How severe is CVE-2026-40308?

CVE-2026-40308 has a CVSS 4.0 base score of 8.8, rated high severity.

Is CVE-2026-40308 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (56th percentile), an estimate of the probability of exploitation in the next 30 days.

How do I fix CVE-2026-40308?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

Does CVE-2026-40308 have an EU (EUVD) identifier?

Yes. CVE-2026-40308 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-23306.

When was CVE-2026-40308 published?

CVE-2026-40308 was published on 2026-04-16 and last updated on 2026-06-17.

CVE-2026-40308

CVE-2026-40308 is a high-severity vulnerability with a CVSS 4.0 base score of 8.8. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-639.

Key facts

Severity: High (CVSS 4.0 base score 8.8)
EPSS exploit prediction: 1% (56th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-23306
Weakness: CWE-639
Published: 2026-04-16
Last modified: 2026-06-17

Description

My Calendar is a WordPress plugin for managing calendar events. In versions 3.7.6 and below, the mc_ajax_mcjs_action AJAX endpoint, registered for unauthenticated users, passes user-supplied arguments through parse_str() without validation, allowing injection of arbitrary parameters including a site value. On WordPress Multisite installations, this enables an unauthenticated attacker to call switch_to_blog() with an arbitrary site ID and extract calendar events from any sub-site on the network, including private or hidden events. On standard Single Site installations, switch_to_blog() does not exist, causing an uncaught PHP fatal error and crashing the worker thread, creating an unauthenticated denial of service vector. This issue has been fixed in version 3.7.7.

Frequently asked questions

What is CVE-2026-40308?: My Calendar is a WordPress plugin for managing calendar events. In versions 3.7.6 and below, the mc_ajax_mcjs_action AJAX endpoint, registered for unauthenticated users, passes user-supplied arguments through parse_str() without validation, allowing injection of arbitrary parameters including a site value. On WordPress Multisite installations, this enables an unauthenticated attacker to call switch_to_blog() with an arbitrary site ID and extract calendar events from any sub-site on the network, including private or hidden events. On standard Single Site installations, switch_to_blog() does not exist, causing an uncaught PHP fatal error and crashing the worker thread, creating an unauthenticated denial of service vector. This issue has been fixed in version 3.7.7.
How severe is CVE-2026-40308?: CVE-2026-40308 has a CVSS 4.0 base score of 8.8, rated high severity.
Is CVE-2026-40308 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (56th percentile), an estimate of the probability of exploitation in the next 30 days.
How do I fix CVE-2026-40308?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2026-40308 have an EU (EUVD) identifier?: Yes. CVE-2026-40308 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-23306.
When was CVE-2026-40308 published?: CVE-2026-40308 was published on 2026-04-16 and last updated on 2026-06-17.

References

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