CVE-2026-55247

Executive Summary

CVE-2026-55247 affects the plone.app.event package, which provides the event content type for the Plone content management system. A logged-in editor with access to the iCalendar import functionality can exploit insufficiently restricted URL handling, unbounded resource consumption, and inadequate event URL validation to achieve server-side request forgery (SSRF), denial of service (DoS), and stored cross-site scripting (XSS). The vulnerability has a CVSS score of 9.1, indicating critical severity, and affects versions prior to 5.2.4 and 6.0.1. Successful exploitation can lead to internal network reconnaissance, complete site unavailability, and execution of attacker-controlled script in victim browsers.

Technical Analysis

The vulnerability resides in the iCalendar import functionality of the plone.app.event package, specifically in the module located at src/plone/app/event/ical/importer.py. The NVD description identifies three distinct but related security weaknesses in this import routine:

1. Insufficient URL Restriction (SSRF): The importer accepts calendar and event URLs without adequate validation of the target. This allows a logged-in editor to supply URLs pointing to internal network resources or local files accessible from the Plone server's filesystem. The absence of allow-listing or scheme/host restrictions permits the server to fetch arbitrary resources on the internal network, potentially revealing internal service details, cloud metadata endpoints, or local file contents.

2. Unbounded Resource Consumption (DoS): The import routine does not enforce limits on the number of bytes downloaded from remote calendar sources, nor does it cap the number of events processed during a single import operation. An attacker can provide a URL to a large or maliciously crafted calendar file that causes the server to exhaust memory, disk space, or CPU resources. Additionally, because the import commits work per event (rather than as an atomic transaction), a partial import can leave the system in an inconsistent state and repeatedly trigger expensive rollback operations.

3. Insecure Event URL Storage (Stored XSS): The importer stores event URLs without validating them against dangerous schemes or payloads. If a malicious URL containing JavaScript (e.g., javascript: URI or crafted HTML/script payloads in the URL string) is imported and later rendered by a victim user, the script executes in the victim's browser context. This persists in the event content, meaning any user who views the imported event becomes a target.

The fix implemented in versions 5.2.4 and 6.0.1 addresses these issues by: restricting accepted URLs to safe schemes and hosts, applying MAXIMUM_ICAL_IMPORT_SIZE_BYTES and MAXIMUM_ICAL_IMPORT_EVENTS limits, using transaction savepoints to enable partial-import rollback, and validating event URLs prior to storage.

The CVSS vector (AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H) reflects: network attack vector, low attack complexity, low privileges required (editor role), no user interaction, scope change (compromise affects resources beyond the vulnerable component), low confidentiality impact, low integrity impact, and high availability impact.

Exploitation Scenarios

No public PoC is available; the attack chain is inferred from the vulnerability class and the NVD description.

Step 1 — Reconnaissance/Privilege Acquisition: An attacker first obtains a legitimate editor account on the target Plone site. Plone deployments commonly have multiple content editors, and account compromise or insider access is a realistic avenue. Alternatively, an attacker with prior credentials from phishing or credential-stuffing campaigns can proceed.

Step 2 — Triggering the Vulnerable Import: With editor privileges, the attacker navigates to the event content type creation interface and initiates the iCalendar import workflow. Instead of supplying a benign public calendar URL, the attacker supplies a crafted URL designed to trigger one or more of the three weakness classes.

Step 3 — Payload Execution at the Vulnerability-Class Level:

• For SSRF: The attacker supplies a URL such as http://169.254.169.254/latest/meta-data/ (cloud metadata) or an internal service address like http://192.168.1.10:8080/admin. The server fetches these resources on the attacker's behalf, and the response content (or error messages) may be reflected in the import result, enabling data exfiltration.
• For DoS: The attacker points the importer at a deliberately large calendar file (e.g., a multi-gigabyte .ics file hosted on an attacker-controlled server) or a URL that streams data indefinitely. The import process consumes server memory and CPU until the site becomes unresponsive.
• For Stored XSS: The attacker creates or hosts a calendar file containing an event with a crafted URL, such as javascript:alert(document.cookie) or a URL containing encoded script. Upon import, the malicious URL is persisted in the event object. Any authenticated or anonymous user who views the event triggers the script.

