CVE-2026-39601

Booking Calendar <= 11.8.4 - Unauthenticated Duplicate Slot Booking

2026-10-02 00:00

Strategic Overview

Status: Patched in 11.9
Affected Plugin: Booking Calendar
Affected Version: <= 11.8.4
CVSS: 5.3Medium
Weakness type: CWE-362 · Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
CVE: CVE-2026-39601

View all Booking Calendar vulnerabilities

At a glance

CVE-2026-39601 is a medium-severity Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition') vulnerability in the Booking Calendar WordPress plugin, affecting versions <= 11.8.4. It carries a CVSS score of 5.3 (reachable over the network; low attack complexity). Exploitation requires no authentication. The issue is fixed in version 11.9; sites on affected versions should update now. Disclosed October 2026, reported by Ananda Dhakal.

Vulnerability Overview

The Booking Calendar plugin for WordPress is vulnerable to Race Condition in all versions up to, and including, 11.8.4. This is due to missing real-time date-availability validation at the moment of date selection, allowing a date shown as available during calendar load to be selected and submitted after a concurrent booking had taken it. This makes it possible for unauthenticated attackers to make duplicate bookings for the same resource slot by exploiting the window between calendar availability display and actual booking submission.

Technical Analysis

The vector marks this flaw as remotely reachable over the network, with low attack complexity — no special timing or configuration is needed, and no privileges on the target site, and no interaction from a victim user.

CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

Remediation

Update to version 11.9, or a newer patched version

How does WordSec protect against this?

This one needs no account at all, which puts it outside what login hardening can reach; WordSec's login security narrows the account-level paths around it, and the firewall is what inspects the request itself. None of that substitutes for the fix: Booking Calendar 11.9 closes this, and updating the plugin is the step that ends it.

Login Security
Alerts

External References

https://www.cve.org/CVERecord?id=CVE-2026-39601 https://www.wordfence.com/threat-intel/vulnerabilities/id/329a886e-35e0-4c31-b9fe-2a82166e948a https://nvd.nist.gov/vuln/detail/CVE-2026-39601

Related records

Other vulnerabilities in Booking Calendar

CVSS Vector

5.3

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

Base

Attack VectorNetwork
Attack ComplexityLow
Privileges RequiredNone
User InteractionNone
ScopeUnchanged
ConfidentialityNone
IntegrityLow
AvailabilityNone

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