CVE-2026-55247: Plone.app.event Denial of Service Vulnerability

CRITICAL
Advisory Identifier: GHSA-r82h-mqw3-fc56
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CVSS 9.1
CWE-400
Published
Advisory Summary

CVE-2026-55247 is a CRITICAL (CVSS 9.1) denial of service vulnerability (CWE-400: Uncontrolled Resource Consumption) affecting Plone.app.event libraries in the PyPI ecosystem. The issue involves plone.app.event vulnerable to denial of service via icalendar import. Remediation is available by upgrading affected packages to version 5.2.4, 6.0.1.

CVE-2026-55247 at a Glance

CVE Identifier CVE-2026-55247
GHSA Identifier GHSA-r82h-mqw3-fc56
Severity CRITICAL
CVSS v3.1 Base Score 9.1
CWE Classification CWE-400: Uncontrolled Resource Consumption
Affected Product Plone.app.event
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 9.1

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required Low (PR:L)
User Interaction None (UI:N)
Scope Changed (S:C)
Confidentiality Low (C:L)
Integrity Low (I:L)
Availability High (A:H)

How to Fix CVE-2026-55247

To remediate this vulnerability, upgrade all affected Plone.app.event dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

plone.app.event fixed: 5.2.4 
pip install plone.app.event>=5.2.4
plone.app.event fixed: 6.0.1 
pip install plone.app.event>=6.0.1
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show plone.app.event

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-55247, inspect your installed packages using pip:

pip show plone.app.event

Analyze the terminal output to locate any version of Plone.app.event in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 5.2.4), your application is affected.

Note: The pip show plone.app.event command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-400: Uncontrolled Resource Consumption, where the product does not properly control the allocation and maintenance of a limited resource. In Plone.app.event, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### 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](mailto:security@plone.org). Thank you!

CVE-2026-55247 Affected Plone.app.event Versions

2 component(s)
Fixed Versions:
5.2.46.0.1
Product / Package Ecosystem Affected Versions Fixed Version
plone.app.event
pypi/plone.app.event
PyPI
< 5.2.4
5.2.4
plone.app.event
pypi/plone.app.event
PyPI
≥ 6.0.0a1
< 6.0.1
6.0.1

References

GitHub Advisory / Wiki ADVISORY
https://github.com/plone/plone.app.event/security/advisories/GHSA-r82h-mqw3-fc56
Security Fix Commit FIX COMMIT
https://github.com/plone/plone.app.event/commit/1e3c83c15a24d1a789cdb012593505bc5620e28e
Security Fix Commit FIX COMMIT
https://github.com/plone/plone.app.event/commit/4de5eb3ea9e4f7f1781622e6d64fc086629d1437
Plone.app.event Source Repository REPOSITORY
https://github.com/plone/plone.app.event
Plone.app.event Release RELEASE
https://github.com/plone/plone.app.event/releases/tag/5.2.4
Plone.app.event Release RELEASE
https://github.com/plone/plone.app.event/releases/tag/6.0.1

Frequently Asked Questions

What is CVE-2026-55247?
CVE-2026-55247 is a security vulnerability in Plone.app.event involving plone.app.event vulnerable to denial of service via iCalendar import under CWE-400: Uncontrolled Resource Consumption.
Which Plone.app.event versions are affected?
Affected versions include releases prior to 5.2.4, 6.0.1 across affected packages: plone.app.event.
What version fixes CVE-2026-55247?
Upgrading to 5.2.4, 6.0.1 (or the latest patched release for your respective branch) resolves the vulnerability.
Is plone.app.event affected?
Yes, plone.app.event is affected in releases prior to 5.2.4, 6.0.1. Upgrade your dependency to 5.2.4, 6.0.1 or later.
What is the CVSS score of CVE-2026-55247?
CVE-2026-55247 has a CVSS v3.1 base score of 9.1 (CRITICAL) with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H.
What CWE is associated with CVE-2026-55247?
CVE-2026-55247 is classified under CWE-400: Uncontrolled Resource Consumption (The product does not properly control the allocation and maintenance of a limited resource.).
How can I check PyPI dependencies for CVE-2026-55247?
Run the dependency tree command: 'pip show plone.app.event'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-r82h-mqw3-fc56",
  "modified" : "2026-08-28T18:59:42Z",
  "published" : "2026-08-28T18:59:42Z",
  "aliases" : [ "CVE-2026-55247" ],
  "summary" : "plone.app.event vulnerable to denial of service via iCalendar import",
  "details" : "### Impact\nBy 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.\n\n### Patches\nThe problem has been patched in `plone.app.event`.\n\n* For Plone 6.2: upgrade to `plone.app.event` 6.0.1\n* For Plone 6.1: upgrade to `plone.app.event` 5.2.4\n* For Plone 6.0: upgrade to `plone.app.event` 5.2.4\n\n### Workarounds\nIn 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.\n\nThere is no workaround for the stored XSS in the URL field of events.\n\nThe vulnerabilities were discovered by Timothy Dudley and responsibly reported to the [Plone Security Team](mailto:security@plone.org). Thank you!",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "plone.app.event"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "5.2.4"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "plone.app.event"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "6.0.0a1"
      }, {
        "fixed" : "6.0.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/plone/plone.app.event/security/advisories/GHSA-r82h-mqw3-fc56"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/plone/plone.app.event/commit/1e3c83c15a24d1a789cdb012593505bc5620e28e"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/plone/plone.app.event/commit/4de5eb3ea9e4f7f1781622e6d64fc086629d1437"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/plone/plone.app.event"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/plone/plone.app.event/releases/tag/5.2.4"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/plone/plone.app.event/releases/tag/6.0.1"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400" ],
    "severity" : "CRITICAL",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-08-28T18:59:42Z",
    "nvd_published_at" : null
  }
}