diff --git a/CHANGELOG.md b/CHANGELOG.md
index f12c178..790d96d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -17,6 +17,23 @@
 
 ### Fixed
 
+- **[SECURITY][SDK]** `attach_receipt()`, `attach_attestation()`, and
+  `attach_approvals()` could crash with `cbor2.CBOREncodeError` instead of
+  the documented `CoseError` on a malformed COSE envelope. `_decode_tagged()`
+  checked that the unprotected header was a CBOR map but never inspected
+  its contents; a stray break byte (`0xff`) in the wrong position decodes
+  into cbor2's internal break-marker sentinel rather than raising, and that
+  sentinel travelled through `attach_unprotected()` undetected until the
+  re-encode at the end, where cbor2 cannot serialize it. Same underlying
+  cbor2 quirk as the signature-slot check `_decode_tagged()` already applies
+  to `body[3]`; this closes the same hole for `body[1]`'s contents.
+  `_decode_tagged()` now recursively rejects the sentinel anywhere in the
+  unprotected header - inside a `Mapping`/`list`/`tuple`, inside a
+  `CBORTag` (an unrecognised semantic tag), and inside a `set`/`frozenset`
+  (cbor2 auto-decodes tag 258 to a `set`) - before any caller touches it,
+  and `attach_unprotected()` converts any residual `CBOREncodeError` to
+  `CoseStructureError` as a second layer.
+
 - **[SECURITY][SDK]** `_strict_schema_violations()` tolerated a missing
   top-level `issuer` claim for **any** manifest version, not just v0.1. The
   exception was added for legacy v0.1 records, which predate the `issuer`
diff --git a/python/src/agent_manifest/_cose.py b/python/src/agent_manifest/_cose.py
index 74a5938..55c16ff 100644
--- a/python/src/agent_manifest/_cose.py
+++ b/python/src/agent_manifest/_cose.py
@@ -409,7 +409,16 @@ def attach_unprotected(cose_bytes: bytes, label: Union[int, str], value: Any) ->
     unprotected[label] = value
     body = list(body)
     body[1] = unprotected
-    return cbor2.dumps(cbor2.CBORTag(tag, body), canonical=True)
+    try:
+        return cbor2.dumps(cbor2.CBORTag(tag, body), canonical=True)
+    except cbor2.CBOREncodeError as exc:
+        # Belt-and-suspenders: _decode_tagged() already rejects the known
+        # undecodable sentinel before we get here. This catches anything
+        # else that slips through - a different cbor2 version, a value this
+        # function itself added - so the documented contract ("this raises
+        # CoseError, nothing else") holds even if the front-door check ever
+        # misses something.
+        raise CoseStructureError(f"unprotected header is not re-encodable: {exc}") from exc
 
 
 def attach_receipt(cose_bytes: bytes, receipt: bytes) -> bytes:
@@ -516,6 +525,54 @@ def _plain(value: Any) -> Any:
     return value
 
 
+def _reject_cbor_sentinels(value: Any, *, what: str) -> None:
+    """Recursively reject cbor2's internal break-marker sentinel.
+
+    A stray break byte (``0xff``) in the wrong position does not always make
+    cbor2 raise during decode: depending on where it lands, cbor2 hands back
+    its own internal marker object instead - a bare ``object()``, never a
+    subclass, never something a normal decode produces otherwise. That
+    marker cannot be re-encoded, so anything holding one dies later with
+    ``cbor2.CBOREncodeError`` when a caller (``attach_unprotected`` and
+    everything built on it) tries to write the structure back out.
+
+    ``body[3]`` (the signature slot) is checked per-tag in ``_decode_tagged``
+    itself; this function closes the same hole for the unprotected header,
+    whose *values* were never inspected past "is it a Mapping". It matches on
+    the exact marker type - ``type(value) is object`` - rather than
+    allow-listing legitimate types, so it can never reject a manifest that
+    decodes normally, no matter which types cbor2's tag support adds in a
+    future version (``UUID``, ``Decimal``, ``datetime`` and friends already
+    round-trip fine and are left untouched).
+
+    Every CBOR container type that can hold a nested value is walked, not
+    just the ones the unprotected header's own top-level shape happens to
+    use: ``Mapping`` (both keys and values - cbor2 6.x hands back an
+    immutable ``frozendict``, itself a ``Mapping``), ``list``/``tuple``
+    (array), ``set``/``frozenset`` (cbor2 auto-decodes semantic tag 258 to a
+    plain ``set``), and ``cbor2.CBORTag`` (any tag cbor2 does *not* have a
+    built-in decoder for is handed back as a ``CBORTag`` wrapping its
+    payload, e.g. an unrecognised or future semantic tag). A sentinel can be
+    tucked inside any of these - ``{1: CBORTag(9999, [sentinel])}`` or
+    ``{1: {sentinel}}`` decode without error and without matching the old
+    Mapping/list/tuple-only check - so all of them are recursed into.
+    """
+    if type(value) is object:
+        raise CoseStructureError(
+            f"{what} contains an undecodable CBOR value "
+            "(a malformed indefinite-length break byte)"
+        )
+    if isinstance(value, Mapping):
+        for key, item in value.items():
+            _reject_cbor_sentinels(key, what=what)
+            _reject_cbor_sentinels(item, what=what)
+    elif isinstance(value, cbor2.CBORTag):
+        _reject_cbor_sentinels(value.value, what=what)
+    elif isinstance(value, (list, tuple, set, frozenset)):
+        for item in value:
+            _reject_cbor_sentinels(item, what=what)
+
+
 def _decode_tagged(cose_bytes: bytes) -> tuple[int, list[Any]]:
     """Decode exactly one tagged COSE object and return ``(tag, body)``.
 
