diff --git a/emrg/sandbox/win32/abi.py b/emrg/sandbox/win32/abi.py
index c25b3d11..6c2ff728 100644
--- a/emrg/sandbox/win32/abi.py
+++ b/emrg/sandbox/win32/abi.py
@@ -8,6 +8,8 @@
 
 from __future__ import annotations
 
+import ctypes
+
 #: OpenProcess access required to query the current process token.
 PROCESS_QUERY_INFORMATION = 0x0400
 #: Token right required by CreateProcessAsUserW.
@@ -135,11 +137,38 @@
 )
 #: ``SECURITY_DESCRIPTOR_REVISION``: the only revision ``InitializeSecurityDescriptor`` accepts.
 SECURITY_DESCRIPTOR_REVISION = 1
+class SecurityDescriptor(ctypes.Structure):
+    """``SECURITY_DESCRIPTOR`` — the absolute form ``InitializeSecurityDescriptor`` builds.
+
+    Declared here, beside the constant it sizes, so that constant can be
+    *measured*: a four-byte header (``Revision``, ``Sbz1``, ``Control``) followed
+    by four pointers is 20 bytes on a 32-bit interpreter and 40 on a 64-bit one,
+    and the DACL pointer — the last field the API writes — starts at offset 32
+    there.  A buffer sized by the 32-bit answer is overrun by half its length on a
+    64-bit host, and because the allocation is a raw byte buffer rather than a
+    Python object's storage, nothing raises: the surplus write lands in the
+    process heap (issue #2509).
+    """
+
+    _fields_ = [
+        ("Revision", ctypes.c_ubyte),
+        ("Sbz1", ctypes.c_ubyte),
+        ("Control", ctypes.c_ushort),
+        ("Owner", ctypes.c_void_p),
+        ("Group", ctypes.c_void_p),
+        ("Sacl", ctypes.c_void_p),
+        ("Dacl", ctypes.c_void_p),
+    ]
+
+
 #: The bytes an absolute ``SECURITY_DESCRIPTOR`` occupies.
 #:
 #: Named by the SDK for exactly this use — the buffer an absolute descriptor is
-#: built in before ``MakeSelfRelativeSD`` measures the form the API wants.
-SECURITY_DESCRIPTOR_MIN_LENGTH = 20
+#: built in before ``MakeSelfRelativeSD`` measures the form the API wants — and
+#: the SDK defines it as ``sizeof(SECURITY_DESCRIPTOR)``, so it is taken from the
+#: struct above and never written down.  A literal ``20`` here is the 32-bit
+#: answer: correct on one interpreter width and a 20-byte overrun on the other.
+SECURITY_DESCRIPTOR_MIN_LENGTH = ctypes.sizeof(SecurityDescriptor)
 
 # --- process creation (Win32 process owner, win32-process/src/abi.ts) -------
 
diff --git a/tests/test_bash_v2_win32.py b/tests/test_bash_v2_win32.py
index a28d34ed..5df8ab4b 100644
--- a/tests/test_bash_v2_win32.py
+++ b/tests/test_bash_v2_win32.py
@@ -76,6 +76,44 @@
 SUB_AUTHORITY_CEILING = 2**30 - 1
 
 
+# ── the ABI declarations ─────────────────────────────────────────────────
+
+
+def test_the_descriptor_buffer_holds_the_whole_struct_the_api_fills():
+    """``SECURITY_DESCRIPTOR_MIN_LENGTH`` is a size, and it is measured, not chosen.
+
+    Issue #2509. The constant was written down as ``20`` — the 32-bit answer —
+    while ``InitializeSecurityDescriptor`` fills the whole struct, whose DACL
+    pointer alone starts at offset 32 on a 64-bit interpreter. The 20-byte buffer
+    ``registry.apply_dacl`` handed the API was overrun by 20 bytes, the surplus
+    write landed in the process heap instead of in an object ctypes bounds, and
+    the crash surfaced in whichever test file the same xdist worker loaded next.
+
+    This guard belongs on every platform, which is why it sits in this file's
+    cross-platform half: the Win32 *calls* need Windows, the struct's layout does
+    not, so the Linux and macOS legs — the ones that stay green — are exactly
+    where a hand-written size has to be caught.
+    """
+    descriptor = abi.SecurityDescriptor
+    pointer = ctypes.sizeof(ctypes.c_void_p)
+    # The property that broke: the buffer is the size of the struct.
+    assert abi.SECURITY_DESCRIPTOR_MIN_LENGTH == ctypes.sizeof(descriptor)
+    # …and the one that still holds where the first is trivially true: the last
+    # field the API writes lives inside the buffer it was handed.
+    assert descriptor.Dacl.offset + pointer <= abi.SECURITY_DESCRIPTOR_MIN_LENGTH
+    # A truncated declaration would shrink both sides together, so pin the shape
+    # the SDK defines: one pointer-size field per member, in order.
+    assert [name for name, _ in descriptor._fields_] == [
+        "Revision",
+        "Sbz1",
+        "Control",
+        "Owner",
+        "Group",
+        "Sacl",
+        "Dacl",
+    ]
+
+
 # ── capability SIDs ───────────────────────────────────────────────────────