diff --git a/.github/workflows/pythontest.yml b/.github/workflows/pythontest.yml
index 64f80820a5..b6c8a9bdcd 100644
--- a/.github/workflows/pythontest.yml
+++ b/.github/workflows/pythontest.yml
@@ -80,7 +80,7 @@ jobs:
       - name: Property-based tests
         run: |
           pip install --upgrade ".[hypothesis]"
-          python -m unittest property_tests.test_config
+          python -m unittest discover property_tests
         env:
           HYPOTHESIS_PROFILE: ci
       - name: Run contrib tests
diff --git a/CONTRIBUTING.rst b/CONTRIBUTING.rst
index 6c0319ce7d..143ecb00b4 100644
--- a/CONTRIBUTING.rst
+++ b/CONTRIBUTING.rst
@@ -142,7 +142,7 @@ them explicitly. Their default Hypothesis profile is deterministic.
 .. code:: console
 
    $ pip install -e ".[hypothesis]"
-   $ python -m unittest property_tests.test_config
+   $ python -m unittest discover property_tests
 
 testr and tox configuration is also present.
 
diff --git a/crates/pack/src/lib.rs b/crates/pack/src/lib.rs
index 19baa3f47b..07c24bbe51 100644
--- a/crates/pack/src/lib.rs
+++ b/crates/pack/src/lib.rs
@@ -170,6 +170,9 @@ fn apply_delta(py: Python, py_src_buf: Py<PyAny>, py_delta: Py<PyAny>) -> PyResu
 
             for i in 0..4 {
                 if cmd & (1 << i) != 0 {
+                    if index >= delta_len {
+                        return Err(ApplyDeltaError::new_err("delta not empty"));
+                    }
                     let x = delta[index] as usize;
                     index += 1;
                     cp_off |= x << (i * 8);
@@ -178,6 +181,9 @@ fn apply_delta(py: Python, py_src_buf: Py<PyAny>, py_delta: Py<PyAny>) -> PyResu
 
             for i in 0..3 {
                 if cmd & (1 << (4 + i)) != 0 {
+                    if index >= delta_len {
+                        return Err(ApplyDeltaError::new_err("delta not empty"));
+                    }
                     let x = delta[index] as usize;
                     index += 1;
                     cp_size |= x << (i * 8);
@@ -193,6 +199,7 @@ fn apply_delta(py: Python, py_src_buf: Py<PyAny>, py_delta: Py<PyAny>) -> PyResu
                 || cp_off > src_size
                 || cp_off > src_size - cp_size
                 || cp_size > dest_size
+                || outindex > dest_size - cp_size
             {
                 break;
             }
@@ -208,6 +215,9 @@ fn apply_delta(py: Python, py_src_buf: Py<PyAny>, py_delta: Py<PyAny>) -> PyResu
             if outindex + cmd as usize > dest_size {
                 return Err(ApplyDeltaError::new_err("Not enough space to copy"));
             }
+            if index + cmd as usize > delta_len {
+                return Err(ApplyDeltaError::new_err("delta not empty"));
+            }
 