Step 4 — Post-Exploitation Impact: Successful SSRF enables internal network discovery and potential lateral movement. The DoS vector achieves complete site unavailability (high availability impact in CVSS). The stored XSS yields session hijacking, privilege escalation within the CMS, or defacement of the Plone site for all visitors.

Proof of Concept Intelligence

There is currently no public proof-of-concept available for CVE-2026-55247. The vendor patch has been released, which indicates the vulnerability is real and confirmed by the maintainers.

Why exploitation is still possible: Patching lag is common in Plone deployments, which are often long-lived institutional systems that undergo infrequent upgrade cycles. The vulnerable import function is a standard feature for editors; widespread deployments mean the attack surface remains significant. The attack vector requires only low-privilege (editor) access, which broadens the pool of potential attackers.

Resources defenders should monitor: Track the following for early warning of PoC disclosure: 1) public exploit repositories (GitHub, ExploitDB, GitLab), 2) Plone security advisories and mailing lists, 3) security researcher write-ups (e.g., on blogs or at conferences), 4) new Nuclei or Metasploit modules referencing CVE-2026-55247, and 5) dark web or criminal forum chatter, as this CVE's high availability impact makes it attractive for extortion-style attacks on public-sector or educational institutions.

Impact Assessment

CVSS Sub-Scores: The base score of 9.1 (Critical) is driven primarily by the availability impact (High), with additional contributions from confidentiality and integrity impacts (Low each). Scope change increases the overall severity, meaning the compromised component (plone.app.event import) can affect resources beyond its own security scope.

Affected Systems: Any Plone site running plone.app.event versions prior to 5.2.4 (Plone 5.x) or 6.0.1 (Plone 6.x) is vulnerable. This includes intranets, public-facing government portals, academic institutions, and non-profit organizations — the typical Plone user base.

Business Impact: The DoS component alone can take the entire site offline, disrupting content publishing, member-facing services, and public information dissemination. Government and educational sites that rely on Plone for compliance documentation face regulatory and reputational damage. SSRF can expose internal network architecture, and subsequent lateral movement can reach databases or other internal services. Stored XSS can be used for spear-phishing campaigns against site users by injecting content into trusted pages.

Blast Radius: Because scope change is present, the blast radius extends beyond the vulnerable application to the broader internal network. An SSRF-harvested internal service listing can be weaponized for additional targeted attacks. The stored XSS affects all users who view the manipulated event content, expanding the victim population to anonymous public visitors.

Detection Guidance

Sigma Rule

title: Plone iCalendar Import - Unusual Event Count or Size
id: 8a41e6e2-7c3b-4f1e-9d2a-5f6b8c1d3e4f
status: experimental
description: Detects excessive event counts or large imports via Plone iCalendar import, indicating potential DoS or SSRF attempts (CVE-2026-55247)
logsource:
    product: plone
    service: application_server
detection:
    selection:
        app.name: "plone.app.event"
    condition: selection
    timeframe: 5m
fields:
    - user.name
    - http.url
    - http.status_code
level: high
falsepositives:
    - Legitimate bulk calendar imports by trusted content managers
    - Site migrations importing large event archives
tags:
    - attack.t1190
    - cve.2026-55247
    - cwe.400

YARA Rule

rule CVE_2026_55247_Plone_ICAL_Import_Exploit
{
    meta:
        author = "ValtersIT SOC"
        description = "Detects indicators of CVE-2026-55247 exploitation in Plone iCalendar import URLs"
        cve = "CVE-2026-55247"
        date = "2026-01-01"
        severity = "high"
    strings:
        $a = "ical/importer" ascii wide
        $b = "MAXIMUM_ICAL_IMPORT" ascii wide
        $c = "plone.app.event" ascii wide
        $d = "ics" ascii wide
        $e = "fetch" ascii wide
        $f = "url" ascii wide
    condition:
        uint16(0) == 0x5A4D and filesize < 500KB and 2 of them
}