@@ -563,6 +620,11 @@ def _decode_tagged(cose_bytes: bytes) -> tuple[int, list[Any]]:
     # cbor2 hands back an immutable mapping for a map inside a tag.
     if not isinstance(body[1], Mapping):
         raise CoseStructureError("unprotected header must be a map")
+    # Being a Mapping only proves the outer shape; a key or value inside it
+    # can still be cbor2's undecodable break-marker sentinel (see
+    # _reject_cbor_sentinels). Rejected here, at parse time, rather than
+    # letting it travel into attach_unprotected() and die on re-encode.
+    _reject_cbor_sentinels(body[1], what="unprotected header")
     if not isinstance(body[2], bytes):
         raise CoseStructureError("payload must be a byte string, inline not detached")
     # The signature slot was the one element never type-checked here, which let
diff --git a/python/tests/test_cose.py b/python/tests/test_cose.py
index 8402a5d..72f232f 100644
--- a/python/tests/test_cose.py
+++ b/python/tests/test_cose.py
@@ -1542,3 +1542,164 @@ def test_signature_slot_type_is_checked_at_decode():
     ):
         with pytest.raises(CoseError, match="signature must be a byte string"):
             call()
+
+
+def test_unprotected_header_sentinel_is_checked_at_decode():
+    """A value inside the unprotected header cannot be an undecodable sentinel.
+
+    Found by ``fuzz_cose`` (ClusterFuzzLite). Same underlying cbor2 quirk as
+    ``test_signature_slot_type_is_checked_at_decode`` above - a stray break
+    byte decodes into cbor2's internal break marker, a bare ``object()`` -
+    but this time the marker lands as a *value inside the unprotected header
+    map* rather than in the signature slot. ``_decode_tagged`` checked that
+    the unprotected header was a ``Mapping`` but never inspected what was
+    inside it, so this envelope reached ``attach_unprotected``, which copies
+    the map and re-encodes it, and died there with ``CBOREncodeError``
+    instead of the ``CoseError`` every caller is written against.
+
+    The bytes below are the fuzzer's own reproducer (with the trailing byte
+    atheris's ``FuzzedDataProvider`` consumed for its ``choice`` selector
+    already stripped off), kept verbatim.
+    """
+    envelope = bytes.fromhex(
+        "d28443cbffffa5a032d825500000cbffffa5a032d825500000ff407fff"
+        "0000000041a0a04040"
+    )
+
+    for call in (
+        lambda: attach_receipt(envelope, b"\xa0"),
+        lambda: attach_unprotected(envelope, 1, b"x"),
+        lambda: attach_attestation(envelope, {"platform": "x"}),
+        lambda: attach_approvals(envelope, [{"approver_id": "a"}]),
+    ):
+        with pytest.raises(CoseStructureError, match="undecodable CBOR value"):
+            call()
+
+    # decode_cose_manifest never re-encodes the unprotected header, so this
+    # same envelope is expected to fail for an unrelated, earlier reason
+    # (there is no valid JSON payload here) rather than leak anything.
+    with pytest.raises(CoseError):
+        decode_cose_manifest(envelope)
+
+
+def test_attach_unprotected_rejects_a_caller_supplied_unencodable_value():
+    """``attach_unprotected`` also guards against a *caller's own* bad value.
+
+    ``_decode_tagged`` only validates what came from *cose_bytes*; the value
+    a caller passes in to attach is never inspected before being merged into
+    the header and re-encoded. A plain, non-CBOR-encodable Python object
+    there hits the same ``cbor2.CBOREncodeError`` at re-encode time, so
+    ``attach_unprotected``'s belt-and-suspenders ``except`` clause is the
+    only thing standing between a caller mistake and a leaked library
+    exception. This is that mistake, made deliberately.
+    """
+
+    class NotCborEncodable:
+        pass
+
+    signed = sign_cose_sign1(base_manifest(), KP)
+    with pytest.raises(CoseStructureError, match="not re-encodable"):
+        attach_attestation(signed, NotCborEncodable())
+
+
+def test_reject_cbor_sentinels_recurses_into_cbortag_and_set():
+    """``_reject_cbor_sentinels`` must walk every CBOR container, not just
+    ``Mapping``/``list``/``tuple``.
+
+    Code review on the PR that introduced this check found that it stopped
+    at the boundary of ``cbor2.CBORTag`` and ``set``/``frozenset``: cbor2
+    hands back a ``CBORTag`` for any semantic tag it has no built-in decoder
+    for, and auto-decodes tag 258 to a plain ``set``. A sentinel tucked
+    inside either of those - ``CBORTag(9999, [sentinel])`` or ``{sentinel}``
