The YML for Yandex Market plugin for WordPress is vulnerable to Sensitive Information Exposure in all versions up to, and including, 5.10.0 via the 'y4ym_check_action' parameter. This makes it possible for unauthenticated attackers to extract the merchant's plaintext Yandex Market API key written to the publicly accessible log file at wp-content/uploads/y4ym/yml-for-yandex-market.log, granting full control over the merchant's marketplace catalogue, prices, and stock. Exploitation requires that the targeted feed has a stored Yandex Market API key and that the y4ym_keeplogs option is enabled — both conditions that represent normal, documented plugin usage actively encouraged by the plugin's own support and feedback workflow.