - Auth Tag - used by non-AEAD profiles only. When RCC is used with AEAD profiles, the ROC is sent here.
 */
 
+// nolint:cyclop
 func (c *Context) decryptRTP(dst, ciphertext []byte, header *rtp.Header, headerLen int) ([]byte, error) {
 	authTagLen, err := c.cipher.AuthTagRTPLen()
 	if err != nil {
@@ -39,7 +40,11 @@ func (c *Context) decryptRTP(dst, ciphertext []byte, header *rtp.Header, headerL
 		return nil, fmt.Errorf("%w: %d", errTooShortRTP, len(ciphertext))
 	}
 
-	ssrcState := c.getSRTPSSRCState(header.SSRC)
+	// The SSRC in the RTP header is unauthenticated at this point. getSRTPSSRCState is
+	// called in read-only mode (existingState tracks whether it was pre-existing) so that
+	// no new map entry is inserted until after the auth tag has been verified. The state
+	// is committed to the map by setSRTPSSRCState only after markAsValid() succeeds below.
+	ssrcState, existingState := c.getSRTPSSRCState(header.SSRC, false)
 
 	var roc uint32
 	var diff int64
@@ -55,6 +60,11 @@ func (c *Context) decryptRTP(dst, ciphertext []byte, header *rtp.Header, headerL
 		diff = int64(ssrcState.index) - int64(index) //nolint:gosec
 	}
 
+	// The replay check is intentionally performed before authentication.
+	// Rejecting already-seen sequence numbers here avoids the CPU cost of
+	// AES decryption and HMAC/GCM verification on flooded duplicate packets.
+	// Safety relies on the replay detector only committing the index as "seen"
+	// when markAsValid() is explicitly called after successful authentication.
 	markAsValid, ok := ssrcState.replayDetector.Check(index)
 	if !ok {
 		return nil, &duplicatedError{
@@ -87,6 +97,10 @@ func (c *Context) decryptRTP(dst, ciphertext []byte, header *rtp.Header, headerL
 	markAsValid()
 	ssrcState.updateRolloverCount(header.SequenceNumber, diff, hasRocInPacket, roc)
 
+	if !existingState {
+		c.setSRTPSSRCState(ssrcState)
+	}
+
 	return dst, nil
 }
 
@@ -134,8 +148,8 @@ func (c *Context) encryptRTP(dst []byte, header *rtp.Header, headerLen int, plai
 		return nil, errUnsupportedHeaderExtension
 	}
 
-	s := c.getSRTPSSRCState(header.SSRC)
-	roc, diff, ovf := s.nextRolloverCount(header.SequenceNumber)
+	ssrcState, _ := c.getSRTPSSRCState(header.SSRC, true)
+	roc, diff, ovf := ssrcState.nextRolloverCount(header.SequenceNumber)
 	if ovf {
 		// ... when 2^48 SRTP packets or 2^31 SRTCP packets have been secured with the same key
 		// (whichever occurs before), the key management MUST be called to provide new master key(s)
@@ -143,7 +157,7 @@ func (c *Context) encryptRTP(dst []byte, header *rtp.Header, headerLen int, plai
 		// https://www.rfc-editor.org/rfc/rfc3711#section-9.2
 		return nil, errExceededMaxPackets
 	}
-	s.updateRolloverCount(header.SequenceNumber, diff, false, 0)
+	ssrcState.updateRolloverCount(header.SequenceNumber, diff, false, 0)
 
 	rocInPacket := c.rccMode != RCCModeNone && header.SequenceNumber%c.rocTransmitRate == 0
 
diff --git a/srtp_test.go b/srtp_test.go
index 95c0cb6..9177f92 100644
--- a/srtp_test.go
+++ b/srtp_test.go
@@ -909,3 +909,42 @@ func TestRTPSwitchMKI(t *testing.T) { //nolint:cyclop
 		assert.NoError(t, err)
 	}
 }
+
+func TestSRTPFailedAuthDoesNotGrowSSRCMap(t *testing.T) {
+	testSRTPFailedAuthDoesNotGrowSSRCMap := func(t *testing.T, profile ProtectionProfile) {
+		t.Helper()
+
+		encryptCtx, err := buildTestContext(profile)
+		assert.NoError(t, err)
+
+		// Use all-zero key and salt so the authentication tag will never match.
+		keyLen, err := profile.KeyLen()
+		assert.NoError(t, err)
+
+		saltLen, err := profile.SaltLen()
+		assert.NoError(t, err)
+
+		decryptCtx, err := CreateContext(make([]byte, keyLen), make([]byte, saltLen), profile)
+		assert.NoError(t, err)
+
+		pkt := &rtp.Packet{
+			Payload: rtpTestCaseDecrypted(),
+			Header:  rtp.Header{SequenceNumber: 5000, SSRC: 0xDEADBEEF},
+		}
+
+		pktRaw, err := pkt.Marshal()
+		assert.NoError(t, err)
+
+		encrypted, err := encryptCtx.EncryptRTP(nil, pktRaw, nil)
+		assert.NoError(t, err)
+
+		_, err = decryptCtx.DecryptRTP(nil, encrypted, nil)
+		assert.Error(t, err, "DecryptRTP must fail when key does not match")
+
+		assert.Empty(t, decryptCtx.srtpSSRCStates,
+			"SRTP SSRC state map must not grow after failed authentication")
+	}
+
+	t.Run("CTR", func(t *testing.T) { testSRTPFailedAuthDoesNotGrowSSRCMap(t, profileCTR) })
+	t.Run("GCM", func(t *testing.T) { testSRTPFailedAuthDoesNotGrowSSRCMap(t, profileGCM) })
+}