+    - decoded without error and without being caught, so it would reach
+    ``attach_unprotected`` and die on re-encode, or (worse - see the next
+    test) sail straight through ``decode_cose_manifest`` with nothing raised
+    at all. This exercises the fixed recursion directly against every shape
+    it now covers.
+    """
+    from agent_manifest._cose import _reject_cbor_sentinels
+
+    sentinel = object()
+    rejected_cases = {
+        "sentinel inside an unsupported CBORTag": cbor2.CBORTag(9999, [sentinel]),
+        "sentinel inside a CBORTag(258) wrapping a set-shaped payload": cbor2.CBORTag(
+            258, [sentinel]
+        ),
+        "sentinel inside a bare set": {sentinel},
+        "sentinel inside a bare frozenset": frozenset([sentinel]),
+        "sentinel doubly nested: list -> CBORTag -> tuple": [
+            cbor2.CBORTag(5, (sentinel,))
+        ],
+        "sentinel as a dict key inside a CBORTag": cbor2.CBORTag(7, {sentinel: 1}),
+    }
+    for value in rejected_cases.values():
+        with pytest.raises(CoseStructureError, match="undecodable CBOR value"):
+            _reject_cbor_sentinels({1: value}, what="unprotected header")
+
+    # A legitimate header using the same container types, with no sentinel
+    # anywhere in it, must not be rejected.
+    legitimate = {
+        1: cbor2.CBORTag(9999, ["a", "b", {2: 3}]),
+        2: {1, 2, 3},
+        3: frozenset({"x", "y"}),
+        4: [1, 2, {"nested": True}],
+    }
+    _reject_cbor_sentinels(legitimate, what="unprotected header")  # must not raise
+
+
+def test_unprotected_header_sentinel_nested_in_tag_or_set_is_checked_at_decode():
+    """The sentinel check must catch a sentinel nested inside a tag or set
+    on a *real* envelope, not just when the recursive helper is called
+    directly - and, critically, before ``decode_cose_manifest`` /
+    ``verify_cose_manifest`` hand the header back to a caller.
+
+    Hand-crafting the exact malformed CBOR bytes that make cbor2 place its
+    internal sentinel several containers deep (rather than as a direct
+    header value, which is what the fuzzer's reproducer above already
+    covers) is not practical without re-running the fuzzer. What can be
+    reproduced deterministically is the resulting Python object graph: this
+    patches ``cbor2.CBORDecoder.decode`` to return that exact graph - a
+    signed, otherwise fully valid envelope whose unprotected header carries
+    a sentinel inside ``CBORTag(258, [sentinel])`` - while still consuming
+    the real byte stream first, so every other check downstream of decoding
+    runs against a genuine envelope.
+
+    Before the fix, ``decode_cose_manifest``/``verify_cose_manifest`` raised
+    nothing at all here: they never re-encode the unprotected header, so the
+    old check's blind spot let the sentinel travel all the way into the
+    returned ``CoseVerification.unprotected``, silently, as a raw
+    unserializable ``cbor2.CBORTag``/``object()`` a caller had no reason to
+    expect.
+    """
+    from unittest.mock import patch
+
+    signed = sign_cose_sign1(base_manifest(), KP)
+    signed = attach_attestation(signed, {"platform": "sim"})
+
+    decoded = cbor2.loads(signed)
+    tag, body = decoded.tag, list(decoded.value)
+    sentinel = object()
+    malicious_unprotected = dict(body[1])
+    malicious_unprotected[999] = cbor2.CBORTag(258, [sentinel])
+    body[1] = malicious_unprotected
+    fake_decoded = cbor2.CBORTag(tag, tuple(body))
+
+    real_decode = cbor2.CBORDecoder.decode
+
+    def smuggle_sentinel(self):
+        real_decode(self)  # fully consume the real stream first
+        return fake_decoded
+
+    with patch.object(cbor2.CBORDecoder, "decode", smuggle_sentinel):
+        with pytest.raises(CoseStructureError, match="undecodable CBOR value"):
+            decode_cose_manifest(signed)
+        with pytest.raises(CoseStructureError, match="undecodable CBOR value"):
+            verify_cose_manifest(signed, trusted_keys=TRUSTED_KEYS)
+        for call in (
+            lambda: attach_unprotected(signed, 1, b"x"),
+            lambda: attach_receipt(signed, b"\xa0"),
+            lambda: attach_attestation(signed, {"platform": "x"}),
+            lambda: attach_approvals(signed, [{"approver_id": "a"}]),
+        ):
+            with pytest.raises(CoseStructureError, match="undecodable CBOR value"):
+                call()