CVE-2026-54506

Executive Summary

CVE-2026-54506 is a stored cross-site scripting (XSS) vulnerability in Vvveb CMS, an open-source content management system with page-builder capabilities. The flaw resides in the sanitizeHTML() function in system/functions.php, which fails to properly sanitize user-supplied content submitted via the user[bio] field in app/controller/user/profile.php. An authenticated user with Author-role privileges or higher can inject malicious JavaScript that executes in the browsers of unauthenticated visitors, administrators, and other users viewing affected pages. With a CVSS score of 7.6 (High), the vulnerability has a network attack vector, low attack complexity, and high confidentiality impact due to session data exposure.

Technical Analysis

The vulnerability is classified as CWE-79 (Improper Neutralization of Input During Web Page Generation), specifically a stored (persistent) XSS variant. The attack surface involves the user profile bio field, which accepts and persists user-controlled content.

According to the NVD description, two distinct flaws exist within the sanitizeHTML() function in system/functions.php:

Flaw 1 — Incomplete event-handler regex: The regular expression responsible for stripping on* event-handler attributes (such as onclick, onerror, onload) omits the forward-slash delimiter from its pattern. This means payloads using a solidus-prefixed event-handler markup can bypass the filter entirely, as the regex does not account for this syntactic variation.

Flaw 2 — Faulty do-while loop condition: The do-while loop within sanitizeHTML() compares the string to itself in its continuation condition. Since a string is always equal to itself, the loop condition is effectively always false after the first iteration, meaning the sanitization routine executes only once. This single-pass behavior means that forbidden nested tags — tags wrapped inside other tags — are removed only at the outermost layer, leaving inner forbidden tags intact after the first pass.

Attack surface: The user[bio] parameter is accepted by app/controller/user/profile.php. Any user with Author-role or higher can submit a profile update containing crafted markup. The content passes through sanitizeHTML() and is then stored persistently.

Rendering sink: The stored bio is rendered without sufficient output encoding on multiple pages:

• /author/{username} — public-facing author page viewable by unauthenticated visitors
• The admin user-management view — viewable by administrators
• Potentially comment displays

Because the output is not adequately encoded at render time, any JavaScript that survives the flawed sanitization executes in the victim's browser context within the application's origin.

Exploitation Scenarios

Prerequisite: The attacker must possess an authenticated account with Author-role or higher privileges on the target Vvveb instance.

1. Reconnaissance / Target Identification: The attacker identifies a Vvveb CMS deployment running a version prior to 1.0.8.5. This can be accomplished through Shodan queries, manual browsing, or fingerprinting the /author/{username} pages and admin interfaces.

2. Triggering the Flaw Class: The attacker authenticates with an Author-role account and navigates to their profile editing functionality. They submit crafted content in the user[bio] field. The payload leverages either:

   • A solidus-prefixed event-handler attribute designed to evade the incomplete regex, or
   • Nested forbidden tags that survive the single-pass do-while loop because the condition fails to re-evaluate properly.

   The malicious markup is accepted by app/controller/user/profile.php, passes through the flawed sanitizeHTML() in system/functions.php, and is stored in the database.

3. Payload Execution (Vulnerability-Class Level): When any user — an unauthenticated visitor, an administrator reviewing the user-management view, or another user reading comments — loads a page that renders the stored bio, the surviving JavaScript executes in their browser within the Vvveb application's origin. No public PoC is available; the attack chain is inferred from the vulnerability class.

4. Post-Exploitation Impact: The executing JavaScript runs in the victim's browser session context. This permits:

   • Session data exposure: The script can access cookies (unless HttpOnly is set), DOM content, and other in-browser session state, satisfying the C:H (High Confidentiality) CVSS subscore.
   • Victim-context account actions: The attacker can perform actions as the victim, including administrative functions if an admin views the payload.
   • Defacement: The attacker can modify page content visible to the victim.
   • Phishing: The attacker can present fraudulent forms or overlays to harvest credentials.

   The CVSS vector indicates scope change (S:C), meaning the vulnerability can impact resources beyond the vulnerable component's security authority — the XSS executes in the context of the Vvveb origin and can affect other users and systems integrated with it. The I:L (Low Integrity) subscore reflects that while integrity of the application data may be affected, the primary impact vector is confidentiality. User interaction (UI:R) is required because a victim must view the crafted content for exploitation to succeed.

Impact Assessment

Business Impact: Successful exploitation enables persistent JavaScript injection into a CMS platform used for websites, blogs, and ecommerce stores. The consequences include session hijacking of authenticated users (including administrators), unauthorized account actions, website defacement, and phishing campaigns hosted on a trusted domain. For ecommerce deployments, this can lead to customer data theft and loss of trust.

