The vulnerability identified as CVE-2025-11723 affects the 'Appointment Booking Calendar — Simply Schedule Appointments Booking' plugin for WordPress, maintained by croixhaug. The root cause is the use of a hardcoded fallback salt in the plugin's implementation of the hash() function, which is intended to generate tokens for securing booking information. This practice violates secure randomness principles (CWE-330), as the fallback salt is predictable and reused across installations that have not explicitly set a unique salt in the wp-config.php configuration file. Consequently, an unauthenticated attacker can replicate the token generation process and produce valid tokens to access sensitive booking data on any vulnerable site. This access can lead to unauthorized viewing and modification of appointment bookings, compromising both confidentiality and integrity. The vulnerability affects all versions up to and including 1.6.9.5. The CVSS 3.1 base score is 6.5 (medium severity), reflecting network attack vector, no privileges required, no user interaction, and partial impact on confidentiality and integrity without affecting availability. No patches or fixes are currently linked, and no exploits have been reported in the wild, but the vulnerability's nature makes it a plausible target for attackers seeking to manipulate appointment data or harvest sensitive client information. The issue underscores the importance of proper cryptographic practices and configuration management in WordPress plugins handling sensitive data.

For European organizations, especially those in sectors relying heavily on appointment scheduling (healthcare, legal, consulting, education), this vulnerability poses a risk of unauthorized disclosure and manipulation of sensitive client booking information. Exposure of booking details can lead to privacy violations, regulatory non-compliance (e.g., GDPR), reputational damage, and potential operational disruptions if attackers alter appointments. The integrity compromise could result in missed or fraudulent bookings, impacting service delivery and trust. Since the vulnerability requires no authentication or user interaction, it can be exploited remotely by attackers scanning for vulnerable WordPress sites. The lack of a patch increases the window of exposure. Organizations using this plugin without a custom salt configured are particularly vulnerable. The impact is heightened in countries with strict data protection laws, where breaches can lead to significant fines and legal consequences.

European organizations should immediately verify if the 'Appointment Booking Calendar — Simply Schedule Appointments Booking' plugin is installed and identify the version in use. If affected (version ≤ 1.6.9.5), they should: 1) Configure a unique, strong salt value in the wp-config.php file to override the hardcoded fallback salt, ensuring token generation uses sufficient randomness. 2) Monitor web server logs and application logs for unusual access patterns or token generation attempts. 3) Restrict access to booking management interfaces via IP whitelisting or additional authentication layers where feasible. 4) Regularly back up booking data to detect and recover from unauthorized modifications. 5) Engage with the plugin vendor or community to track the release of official patches or updates addressing this vulnerability. 6) Educate site administrators on secure configuration practices and the risks of default settings. 7) Consider alternative booking plugins with stronger security postures if remediation is delayed. These steps go beyond generic advice by focusing on configuration hardening, monitoring, and layered access controls specific to this vulnerability.