python
diff --git a/‎Misc/NEWS.d/next/Library/2026-08-15-13-12-09.gh-issue-155835.DsbSmy.rst‎
Lines changed: 3 additions & 0 deletions b/‎Misc/NEWS.d/next/Library/2026-08-15-13-12-09.gh-issue-155835.DsbSmy.rst‎
Lines changed: 3 additions & 0 deletions
diff --git a/‎Modules/sha3module.c‎
Lines changed: 20 additions & 14 deletions b/‎Modules/sha3module.c‎
Lines changed: 20 additions & 14 deletions
@@ -0,0 +1,3 @@
+:mod:`hashlib`: Fix data races when accessing
+:attr:`~hashlib.hash.digest_size` and :attr:`~hashlib.hash.block_size` on
+SHA-3 objects. Patch by Bénédikt Tran.
@@ -67,6 +67,12 @@ sha3_get_state(PyObject *module)
 typedef struct {
     HASHLIB_OBJECT_HEAD
     Hacl_Hash_SHA3_state_t *hash_state;
+    // HACL* update functions entirely replace the state, which can lead
+    // to races on the free-threaded build. Since the kind of hash is static,
+    // we can store its corresponding metadata once.
+    uint32_t digest_size;
+    uint32_t block_size;
+    int is_shake;
 } SHA3object;
 
 #define _SHA3object_CAST(op)    ((SHA3object *)(op))
@@ -96,7 +102,7 @@ newSHA3object(PyTypeObject *type)
         return NULL;
     }
     HASHLIB_INIT_MUTEX(newobj);
-
+    newobj->digest_size = newobj->block_size = 0;
     PyObject_GC_Track(newobj);
     return newobj;
 }
@@ -179,6 +185,11 @@ py_sha3_new_impl(PyTypeObject *type, PyObject *data_obj, int usedforsecurity,
         goto error;
     }
 
+    // set the metadata once we know that the state is valid
+    int is_shake = Hacl_Hash_SHA3_is_shake(self->hash_state);
+    self->digest_size = is_shake ? 0 : Hacl_Hash_SHA3_hash_len(self->hash_state);
+    self->block_size = Hacl_Hash_SHA3_block_len(self->hash_state);
+
     if (data) {
         GET_BUFFER_VIEW_OR_ERROR(data, &buf, goto error);
         /* Do not use self->mutex here as this is the constructor
@@ -253,6 +264,8 @@ _sha3_sha3_224_copy_impl(SHA3object *self, PyTypeObject *cls)
         Py_DECREF(newobj);
         return PyErr_NoMemory();
     }
+    newobj->digest_size = self->digest_size;
+    newobj->block_size = self->block_size;
     return (PyObject *)newobj;
 }
 
@@ -273,8 +286,7 @@ _sha3_sha3_224_digest_impl(SHA3object *self)
     HASHLIB_ACQUIRE_LOCK(self);
     (void)Hacl_Hash_SHA3_digest(self->hash_state, digest);
     HASHLIB_RELEASE_LOCK(self);
-    return PyBytes_FromStringAndSize((const char *)digest,
-        Hacl_Hash_SHA3_hash_len(self->hash_state));
+    return PyBytes_FromStringAndSize((const char *)digest, self->digest_size);
 }
 
 
@@ -292,8 +304,7 @@ _sha3_sha3_224_hexdigest_impl(SHA3object *self)
     HASHLIB_ACQUIRE_LOCK(self);
     (void)Hacl_Hash_SHA3_digest(self->hash_state, digest);
     HASHLIB_RELEASE_LOCK(self);
-    return _Py_strhex((const char *)digest,
-        Hacl_Hash_SHA3_hash_len(self->hash_state));
+    return _Py_strhex((const char *)digest, self->digest_size);
 }
 
 
@@ -334,8 +345,7 @@ static PyObject *
 SHA3_get_block_size(PyObject *op, void *Py_UNUSED(closure))
 {
     SHA3object *self = _SHA3object_CAST(op);
-    uint32_t rate = Hacl_Hash_SHA3_block_len(self->hash_state);
-    return PyLong_FromLong(rate);
+    return PyLong_FromLong(self->block_size);
 }
 
 
@@ -371,18 +381,15 @@ SHA3_get_digest_size(PyObject *op, void *Py_UNUSED(closure))
 {
     // Preserving previous behavior: variable-length algorithms return 0
     SHA3object *self = _SHA3object_CAST(op);
-    if (Hacl_Hash_SHA3_is_shake(self->hash_state))
-      return PyLong_FromLong(0);
-    else
-      return PyLong_FromLong(Hacl_Hash_SHA3_hash_len(self->hash_state));
+    return PyLong_FromLong(self->digest_size);
 }
 
 
 static PyObject *
 SHA3_get_capacity_bits(PyObject *op, void *Py_UNUSED(closure))
 {
     SHA3object *self = _SHA3object_CAST(op);
-    uint32_t rate = Hacl_Hash_SHA3_block_len(self->hash_state) * 8;
+    uint32_t rate = self->block_size * 8;
     assert(rate <= 1600);
     int capacity = 1600 - rate;
     return PyLong_FromLong(capacity);
@@ -393,8 +400,7 @@ static PyObject *
 SHA3_get_rate_bits(PyObject *op, void *Py_UNUSED(closure))
 {
     SHA3object *self = _SHA3object_CAST(op);
-    uint32_t rate = Hacl_Hash_SHA3_block_len(self->hash_state) * 8;
-    return PyLong_FromLong(rate);
+    return PyLong_FromLong(self->block_size * 8);
 }
 
 static PyObject *
Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	+:mod:`hashlib`: Fix data races when accessing
	`2`	+:attr:`~hashlib.hash.digest_size` and :attr:`~hashlib.hash.block_size` on
	`3`	`+SHA-3 objects. Patch by Bénédikt Tran.`