diff --git a/src/borg/_hashindex.c b/src/borg/_hashindex.c
index 5210fae00..636b85b99 100644
--- a/src/borg/_hashindex.c
+++ b/src/borg/_hashindex.c
@@ -87,7 +87,7 @@ static int hash_sizes[] = {
 #define EPRINTF_PATH(path, msg, ...) fprintf(stderr, "hashindex: %s: " msg " (%s)\n", path, ##__VA_ARGS__, strerror(errno))
 
 #ifndef BORG_NO_PYTHON
-static HashIndex *hashindex_read(PyObject *file_py, int permit_compact);
+static HashIndex *hashindex_read(PyObject *file_py, int permit_compact, int expected_key_size, int expected_value_size);
 static void hashindex_write(HashIndex *index, PyObject *file_py);
 #endif
 
@@ -259,7 +259,7 @@ count_empty(HashIndex *index)
 
 #ifndef BORG_NO_PYTHON
 static HashIndex *
-hashindex_read(PyObject *file_py, int permit_compact)
+hashindex_read(PyObject *file_py, int permit_compact, int expected_key_size, int expected_value_size)
 {
     Py_ssize_t length, buckets_length, bytes_read;
     Py_buffer header_buffer;
@@ -336,6 +336,36 @@ hashindex_read(PyObject *file_py, int permit_compact)
         goto fail_release_header_buffer;
     }
 
+    // check if key / value sizes match the expected sizes.
+    if (expected_key_size != -1 && header->key_size != expected_key_size) {
+        PyErr_Format(PyExc_ValueError, "Expected key size %d, got %d.",
+                     expected_key_size, header->key_size);
+        goto fail_decref_header;
+    }
+    if (expected_value_size != -1 && header->value_size != expected_value_size) {
+        PyErr_Format(PyExc_ValueError, "Expected value size %d, got %d.",
+                     expected_value_size, header->value_size);
+        goto fail_decref_header;
+    }
+    // in any case, the key and value sizes must be both >= 4, the code assumes this.
+    if (header->key_size < 4) {
+        PyErr_Format(PyExc_ValueError, "Expected key size >= 4, got %d.", header->key_size);
+        goto fail_decref_header;
+    }
+    if (header->value_size < 4) {
+        PyErr_Format(PyExc_ValueError, "Expected value size >= 4, got %d.", header->value_size);
+        goto fail_decref_header;
+    }
+    // sanity check for num_buckets and num_entries.
+    if (header->num_buckets < 1) {
+        PyErr_Format(PyExc_ValueError, "Expected num_buckets >= 1, got %d.", header->num_buckets);
+        goto fail_decref_header;
+    }
+    if ((header->num_entries < 0) || (header->num_entries > header->num_buckets)) {
+        PyErr_Format(PyExc_ValueError, "Expected 0 <= num_entries <= num_buckets, got %d.", header->num_entries);
+        goto fail_decref_header;
+    }
+
     buckets_length = (Py_ssize_t)_le32toh(header->num_buckets) * (header->key_size + header->value_size);
     if((Py_ssize_t)length != (Py_ssize_t)sizeof(HashHeader) + buckets_length) {
         PyErr_Format(PyExc_ValueError, "Incorrect file length (expected %zd, got %zd)",
diff --git a/src/borg/hashindex.pyx b/src/borg/hashindex.pyx
index 182018ed2..28f60ef9c 100644
--- a/src/borg/hashindex.pyx
+++ b/src/borg/hashindex.pyx
@@ -23,7 +23,8 @@ cdef extern from "_hashindex.c":
         uint32_t version
         char hash[16]
 
-    HashIndex *hashindex_read(object file_py, int permit_compact) except *
+    HashIndex *hashindex_read(object file_py, int permit_compact,
+                              int expected_key_size, int expected_value_size) except *
     HashIndex *hashindex_init(int capacity, int key_size, int value_size)
     void hashindex_free(HashIndex *index)
     int hashindex_len(HashIndex *index)
@@ -95,9 +96,9 @@ cdef class IndexBase:
         if path:
             if isinstance(path, (str, bytes)):
                 with open(path, 'rb') as fd:
-                    self.index = hashindex_read(fd, permit_compact)
+                    self.index = hashindex_read(fd, permit_compact, self.key_size, self.value_size)
             else:
-                self.index = hashindex_read(path, permit_compact)
+                self.index = hashindex_read(path, permit_compact, self.key_size, self.value_size)
             assert self.index, 'hashindex_read() returned NULL with no exception set'
         else:
             self.index = hashindex_init(capacity, self.key_size, self.value_size)