Affected Systems: All Vvveb CMS installations prior to version 1.0.8.5 that permit Author-role or higher users to edit profile bios and that render those bios without sufficient output encoding.

Blast Radius: The scope change (S:C) in the CVSS vector indicates that exploitation can affect resources beyond the initially vulnerable component. Because stored XSS affects every viewer of the compromised page — including unauthenticated visitors, authenticated users, and administrators — the blast radius is broad. An administrator viewing an infected bio in the user-management view could have their privileged session compromised, leading to full site takeover.

CVSS Subscores: AV:N (network-exploitable), AC:L (no special conditions), PR:L (requires low-privileged Author account), UI:R (victim must view content), S:C (scope change), C:H (high confidentiality impact), I:L (low integrity impact), A:N (no availability impact). Total: 7.6 High.

Detection Guidance

Sigma Rule

title: Vvveb CMS Stored XSS via Profile Bio Field
id: 8f3a2b1c-4d5e-6f7a-8b9c-0d1e2f3a4b5c
status: experimental
description: Detects attempts to store malicious event-handler or nested tag payloads in the Vvveb CMS user bio field that may bypass sanitizeHTML()
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-54506
author: Valters IT Threat Intelligence
date: 2026/01/15
tags:
  - attack.t1190
  - attack.t1059.007
logsource:
  category: web
  product: vvveb
detection:
  selection_method:
    cs-method:
      - 'POST'
      - 'PUT'
  selection_uri:
    cs-uri-stem|contains:
      - '/user/profile'
      - '/profile.php'
  selection_payload:
    cs-uri-query|contains:
      - 'user[bio]='
  selection_keywords:
    cs-uri-query|contains:
      - 'onerror'
      - 'onload'
      - 'onclick'
      - 'onmouseover'
      - '<script'
      - '<svg'
      - '<img'
  condition: selection_method and selection_uri and selection_payload and selection_keywords
falsepositives:
  - Legitimate users discussing HTML/JavaScript in bio text
level: medium

YARA Rule

rule CVE_2026_54506_Vvveb_Stored_XSS
{
    meta:
        description = "Detects stored XSS payloads targeting Vvveb CMS sanitizeHTML() bypass"
        author = "Valters IT Threat Intelligence"
        date = "2026-01-15"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-54506"
        severity = "high"

    strings:
        $vvveb_path1 = "app/controller/user/profile.php" ascii
        $vvveb_path2 = "system/functions.php" ascii
        $func_sanitize = "sanitizeHTML" ascii
        $field_bio = "user[bio]" ascii

        $evt1 = /\/on[a-z]+\s*=/ nocase
        $evt2 = "onerror" nocase
        $evt3 = "onload" nocase
        $evt4 = "onclick" nocase
        $evt5 = "onmouseover" nocase

        $tag1 = "<script" nocase
        $tag2 = "<svg" nocase
        $tag3 = "<img" nocase
        $tag4 = "<iframe" nocase

        $nested1 = "<div><script" nocase
        $nested2 = "<span><img" nocase
        $nested3 = "<p><svg" nocase

    condition:
        (
            ($vvveb_path1 or $vvveb_path2 or $func_sanitize or $field_bio)
            and
            (any of ($evt*) or any of ($tag*) or any of ($nested*))
        )
        or
        (
            2 of ($evt*) and any of ($tag*)
        )
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-54506 Vvveb CMS Stored XSS via Profile Bio Field"; flow:established,to_server; http.method; content:"POST"; http.uri; content:"profile"; nocase; http.request_body; content:"user[bio]"; nocase; http.request_body; pcre:"/(?:\/on[a-z]+\s*=|<script|<svg|<img|<iframe)/i"; reference:cve,CVE-2026-54506; classtype:web-application-attack; sid:202654506; rev:1;)

Elastic Detection

http where http.request.method in ("POST", "PUT")
  and http.request.body.text contains "user[bio]"
  and (
    http.request.body.text regex~ "/on[a-z]+\\s*=" or
    http.request.body.text contains "<script" or
    http.request.body.text contains "<svg" or
    http.request.body.text contains "<img" or
    http.request.body.text contains "<iframe"
  )
  and (
    url.path contains "/profile" or
    url.path contains "/user"
  )

Splunk SPL Query

index=web_logs (sourcetype="access_combined" OR sourcetype="nginx:plus:kv" OR sourcetype="proxy")
| eval body = coalesce(form_data, request_body, uri_query)
| where match(body, "(?i)user\[bio\]")
| where match(body, "(?i)(/on[a-z]+\s*=|%2Fon[a-z]+%3D|<script|<svg|<img|<iframe|&lt;script)")
| stats count min(_time) as first_seen max(_time) as last_seen values(clientip) as src_ips values(uri_path) as target_uris by user, host
| convert ctime(first_seen) ctime(last_seen)
| where count >= 1
| sort - count

