Chloé Marcie Small Saddle Bag
Madeleine Swann (played by Léa Seydoux) can be seen holding a brown Chloé Marcie Small Saddle Bag in the Bond Women video blog and promotional images for movie SPECTRE.
The color of the crossbody style bag is probably "Tan", a soft, supple lambskin with matte goldtone hardware. The ridged front has a hidden pocket underneath flap. The fold-over flap has an o-ring and leather strap closure. Buckle adjustable shoulder strap. Fabric lined with single patch pocket.
The bag measures approximately 7'' (19cm) at widest x 6'' (16cm) tall at center x 3½'' (9cm) deep; weighs 1 lb (450gram).
Note that there are different sizes available for the Marcie Saddle Bag, it looks like Madeleine Swann is using the Small version.
Retail price of the bag is around $775. A tan (and other colors) leather small Chloe Marcie bag is available online at Harrods (£485).