diff --git a/libavcodec/cbs_h2645.c b/libavcodec/cbs_h2645.c
index 095e449ddc..b432921ecc 100644
--- a/libavcodec/cbs_h2645.c
+++ b/libavcodec/cbs_h2645.c
@@ -553,6 +553,7 @@ static void cbs_h265_free_sei_payload(H265RawSEIPayload *payload)
         av_buffer_unref(&payload->payload.other.data_ref);
         break;
     }
+    av_buffer_unref(&payload->extension_data.data_ref);
 }
 
 static void cbs_h265_free_sei(void *opaque, uint8_t *content)
diff --git a/libavcodec/cbs_h265.h b/libavcodec/cbs_h265.h
index 2c1e153ad9..73897f77a4 100644
--- a/libavcodec/cbs_h265.h
+++ b/libavcodec/cbs_h265.h
@@ -715,6 +715,7 @@ typedef struct H265RawSEIPayload {
             AVBufferRef *data_ref;
         } other;
     } payload;
+    H265RawExtensionData extension_data;
 } H265RawSEIPayload;
 
 typedef struct H265RawSEI {
diff --git a/libavcodec/cbs_h265_syntax_template.c b/libavcodec/cbs_h265_syntax_template.c
index ed08b06e9c..45838467b7 100644
--- a/libavcodec/cbs_h265_syntax_template.c
+++ b/libavcodec/cbs_h265_syntax_template.c
@@ -1564,7 +1564,8 @@ static int FUNC(slice_segment_header)(CodedBitstreamContext *ctx, RWContext *rw,
 
 static int FUNC(sei_buffering_period)(CodedBitstreamContext *ctx, RWContext *rw,
                                       H265RawSEIBufferingPeriod *current,
-                                      uint32_t *payload_size)
+                                      uint32_t *payload_size,
+                                      int *more_data)
 {
     CodedBitstreamH265Context *h265 = ctx->priv_data;
     const H265RawSPS *sps;
@@ -1658,8 +1659,15 @@ static int FUNC(sei_buffering_period)(CodedBitstreamContext *ctx, RWContext *rw,
     else
         infer(use_alt_cpb_params_flag, 0);
 #else
-    if (current->use_alt_cpb_params_flag)
+    // If unknown extension data exists, then use_alt_cpb_params_flag is
+    // coded in the bitstream and must be written even if it's 0.
+    if (current->use_alt_cpb_params_flag || *more_data) {
         flag(use_alt_cpb_params_flag);
+        // Ensure this bit is not the last in the payload by making the
+        // more_data_in_payload() check evaluate to true, so it may not
+        // be mistaken as something else by decoders.
+        *more_data = 1;
+    }
 #endif
 
     return 0;
@@ -2057,11 +2065,48 @@ static int FUNC(sei_alpha_channel_info)(CodedBitstreamContext *ctx,
     return 0;
 }
 
+static int FUNC(payload_extension)(CodedBitstreamContext *ctx, RWContext *rw,
+                                   H265RawExtensionData *current, uint32_t payload_size,
+                                   int cur_pos)
+{
+    int err;
+    size_t byte_length, k;
+
+#ifdef READ
+    GetBitContext tmp;
+    int bits_left, payload_zero_bits;
+
+    if (!cbs_h265_payload_extension_present(rw, payload_size, cur_pos))
+        return 0;
+
+    bits_left = 8 * payload_size - cur_pos;
+    tmp = *rw;
+    if (bits_left > 8)
+        skip_bits_long(&tmp, bits_left - 8);
+    payload_zero_bits = get_bits(&tmp, FFMIN(bits_left, 8));
+    if (!payload_zero_bits)
+        return AVERROR_INVALIDDATA;
+    payload_zero_bits = ff_ctz(payload_zero_bits);
+    current->bit_length = bits_left - payload_zero_bits - 1;
+    allocate(current->data, (current->bit_length + 7) / 8);
+#endif
+
+    byte_length = (current->bit_length + 7) / 8;
+    for (k = 0; k < byte_length; k++) {
+        int length = FFMIN(current->bit_length - k * 8, 8);
+        xu(length, reserved_payload_extension_data, current->data[k],
+           0, MAX_UINT_BITS(length), 0);
+    }
+
+    return 0;
+}
+
 static int FUNC(sei_payload)(CodedBitstreamContext *ctx, RWContext *rw,
                              H265RawSEIPayload *current, int prefix)
 {
     int err, i;
-    int start_position, end_position;
+    int start_position, current_position, end_position;
+    int more_data = !!current->extension_data.bit_length;
 
 #ifdef READ
     start_position = get_bits_count(rw);
@@ -2093,8 +2138,15 @@ static int FUNC(sei_payload)(CodedBitstreamContext *ctx, RWContext *rw,
         CHECK(FUNC(sei_ ## name)(ctx, rw, &current->payload.name, \
                                  &current->payload_size)); \
         break
+#define SEI_TYPE_E(type, prefix_valid, suffix_valid, name) \
+    case HEVC_SEI_TYPE_ ## type: \
+        SEI_TYPE_CHECK_VALID(name, prefix_valid, suffix_valid); \
+        CHECK(FUNC(sei_ ## name)(ctx, rw, &current->payload.name, \
+                                 &current->payload_size, \
+                                 &more_data)); \
+        break
 
-        SEI_TYPE_S(BUFFERING_PERIOD,         1, 0, buffering_period);
+        SEI_TYPE_E(BUFFERING_PERIOD,         1, 0, buffering_period);
         SEI_TYPE_N(PICTURE_TIMING,           1, 0, pic_timing);
         SEI_TYPE_N(PAN_SCAN_RECT,            1, 0, pan_scan_rect);
         SEI_TYPE_S(USER_DATA_REGISTERED_ITU_T_T35,
@@ -2125,7 +2177,16 @@ static int FUNC(sei_payload)(CodedBitstreamContext *ctx, RWContext *rw,
         }
     }
 
-    if (byte_alignment(rw)) {
+    // more_data_in_payload()
+#ifdef READ
+    current_position = get_bits_count(rw) - start_position;
+    if (current_position != 8 * current->payload_size) {
+#else
+    current_position = put_bits_count(rw) - start_position;
+    if (byte_alignment(rw) || more_data) {
+#endif
+        CHECK(FUNC(payload_extension)(ctx, rw, &current->extension_data,
+                                      current->payload_size, current_position));
         fixed(1, bit_equal_to_one, 1);
         while (byte_alignment(rw))
             fixed(1, bit_equal_to_zero, 0);