Wazuh OSSEC Rule

<group name="web,vvveb,xss,">
  <rule id="100506" level="10">
    <if_sid>31100,31101,31103,31104,31108</if_sid>
    <field name="url" type="pcre2">(?i)(profile|user).*user\[bio\]</field>
    <field name="url" type="pcre2">(?i)(\/on[a-z]+\s*=|%2Fon[a-z]+%3D)</field>
    <description>CVE-2026-54506: Vvveb CMS stored XSS attempt via profile bio field (event-handler bypass) - possible exploitation</description>
    <mitre>
      <id>T1190</id>
      <id>T1059.007</id>
    </mitre>
    <group>attack,web_attack,</group>
  </rule>

  <rule id="100507" level="10">
    <if_sid>31100,31101,31103,31104,31108</if_sid>
    <field name="url" type="pcre2">(?i)(profile|user).*user\[bio\]</field>
    <field name="url" type="pcre2">(?i)(<script|<svg|<img|<iframe|%3Cscript|%3Csvg|%3Cimg)</field>
    <description>CVE-2026-54506: Vvveb CMS stored XSS attempt via profile bio field (nested tag bypass) - possible exploitation</description>
    <mitre>
      <id>T1190</id>
      <id>T1059.007</id>
    </mitre>
    <group>attack,web_attack,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED. No patch evidence has been confirmed from tracked sources. While the NVD description states the issue is fixed in version 1.0.8.5, no vendor advisory, commit reference, or patch artifact has been verified. Organizations should not assume patch availability without independent confirmation.

1) Immediate Mitigations:

• Restrict Author-role and higher account creation to trusted personnel only. Since exploitation requires an authenticated Author-or-higher account, reducing the number of such accounts narrows the attack surface.
• Disable or restrict public author pages (/author/{username}) if not required for business operations.
• Apply Content Security Policy (CSP) headers that disallow inline JavaScript execution (script-src 'self' without 'unsafe-inline'). This can prevent payload execution even if storage succeeds.
• Deploy a Web Application Firewall (WAF) with custom rules to detect on* event handlers and nested script tags in POST bodies targeting profile endpoints.
• Set HttpOnly and Secure flags on session cookies to limit session theft via document.cookie access.

2) Patch/Upgrade Steps:

• Monitor the official Vvveb CMS repository and vendor channels for version 1.0.8.5 or later. The NVD description indicates 1.0.8.5 contains the fix.
• Test the upgrade in a staging environment before production deployment, verifying that the sanitization functions have been corrected (specifically the event-handler regex and the do-while loop condition).
• After upgrade, audit existing stored bios for residual malicious content, as patching the function does not retroactively clean already-stored payloads. Query the database for bio fields containing on*= patterns, <script, <svg, <img, and nested tag structures.

3) Long-Term Hardening:

• Enforce context-aware output encoding at all rendering sinks rather than relying solely on input sanitization. Sanitization at input is defense-in-depth; encoding at output is the primary control.
• Adopt a well-maintained HTML sanitization library (e.g., DOMPurify on the client side, HTML Purifier on the server side) instead of custom regex-based sanitizers, which are prone to bypasses.
• Implement a strict Content Security Policy and consider a Trusted Types policy to prevent DOM-based XSS sinks.
• Conduct regular code review of sanitization routines with unit tests specifically targeting bypass techniques (solidus-prefixed handlers, nested tags, encoding variations).
• Enforce the principle of least privilege for CMS roles; the Author role should not have unrestricted HTML input capabilities unless required.

References

1. NVD — CVE-2026-54506: https://nvd.nist.gov/vuln/detail/CVE-2026-54506
2. Vvveb CMS Official Project: https://www.vvveb.com/
3. Vvveb CMS GitHub Repository (source for patch verification): https://github.com/givanz/Vvveb
4. OWASP — Cross Site Scripting Prevention Cheat Sheet: https://cheatsheetseries.owasp.org/cheatsheets/Cross_Site_Scripting_Prevention_Cheat_Sheet.html
5. CWE-79 — Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting'): https://cwe.mitre.org/data/definitions/79.html

Threat Actor / Exploit-in-the-Wild Status: No CISA KEV listing. No public PoC, exploit code, or Nuclei template identified at time of writing. No known threat actor campaigns associated with this CVE. EPSS score not yet assigned by FIRST.org.