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Severity
High
Analysis Summary
A critical vulnerability in Microsoft Bookings, identified in late 2024, stemmed from insufficient input validation within its API, specifically affecting fields such as appointment.serviceNotes, appointment, additionalNotes, and appointment.body.content. These input fields failed to properly sanitize user-supplied content, allowing attackers to inject arbitrary HTML. This flaw posed a significant risk to organizations using Microsoft Bookings within the Microsoft 365 ecosystem, as malicious content could be embedded into booking confirmations and calendar invites, turning them into vectors for phishing or social engineering attacks.
According to the Researcher, vulnerability was particularly exploitable through the platform’s “Reschedule” feature. When users received a confirmation email with a rescheduling link, any unsanitized HTML embedded in the original booking would persist and be reflected in a subsequent PUT request. This meant attackers could create deceptive meeting content that remained intact through the rescheduling process. Furthermore, attackers could abuse the joinWebUrl parameter to insert misleading or malicious meeting links and images, potentially impersonating legitimate Microsoft Teams invites to trick victims.
Beyond email manipulation, attackers were also able to tamper with calendar attachments. Using headers like X-ALT-DESC and crafting additional ORGANIZER entries in ICS files, they could customize calendar entries in a way that misled users and disguised the attack. The potential impact included altered meeting details, phishing links masquerading under trusted domains, modified booking durations that caused resource exhaustion, and even the creation of hidden mailboxes that bypassed administrative controls, highlighting the extensive threat this vulnerability posed to data integrity and organizational security.
While Microsoft addressed most aspects of the vulnerability by February 2025, following its initial disclosure in December 2024, some parameters, such as additionalRecipients, startTime, and endTime, reportedly remained vulnerable. Security experts have recommended adherence to CWE-20 guidelines on input validation and encouraged administrators to follow Microsoft's updated security best practices, released in March 2025. These include restricting public access to booking pages, enforcing strict naming conventions, and closely monitoring appointment activity to detect anomalies indicative of potential exploitation.
Impact
- Gain Access
Remediation
- Apply the latest Microsoft security patches for Microsoft Bookings to ensure all known vulnerabilities are addressed.
- Enforce strong input validation and sanitization for all user-supplied fields, in alignment with CWE-20: Improper Input Validation.
- Restricting access to booking pages (e.g., authenticated users only).
- Implementing strict naming policies for appointments and users.
- Monitor booking activity logs for unusual patterns such as long-duration appointments, mass reschedules, or unknown recipient email addresses.
- Regularly audit ICS attachments and calendar invites for unexpected or unauthorized metadata (e.g., custom headers, altered organizers).
- Limit use of the “Reschedule” feature or sanitize HTML content before reflecting it in PUT requests or confirmation emails.
