CVE-2023-43658 – This vulnerability in the discourse-calendar pl... (How to Fix)

📋 TL;DR

This vulnerability in the discourse-calendar plugin allows attackers to inject malicious scripts into event titles, leading to cross-site scripting (XSS) attacks in the email preview interface. Only Discourse sites with Content Security Policy (CSP) disabled are affected, which is a non-default configuration. Most Discourse installations are protected by default CSP settings.

💻 Affected Systems

Products:

discourse-calendar plugin for Discourse

Versions: All versions before the fix commit 9788310906febb36822d6823d14f1059c39644de

Operating Systems: All platforms running Discourse

Default Config Vulnerable: ✅ No

Notes: Only vulnerable when Content Security Policy (CSP) is disabled on the Discourse forum, which is not the default configuration.

📦 What is this software?

Discourse Calendar by Discourse

View all CVEs affecting Discourse Calendar →

⚠️ Risk & Real-World Impact

🔴

Worst Case

Attackers could steal session cookies, perform actions as authenticated users, or redirect users to malicious sites if CSP is disabled and XSS is successfully exploited.

🟠

Likely Case

Limited impact since most Discourse sites have CSP enabled by default, preventing the XSS from executing even if injection occurs.

🟢

If Mitigated

No impact when CSP is enabled, as modern browsers will block the malicious script execution.

🌐 Internet-Facing: MEDIUM - Only affects internet-facing Discourse forums with CSP disabled, which is uncommon.

🏢 Internal Only: LOW - Internal Discourse instances with CSP disabled could be affected, but this configuration is rare.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ✅ No

Complexity: LOW

Exploitation requires the attacker to create or modify calendar events with malicious titles and the victim to view email previews. CSP provides strong protection even if injection occurs.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Version containing commit 9788310906febb36822d6823d14f1059c39644de

Vendor Advisory: https://github.com/discourse/discourse-calendar/security/advisories/GHSA-3fwj-f6ww-7hr6

Restart Required: Yes

Instructions:

1. Update discourse-calendar plugin to latest version via Discourse admin interface
2. Restart Discourse application
3. Verify CSP remains enabled (default setting)

🔧 Temporary Workarounds

Enable Content Security Policy

all

Ensure CSP is enabled on Discourse forum to block XSS execution even if injection occurs

Check Discourse admin settings for CSP configuration

🧯 If You Can't Patch

Ensure Content Security Policy (CSP) is enabled and properly configured on the Discourse forum
Restrict calendar event creation/modification permissions to trusted users only

🔍 How to Verify

Check if Vulnerable:

Check if discourse-calendar plugin version is older than commit 9788310906febb36822d6823d14f1059c39644de and CSP is disabled

Check Version:

Check Discourse admin panel → Plugins → discourse-calendar version

Verify Fix Applied:

Verify plugin has been updated to include the proper escaping fix and CSP is enabled

📡 Detection & Monitoring

Log Indicators:

Unusual calendar event creation/modification patterns
Suspicious script-like content in event titles

Network Indicators:

Unexpected script tags in email preview responses

SIEM Query:

Search for calendar event modifications containing script tags or JavaScript code in titles

📊 Metadata

CVE ID: CVE-2023-43658

CVSS v3 Score: 8.0 (HIGH)

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H

CWE: CWE-79

Published: October 16, 2023

Last Updated: November 21, 2024

🔗 References

https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP Third Party Advisory
https://github.com/discourse/discourse-calendar/commit/9788310906febb36822d6823d14f1059c39644de Patch
https://github.com/discourse/discourse-calendar/security/advisories/GHSA-3fwj-f6ww-7hr6 Vendor Advisory
https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP Third Party Advisory
https://github.com/discourse/discourse-calendar/commit/9788310906febb36822d6823d14f1059c39644de Patch
https://github.com/discourse/discourse-calendar/security/advisories/GHSA-3fwj-f6ww-7hr6 Vendor Advisory

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🔗 Related Vulnerabilities

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