Suricata IDS Rule

alert http any any -> $HOME_NET any (msg:"CVE-2026-55247 Plone iCalendar Import Potential SSRF/DoS"; flow:established,to_server; content:"POST"; http_method; content:"/ical"; http_uri; content:"import"; http_uri; content:"http://"; http_client_body; nocase; content:"javascript:"; http_client_body; nocase; reference:cve,2026-55247; classtype:attempted-admin; sid:9912345; rev:1;)

Elastic Detection

sequence by process.entity_id
    [network where event.type == "start" and process.name == "python" and destination.port == 80]
    [file where event.action == "write" and filename like "*.ics"]

// Alternative EQL rule targeting URL-handling behavior
any where event.category == "http" and url.path contains "ical" and url.path contains "import"

Splunk SPL Query

index=apache sourcetype=access_combined
| search uri="*/ical*import*" 
| stats count by src_ip, useragent, uri, status
| where count > 20
| head 100

Wazuh OSSEC Rule

<rule id="551234" level="12">
    <decoded>plone.ical.import</decoded>
    <description>Plone iCalendar import server-side exception - possible CVE-2026-55247 exploitation attempt</description>
    <group>local_attack,pci_dss_6.5,</group>
    <mitre>
        <id>T1190</id>
    </mitre>
</rule>

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED — no patch evidence from any tracked source. The vendor has released fixed versions 5.2.4 and 6.0.1, but the patch sources list is empty and the availability of the patch has not been confirmed by any tracked source.

1. Immediate Mitigations:

   • Restrict Editor Roles: Temporarily revoke editor privileges for the iCalendar import feature or disable the import functionality entirely in production until patching is complete.
   • Network Egress Control: Implement strict egress filtering on the Plone server to block outbound connections to non-public IP ranges (RFC 1918 addresses, 169.254.0.0/16), preventing SSRF to internal resources.
   • Web Application Firewall (WAF): Configure WAF rules to inspect request bodies for excessively long URLs, multiple event entries, or javascript: URI schemes.
   • Request Size Limits: Place a load balancer or web server-level limit on POST request body sizes to mitigate the DoS component.

2. Patch/Upgrade Steps:

   • Update plone.app.event to version 5.2.4 (for Plone 5.x) or 6.0.1 (for Plone 6.x) once availability is verified. The fix introduces MAXIMUM_ICAL_IMPORT_SIZE_BYTES and MAXIMUM_ICAL_IMPORT_EVENTS configuration limits for administrators to tune.
   • Verify the upgrade in a staging environment to ensure compliance with existing customizations prior to production deployment.
   • After patching, audit existing event content for previously imported malicious URLs.

3. Long-Term Hardening:

   • Implement a least-privilege access model for content editors; require additional approval workflows for calendar imports.
   • Monitor Plone security advisories regularly and participate in the Plone security mailing list.
   • Deploy a comprehensive patching cycle that covers add-on packages (not just the core CMS) on a monthly or quarterly basis.
   • Maintain comprehensive logging of all import activities, including source URLs, event counts, and byte sizes.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-55247 (CVE description, CVSS data, CWE-400 classification)
2. Plone Security Advisories: https://plone.org/security/ (official advisory list; the specific advisory for plone.app.event security fix)
3. Plone Documentation — plone.app.event: https://pypi.org/project/plone.app.event/ (package version history and release notes confirming 5.2.4/6.0.1 fix)
4. MITRE ATT&CK — T1190 Exploit Public-Facing Application: https://attack.mitre.org/techniques/T1190/ (relevance for the SSRF/DoS vector in content management systems)

No known threat actor campaigns or exploit-in-the-wild evidence exist at the time of this report. The CVE is not listed on CISA's Known Exploited Vulnerabilities (KEV) catalog.