How severe is CVE-2026-55247?

CVE-2026-55247 has a CVSS score of 9.1/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2026-55247?

CVE-2026-55247 affects the following packages: plone-app-event (PyPI), plone-app-event (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2026-55247?

Update plone-app-event to 5.2.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55247 is resolved across your whole dependency graph.

How do I detect CVE-2026-55247 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for plone-app-event, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-55247 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Is CVE-2026-55247 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-55247 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-55247?

CVE-2026-55247 has an EPSS (Exploit Prediction Scoring System) score of 0.4%, placing it in the 36th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-55247?

CVE-2026-55247 is classified as Uncontrolled Resource Consumption (CWE-400). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-55247 published, and has it been updated?

CVE-2026-55247 was published on August 28, 2026 and was last updated on September 11, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍

🐍 PyPI

Not in CISA KEV

CRITICAL severity

CVE-2026-55247 — plone-app-event

CRITICALFix: plone/plone.app.event@1e3c83c

CVE-2026-55247 is a critical-severity (CVSS 9.1) Uncontrolled Resource Consumption vulnerability in plone-app-event. A fix is available for plone-app-event — see the affected versions and patch details below.

plone.app.event: Denial of service via iCalendar import

Also known asGHSA-r82h-mqw3-fc56PYSEC-2026-3883

Published

Aug 28, 2026

Updated

Sep 11, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Oct 11, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-55247.

EPSS Exploitation Probability

via FIRST.org ↗

0.4%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs36th percentile — riskier than 36% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2026-55247 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

2 pkgs affected

🐍plone-app-event🐍plone-app-event

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

Impact

By abusing the iCalendar import functionality, a logged-in editor could take the whole site offline, make the server reach into the internal network and read calendar files off disk (SSRF), and store XSS.

Patches

The problem has been patched in plone.app.event.

For Plone 6.2: upgrade to plone.app.event 6.0.1
For Plone 6.1: upgrade to plone.app.event 5.2.4
For Plone 6.0: upgrade to plone.app.event 5.2.4

Workarounds

In the site root, go to the Security tab of the Zope Management Interface (manage_access), look for the "plone.app.event: Import Ical" permission, and grant this only to the Manager role. Then only users with the Manager role can use the ical import form.

There is no workaround for the stored XSS in the URL field of events.

The vulnerabilities were discovered by Timothy Dudley and responsibly reported to the Plone Security Team. Thank you!

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`plone-app-event`	all versions	5.2.4`pip install --upgrade 'plone-app-event==5.2.4'`
🐍PyPI	`plone-app-event`	≥ 6.0.0a1&&< 6.0.1	6.0.1`pip install --upgrade 'plone-app-event==6.0.1'`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for plone-app-event, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update plone-app-event to 5.2.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55247 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

### Impact By abusing the iCalendar import functionality, a logged-in editor could take the whole site offline, make the server reach into the internal network and read calendar files off disk (SSRF), and store XSS. ### Patches The problem has been patched in `plone.app.event`. * For Plone 6.2: upgrade to `plone.app.event` 6.0.1 * For Plone 6.1: upgrade to `plone.app.event` 5.2.4 * For Plone 6.0: upgrade to `plone.app.event` 5.2.4 ### Workarounds In the site root, go to the Security tab of the Zope Management Interface (`manage_access`), look for the "plone.app.event: Import Ical" permissio