TL;DR (Résumé exécutif)

• PHP Object Injection (CVE-2025-68526) exists in Modal Popup Box (≤ 1.6.1). Fixed in 1.6.2.
• Exploit requires an authenticated account with Contributor-equivalent privileges; malicious serialized payloads are accepted and unserialized without adequate restrictions.
• Potential impact: RCE, arbitrary file writes, data exfiltration, privilege escalation, path traversal, DoS — depending on available gadget classes.
• Primary mitigation: update Modal Popup Box to 1.6.2 immediately. If unable to update, deactivate the plugin or apply targeted HTTP-layer protections (WAF/virtual patching) and restrict Contributor access until patched.

Qu'est-ce que l'injection d'objet PHP (POI) ?

POI occurs when untrusted input is passed to PHP’s unserialize() (or equivalent) without sufficient restriction. Unserializing attacker-controlled data can instantiate objects of existing classes; magic methods like __wakeup(), __destruct() and others may run automatically and be abused to perform unintended actions (POP — property-oriented programming — using gadget chains).

Points clés :

• POI can lead to more than data corruption — attackers may leverage existing classes as gadgets to perform file writes, execute code, or interact with system resources.
• Risk depends on what classes exist in the PHP environment: even small plugins, themes or libraries can introduce gadgets.
• Modern PHP provides safer unserialize options (e.g. option allowed_classes), and safer formats (JSON) are generally preferred for data interchange.

How this specific vulnerability works (high-level)

Summary of the exploitation flow based on disclosure details:

• The plugin accepts input from authenticated users (Contributor-level). Somewhere in the request processing path the plugin calls unserialize() (or equivalent) on attacker-controlled data.
• Parce que option allowed_classes filtering is not used or not restrictive, an attacker can craft serialized payloads that cause PHP to instantiate objects and trigger magic methods.
• If suitable gadget classes exist (in the plugin, theme or other libraries), a POP chain can be constructed to achieve high-impact outcomes such as RCE or arbitrary file manipulation.

Why Contributor-level matters: Contributor is a low-privilege WordPress role (can create/edit posts but cannot install plugins or change site settings). Many sites grant Contributor access to guest writers or external authors; such accounts are often easier to compromise or supply from public registration flows, lowering exploitation friction.

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

Real-world impact & typical exploitation goals

With a working gadget chain, an attacker may achieve:

• Arbitrary file writes (e.g., webshells under /wp-content/uploads/).
• Remote code execution via gadget-triggered execution or file inclusion.
• Data theft — reading wp-config.php or dumping database contents.
• Privilege escalation — creating admin accounts or altering roles.
• SQL-like manipulation by abusing internal DB interaction paths.
• Path traversal or sensitive file exposure.
• Denial of service — resource exhaustion or destructive file operations.

This vulnerability is high risk for production sites, particularly multisite installations or sites with multiple third-party themes/plugins that increase the likelihood of available gadget classes.

Qui est à risque ?

• Sites running Modal Popup Box ≤ 1.6.1.
• Sites that allow external/semi-trusted users to obtain Contributor-equivalent roles.
• Sites that include plugins/themes or libraries that expose gadget classes or misuse PHP magic methods.
• Hosted WordPress instances where administrators have not applied the patch or mitigations.

If your site permits multiple content authors, guest contributors or public registration flows that elevate users to Contributor, treat this vulnerability as urgent.

Immediate action checklist (site owners / administrators)

1. Check plugin version: If Modal Popup Box is installed and version ≤ 1.6.1, proceed immediately.
2. Update: Upgrade Modal Popup Box to version 1.6.2 or later — this is the primary remediation.
3. Si vous ne pouvez pas mettre à jour immédiatement :
   • Désactivez temporairement le plugin.
   • If deactivation breaks critical functionality, block access to the plugin’s vulnerable endpoints at the HTTP layer (see WAF/virtual patching section below).
   • Restrict or remove Contributor-level users until the site is patched.
4. Review user accounts: Audit Contributor and higher roles; remove or revalidate suspicious accounts and rotate credentials where appropriate.
5. Scan for signs of compromise (see Detection section). If you find indicators, follow the incident response checklist below.
6. Enable monitoring: file integrity checks, plugin/theme version alerts, and server log monitoring.
7. Apply targeted HTTP-layer protections where required (WAF rules or similar virtual patching techniques).

