diff --git a/src/mm-base-sim.c b/src/mm-base-sim.c
index 1498fb0f..6740c585 100644
--- a/src/mm-base-sim.c
+++ b/src/mm-base-sim.c
@@ -261,6 +261,13 @@ handle_change_pin_auth_ready (MMAuthProvider *authp,
         return;
     }
 
+    if (!mm_utils_is_numeric (ctx->old_pin) || !mm_utils_is_numeric (ctx->new_pin)) {
+        mm_dbus_method_invocation_return_error_literal (ctx->invocation, MM_CORE_ERROR, MM_CORE_ERROR_INVALID_ARGS,
+                                                       "PIN must be numeric");
+        handle_change_pin_context_free (ctx);
+        return;
+    }
+
     /* If changing PIN is not implemented, report an error */
     if (!MM_BASE_SIM_GET_CLASS (ctx->self)->change_pin ||
         !MM_BASE_SIM_GET_CLASS (ctx->self)->change_pin_finish) {
@@ -445,6 +452,13 @@ handle_enable_pin_auth_ready (MMAuthProvider *authp,
         return;
     }
 
+    if (!mm_utils_is_numeric (ctx->pin)) {
+        mm_dbus_method_invocation_return_error_literal (ctx->invocation, MM_CORE_ERROR, MM_CORE_ERROR_INVALID_ARGS,
+                                                       "PIN must be numeric");
+        handle_enable_pin_context_free (ctx);
+        return;
+    }
+
     /* If changing PIN is not implemented, report an error */
     if (!MM_BASE_SIM_GET_CLASS (ctx->self)->enable_pin ||
         !MM_BASE_SIM_GET_CLASS (ctx->self)->enable_pin_finish) {
@@ -869,6 +883,13 @@ handle_send_pin_auth_ready (MMAuthProvider *authp,
         return;
     }
 
+    if (!mm_utils_is_numeric (ctx->pin)) {
+        mm_dbus_method_invocation_return_error_literal (ctx->invocation, MM_CORE_ERROR, MM_CORE_ERROR_INVALID_ARGS,
+                                                       "PIN must be numeric");
+        handle_send_pin_context_free (ctx);
+        return;
+    }
+
     if (!mm_gdbus_sim_get_active (MM_GDBUS_SIM (ctx->self))) {
         mm_dbus_method_invocation_return_error_literal (ctx->invocation, MM_CORE_ERROR, MM_CORE_ERROR_UNSUPPORTED,
                                                         "Cannot send PIN: SIM not currently active");
@@ -974,6 +995,13 @@ handle_send_puk_auth_ready (MMAuthProvider *authp,
         return;
     }
 
+    if (!mm_utils_is_numeric (ctx->puk) || !mm_utils_is_numeric (ctx->new_pin)) {
+        mm_dbus_method_invocation_return_error_literal (ctx->invocation, MM_CORE_ERROR, MM_CORE_ERROR_INVALID_ARGS,
+                                                       "PIN/PUK must be numeric");
+        handle_send_puk_context_free (ctx);
+        return;
+    }
+
     if (!mm_gdbus_sim_get_active (MM_GDBUS_SIM (ctx->self))) {
         mm_dbus_method_invocation_return_error_literal (ctx->invocation, MM_CORE_ERROR, MM_CORE_ERROR_UNSUPPORTED,
                                                         "Cannot send PUK: SIM not currently active");
diff --git a/src/mm-modem-helpers.c b/src/mm-modem-helpers.c
index bbb9606f..8be5fb63 100644
--- a/src/mm-modem-helpers.c
+++ b/src/mm-modem-helpers.c
@@ -5977,3 +5977,18 @@ mm_utils_is_valid_fqdn (const gchar *fqdn)
     }
     return TRUE;
 }
+
+gboolean
+mm_utils_is_numeric (const gchar *str)
+{
+    const gchar *c;
+
+    if (!str || !str[0])
+        return FALSE;
+
+    for (c = str; *c; c++) {
+        if (!g_ascii_isdigit (*c))
+            return FALSE;
+    }
+    return TRUE;
+}
diff --git a/src/mm-modem-helpers.h b/src/mm-modem-helpers.h
index bf102251..e21ec7eb 100644
--- a/src/mm-modem-helpers.h
+++ b/src/mm-modem-helpers.h
@@ -573,6 +573,7 @@ gboolean mm_parse_supl_address (const gchar  *supl,
 
 void mm_utils_remove_control_characters (gchar *str);
 gboolean mm_utils_is_valid_fqdn (const gchar *fqdn);
+gboolean mm_utils_is_numeric (const gchar *str);
 
 gboolean mm_validate_cbs_channels (GArray   *channels,
                                    GError  **error);
diff --git a/src/tests/test-modem-helpers.c b/src/tests/test-modem-helpers.c
index 03785791..556d9606 100644
--- a/src/tests/test-modem-helpers.c
+++ b/src/tests/test-modem-helpers.c
@@ -4908,6 +4908,21 @@ test_is_valid_fqdn (void *f, gpointer d)
     g_assert (!mm_utils_is_valid_fqdn ("host/com"));
 }
 
+static void
+test_is_numeric (void *f, gpointer d)
+{
+    g_assert (mm_utils_is_numeric ("123456"));
+    g_assert (mm_utils_is_numeric ("0"));
+    g_assert (mm_utils_is_numeric ("0123456789"));
+
+    g_assert (!mm_utils_is_numeric (NULL));
+    g_assert (!mm_utils_is_numeric (""));
+    g_assert (!mm_utils_is_numeric ("123a45"));
+    g_assert (!mm_utils_is_numeric ("123 45"));
+    g_assert (!mm_utils_is_numeric ("123\r45"));
+    g_assert (!mm_utils_is_numeric ("+123"));
+}
+
 /*****************************************************************************/
 
 typedef struct {
@@ -5456,6 +5471,7 @@ int main (int argc, char **argv)
     g_test_suite_add (suite, TESTCASE (test_at_quote_string, NULL));
     g_test_suite_add (suite, TESTCASE (test_remove_control_characters, NULL));
     g_test_suite_add (suite, TESTCASE (test_is_valid_fqdn, NULL));
+    g_test_suite_add (suite, TESTCASE (test_is_numeric, NULL));
 
     g_test_suite_add (suite, TESTCASE (test_cpol_response, NULL));