@@ -1258,14 +1258,20 @@ def unpack(self, length: 'Optional[int]' = None, **kwargs: 'Any') -> 'Data_PCAPN
             global packet data to underlying methods. This is useful when
             the packet data is not available in the current instance.
 
+            A block handed to :meth:`make` as raw :obj:`bytes` is written back
+            verbatim past the Block Type, so the schema :meth:`make` returns
+            holds the octets rather than the block's fields. Those octets are
+            parsed here, off the buffer :meth:`pack` has just filled, which is
+            what a reader of the file would do with them (:issue:`2354`).
+
         """
-        if cast('Optional[Schema_PCAPNG]', self.__header__) is None:
+        header = cast('Optional[Schema_PCAPNG]', self.__header__)
+        if header is None or isinstance(header.block, bytes):
             packet = kwargs.get('__packet__', {})  # packet data
 
-            # NOTE: Reached on the parsing path only, with no flag needed: the
-            # construction path sets ``__header__`` from ``make`` while packing
-            # its own buffer, so the branch above is already false there. A
-            # ``_read`` guard would be a branch that can never be taken.
+            # NOTE: A tail too short for any block is refused here, on the
+            # construction path as on the parsing one: a caller's raw octets
+            # are no more trustworthy than a file's.
             self._check_block_floor(length)
 
             if self._ctx is not None:
@@ -1636,12 +1642,15 @@ def _check_block_floor(self, length: 'Optional[int]') -> 'None':
         passes that check and is caught here instead.
 
         Note:
-            This runs on the parsing path only, since :meth:`unpack` reaches it
-            only when no schema has been built yet and the construction path
-            builds one from :meth:`make` while packing its own buffer. That is
-            what it should do: a constructed block short enough to trip this floor
-            is already reported by :meth:`read`'s own Block Total Length check,
-            which names the length and so says more than an end-of-stream would.
+            This runs on both of :meth:`unpack`'s paths. On the parsing path the
+            tail is what the file has left to read; on the construction path it is
+            the octets of a block handed to :meth:`make` as raw :obj:`bytes`, which
+            :meth:`unpack` parses off the buffer :meth:`pack` has just filled
+            (:issue:`2354`) -- a caller's octets are no more trustworthy than a
+            file's. Octets too short for this floor are therefore refused here
+            rather than by :meth:`read`'s own Block Total Length check, which sees
+            only what gets past the floor and so names an impossible *declared*
+            length, never a tail too short to declare one.
 
             Measuring an undeclared length needs the stream to seek, and nothing
             guards that here because :meth:`__post_init__` has already called
 