             out[outindex..outindex + cmd as usize]
                 .copy_from_slice(&delta[index..index + cmd as usize]);
diff --git a/property_tests/test_pack.py b/property_tests/test_pack.py
new file mode 100644
index 0000000000..68be222ca4
--- /dev/null
+++ b/property_tests/test_pack.py
@@ -0,0 +1,111 @@
+# test_pack.py -- property tests for pack.py
+# Copyright (C) 2026 The Dulwich contributors
+#
+# SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+# Dulwich is dual-licensed under the Apache License, Version 2.0 and the GNU
+# General Public License as published by the Free Software Foundation; version 2.0
+# or (at your option) any later version. You can redistribute it and/or
+# modify it under the terms of either of these two licenses.
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# You should have received a copy of the licenses; if not, see
+# <http://www.gnu.org/licenses/> for a copy of the GNU General Public License
+# and <http://www.apache.org/licenses/LICENSE-2.0> for a copy of the Apache
+# License, Version 2.0.
+#
+
+"""Property tests for pack helpers."""
+
+import os
+from collections.abc import Iterable
+
+from hypothesis import example, given, settings
+from hypothesis import strategies as st
+
+from dulwich.errors import ApplyDeltaError
+from dulwich.pack import _create_delta_py, _delta_encode_size, apply_delta, create_delta
+from tests import TestCase
+
+
+def _delta_to_bytes(delta: bytes | Iterable[bytes]) -> bytes:
+    if isinstance(delta, bytes):
+        return delta
+    return b"".join(delta)
+
+
+settings.register_profile(
+    "deterministic", max_examples=50, deadline=None, derandomize=True
+)
+settings.register_profile("ci", max_examples=50, deadline=None, derandomize=True)
+settings.register_profile(
+    "local-deep", max_examples=1000, deadline=None, derandomize=True
+)
+settings.load_profile(os.environ.get("HYPOTHESIS_PROFILE", "deterministic"))
+
+byte_strings = st.binary(max_size=256)
+
+
+@st.composite
+def delta_pairs(draw) -> tuple[bytes, bytes]:
+    """Generate byte pairs with some shared content for delta copies."""
+    prefix = draw(st.binary(max_size=128))
+    suffix = draw(st.binary(max_size=128))
+    base_middle = draw(st.binary(max_size=128))
+    target_middle = draw(st.binary(max_size=128))
+    return prefix + base_middle + suffix, prefix + target_middle + suffix
+
+
+@st.composite
+def bounded_delta_inputs(draw) -> tuple[bytes, bytes]:
+    """Generate arbitrary delta op streams with bounded output sizes."""
+    base = draw(byte_strings)
+    dest_size = draw(st.integers(min_value=0, max_value=512))
+    ops = draw(st.binary(max_size=256))
+    delta = _delta_encode_size(len(base)) + _delta_encode_size(dest_size) + ops
+    return base, delta
+
+
+byte_pairs = st.one_of(st.tuples(byte_strings, byte_strings), delta_pairs())
+
+
+class PackPropertyTests(TestCase):
+    """Property tests for pack helpers."""
+
+    @given(byte_pairs)
+    @example((b"", b""))
+    @example((b"", b"Z" * 8192))
+    @example((b"Z" * 8192, b"Z" * 8192))
+    @example((b"Z" * 70000 + b"a", b"Z" * 70000 + b"b"))
+    def test_create_delta_roundtrip(self, pair: tuple[bytes, bytes]) -> None:
+        """Check that generated deltas apply back to the target."""
+        base, target = pair
+        delta = _delta_to_bytes(create_delta(base, target))
+        self.assertEqual(target, b"".join(apply_delta(base, delta)))
+
+    @given(byte_pairs)
+    @example((b"", b""))
+    @example((b"", b"Z" * 8192))
+    @example((b"Z" * 8192, b"Z" * 8192))
+    @example((b"Z" * 70000 + b"a", b"Z" * 70000 + b"b"))
+    def test_create_delta_py_roundtrip(self, pair: tuple[bytes, bytes]) -> None:
+        """Check that pure Python generated deltas apply to the target."""
+        base, target = pair
+        delta = _delta_to_bytes(_create_delta_py(base, target))
+        self.assertEqual(target, b"".join(apply_delta(base, delta)))
+
+    @given(bounded_delta_inputs())
+    @example((b"", b"\x00\x01\x01"))
+    def test_apply_delta_only_raises_apply_delta_error(
+        self, base_and_delta: tuple[bytes, bytes]
+    ) -> None:
+        """Check that malformed deltas use the delta error type."""
+        base, delta = base_and_delta
+        try:
+            apply_delta(base, delta)
+        except ApplyDeltaError:
+            pass
diff --git a/tests/test_pack.py b/tests/test_pack.py
index eb0dfee341..321d17a531 100644
--- a/tests/test_pack.py
+++ b/tests/test_pack.py
@@ -248,6 +248,10 @@ def test_apply_delta_invalid_opcode(self) -> None:
         # Should raise ApplyDeltaError
         self.assertRaises(ApplyDeltaError, apply_delta, base, invalid_delta)
 
+    def test_apply_delta_truncated_insert(self) -> None:
+        """Test apply_delta with a truncated insert operation."""
+        self.assertRaises(ApplyDeltaError, apply_delta, b"", b"\x00\x01\x01")
+
     def test_create_delta_insert_only(self) -> None:
         """Test create_delta when only insertions are required."""
         base = b""