@@ -0,0 +1,160 @@
+# -*- coding: utf-8 -*-
+"""A PCAP-NG block handed over as raw bytes parses and rebuilds.
+
+This guards against GitHub issue #2354. ``block`` is typed to accept
+:obj:`bytes`, and the header schema writes such a block back verbatim past the
+Block Type -- but the schema the maker returned then held octets where the
+reader expected fields, so a Decryption Secrets Block built that way raised
+``AttributeError: 'bytes' object has no attribute 'length'``, and so did every
+other block type that got as far as the reader.
+
+The octets are parsed off the buffer the construction path has just packed, the
+way the short-block path parses the body it kept: the block that comes back is
+the one the octets describe, and it rebuilds byte for byte. Octets that are no
+block at all are refused with the library's own errors rather than an
+:exc:`AttributeError`.
+
+A Section Header Block handed raw octets with no section context is the one
+case left out, and it is not pinned here: it never reaches the reader, raising
+``AttributeError: 'PCAPNG' object has no attribute '_byte'`` out of the packer
+instead, since the byte order a section header carries is the one its own maker
+chose and no maker runs for raw octets. That is tracked separately.
+
+Every case builds its own octets in memory and reads no capture. :mod:`pcapkit`
+is imported inside each test, after
+:func:`~tests._support.reimport_once_per_class`.
+
+"""
+
+import unittest
+import warnings
+from typing import TYPE_CHECKING
+
+from tests._support import reimport_once_per_class
+
+if TYPE_CHECKING:
+    from typing import Any
+
+#: Test tier (#1958): a hand-picked round trip, one case per block type.
+__tier__ = 'ALL'
+
+#: A TLS key log, four-octet aligned so the Decryption Secrets Block needs no
+#: padding.
+TLS_LOG = b'CLIENT_RANDOM 00 11\n'
+#: ``TLSK``, the Secrets Type of a TLS key log, as the field carries it.
+TLS_KEY_LOG = 0x544C534B
+
+#: Keywords that build one block for each of the types below, for the round trip
+#: that follows. A Section Header Block is not among them, for the reason the
+#: module docstring gives. A Packet Block and an Enhanced Packet Block both need
+#: a timestamp, their makers reading the parsed data model for a resolution
+#: without one.
+CASES = {
+    'IDB': ('Interface_Description_Block', {'linktype': 1, 'snaplen': 0xFFFF}),
+    'EPB': ('Enhanced_Packet_Block', {'packet_data': b'abcd', 'timestamp': 0}),
+    'PB': ('Packet_Block', {'packet_data': b'abcd', 'timestamp': 0}),
+    'SPB': ('Simple_Packet_Block', {'packet_data': b'abcde'}),
+    'ISB': ('Interface_Statistics_Block', {'interface_id': 0, 'timestamp': 0}),
+    'systemd': ('systemd_Journal_Export_Block', {'entries': b'MESSAGE=hi\n'}),
+    'CB': ('Custom_Block_that_rewriters_can_copy_into_new_files',
+           {'pen': 1, 'data': b'abcd'}),
+    'unknown': ('Reserved_0x00000000', {'data': b'abcd'}),
+}
+
+
+def _dsb_body(secrets: 'bytes') -> 'bytes':
+    """A little-endian Decryption Secrets Block past its Block Type."""
+    assert len(secrets) % 4 == 0, len(secrets)
+    length = 20 + len(secrets)
+    return b''.join((
+        length.to_bytes(4, 'little'),
+        TLS_KEY_LOG.to_bytes(4, 'little'),
+        len(secrets).to_bytes(4, 'little'),
+        secrets,
+        length.to_bytes(4, 'little'),
+    ))
+
+
+class TestPCAPNGRawBlockBytes(unittest.TestCase):
+    """Pin that ``make(block=<bytes>)`` builds a block rather than raising."""
+
+    def setUp(self) -> None:
+        reimport_once_per_class(self)
+
+    def _context(self) -> 'Any':
+        """A one-section context with one Ethernet interface, as the engine builds it."""
+        from pcapkit.const.pcapng.block_type import BlockType
+        from pcapkit.const.reg.linktype import LinkType
+        from pcapkit.foundation.engines.pcapng import Context
+        from pcapkit.protocols.misc.pcapng import PCAPNG
+
+        section = PCAPNG(num=0, sct=1, ctx=None, type=BlockType.Section_Header_Block, block={})
+        context = Context(section.info)  # type: Any
+        interface = PCAPNG(num=1, sct=1, ctx=context, type=BlockType.Interface_Description_Block,
+                           block={'linktype': LinkType.ETHERNET, 'snaplen': 0x40000})
+        context.interfaces.append(interface.info)
+        return context
+
+    def test_a_dsb_handed_its_octets_parses_them(self) -> None:
+        """#2354: the issue's own case, a DSB built from raw bytes."""
+        from pcapkit.const.pcapng.block_type import BlockType
+        from pcapkit.const.pcapng.secrets_type import SecretsType
+        from pcapkit.const.pcapng.tls_key_label import TLSKeyLabel
+        from pcapkit.protocols.misc.pcapng import PCAPNG
+
+        body = _dsb_body(TLS_LOG)
+        block = PCAPNG(num=2, sct=1, ctx=self._context(),
+                       type=BlockType.Decryption_Secrets_Block, block=body)
+
+        self.assertEqual(block.info.secrets_type, SecretsType.TLS_Key_Log)
+        self.assertEqual(block.info.secrets_length, len(TLS_LOG))
+        self.assertEqual(block.info.secrets_data.data, TLS_LOG.decode())
+        self.assertIn(TLSKeyLabel.CLIENT_RANDOM, block.info.secrets_data.entries)
+        # The octets are written back exactly as handed over, after the type.
+        self.assertEqual(block.data,
+                         int(BlockType.Decryption_Secrets_Block).to_bytes(4, 'little') + body)
+
+    def test_every_block_type_handed_its_octets_round_trips(self) -> None:
+        """#2354: raw octets parse as the block they describe, and rebuild byte for byte."""
+        from pcapkit.const.pcapng.block_type import BlockType
+        from pcapkit.protocols.misc.pcapng import PCAPNG
+
+        for label, (name, keywords) in CASES.items():
+            with self.subTest(block=label):
+                type_ = getattr(BlockType, name)
+                context = self._context()
+                with warnings.catch_warnings():
+                    # a Packet Block warns that the block is obsolete
+                    warnings.simplefilter('ignore')
+                    built = PCAPNG(num=2, sct=1, ctx=context, type=type_, block=keywords)
+                    again = PCAPNG(num=2, sct=1, ctx=context, type=type_, block=built.data[4:])
+
+                self.assertEqual(again.data, built.data)
+                self.assertEqual(str(again.info), str(built.info))
+
+    def test_octets_that_are_no_block_are_refused(self) -> None:
+        """#2354: a short or impossible block raises the library's own error."""
+        from pcapkit.const.pcapng.block_type import BlockType
+        from pcapkit.protocols.misc.pcapng import PCAPNG
+        from pcapkit.utilities.exceptions import ProtocolError, StreamEOFError
+
+        context = self._context()
+        # Shorter than the twelve octets a block takes, the Block Type counted.
+        for body in (b'', b'raw', b'\x00' * 7):
+            with self.subTest(body=body):
+                with self.assertRaisesRegex(StreamEOFError, r'fewer than the 12 a block needs'):
+                    PCAPNG(num=2, sct=1, ctx=context,
+                           type=BlockType.Decryption_Secrets_Block, block=body)
+
+        # Long enough, but declaring a Block Total Length of zero.
+        for body in (b'\x00' * 8, b'\x00' * 12):
+            with self.subTest(body=body):
+                with warnings.catch_warnings():
+                    warnings.simplefilter('ignore')
+                    with self.assertRaisesRegex(ProtocolError, r'invalid length: 0'):
+                        PCAPNG(num=2, sct=1, ctx=context,
+                               type=BlockType.Decryption_Secrets_Block, block=body)
+
+
+if __name__ == '__main__':
+    unittest.main()