Detection: Signs of exploitation

Immediate red flags to investigate:

• Unexpected POST requests containing serialized PHP data (patterns like O:\d+:"ClassName":).
• Nouveaux fichiers ou fichiers modifiés dans /wp-content/uploads/, /wp-content/plugins/, /wp-content/themes/ or root directories.
• New admin users or sudden role changes.
• Unexplained scheduled events (WP-Cron/cron entries).
• PHP errors in logs referencing unserialize, magic-method-related errors, or fatal errors triggered by unexpected objects.
• Connexions sortantes inattendues depuis le serveur web.
• Database changes consistent with content manipulation by unknown users.
• Spikes in CPU or memory usage indicating possible DoS activity.
• Fichiers contenant eval, base64_decode, système, passthru, exec or similar — potential webshells/backdoors.

Conseils de chasse :

• Search logs for serialized object patterns: O:\d+:"[A-Za-z0-9_\\x5c]+" ou s:\d+:"..."; a:\d+:{.
• Search the database (especially wp_options) for suspicious serialized values.
• Check recent file modification timestamps: find . -type f -mtime -14.

Short-term mitigation via WAF / virtual patching

If immediate plugin update is not possible, mitigate at the HTTP layer by blocking or challenging malicious requests targeting the vulnerable endpoints. The following guidance is vendor-agnostic.

1. Block requests containing serialized object patterns targeting plugin endpoints:
   • Detection regex (conceptual): O:\d+:"[A-Za-z0-9_\\]+";t|O:\d+:"[A-Za-z0-9_\\]+"?:\d+:{
   • Generic regex to detect serialized objects in POST bodies: /(O:\d+:"[A-Za-z0-9_\\]+":\d+:\{)/i
   • Apply such checks only for requests to plugin admin endpoints (admin-ajax actions or plugin-specific URLs).
2. Make rules path-specific:
   • Avoid blocking serialized content site-wide; focus on the plugin’s endpoints and admin-ajax actions used by Modal Popup Box.
3. Detect Base64-encoded serialized payloads:
   • Some attackers encode payloads; if your HTTP protection can decode Base64 or gzip, enable decoding before inspection and match serialized patterns.
4. Limit access to sensitive admin endpoints:
   • Restrict access to plugin admin pages from untrusted networks, require authentication cookies, or block requests from external IPs that do not need access.
5. Rate-limit or challenge repeated POST requests with serialized content — consider CAPTCHA or throttling on suspicious traffic.
6. Sanitize request parameters where feasible — strip or neutralise suspicious serialized patterns for legacy endpoints if safe to do so.

Important caution: blocking serialized data globally can break legitimate operations for plugins that legitimately use serialization. Keep rules targeted and test in a staging environment where possible.

Example WAF rule (conceptual)

Conceptual pseudo-rule — adapt to your HTTP protection engine and test before deployment:

Name: Block_POI_Serialized_Object_ModalPopupBox

Conditions:
 - REQUEST_URI contains /wp-admin/admin-ajax.php OR /wp-admin/admin.php? (plugin admin endpoints) OR REQUEST_URI contains /wp-content/plugins/modal-popup-box/
 - REQUEST_METHOD is POST
 - REQUEST_BODY matches regex /(O:\d+:"[A-Za-z0-9_\\]+":\d+:\{)/

Action: Block (HTTP 403) or challenge (CAPTCHA/403)
Logging: log full request body and source IP, notify admin
    

If your protection can decode Base64/gzip, enable those decoders to detect obfuscated payloads.

Developer guidance (how plugin authors should fix this)

If you maintain a plugin, library or theme, the following measures will greatly reduce POI risk:

1. Éviter unserialize() on untrusted data. Prefer JSON (json_encode/json_decode) for data interchange.
2. Si unserialize() is unavoidable:
   • Use the option allowed_classes option in PHP 7+: $data = @unserialize($input, ['allowed_classes' => false]);
   • Or whitelist specific safe classes: $data = @unserialize($input, ['allowed_classes' => ['MySafeClass']]);
3. Enforce strict server-side capability checks — require an appropriate capability (e.g. current_user_can('gérer_options')) for operations that process serialized data.
4. Validate and sanitize input: reject data that fails schema/type checks.
5. Never evaluate or include user-supplied content (avoid eval(), dynamic includes based on user input, etc.).
6. Add nonce checks and CSRF protection for admin-facing actions.
7. Conduct code reviews and automated scans for risky constructs (unserialize, eval, create_function, etc.).
8. Add unit tests that feed malformed serialized data to ensure safe failure modes.
9. Document security considerations in plugin documentation and changelogs.

Example secure deserialization pattern:

// Safer (PHP 7+):
$unserialized = @unserialize($input, ['allowed_classes' => false]);
if ($unserialized === false && $input !== 'b:0;') {
    // handle error
}

// Best: use JSON for structured data:
// When saving:
$json = wp_json_encode($data);
// When reading:
$data = json_decode($json, true);
    

Liste de contrôle de réponse aux incidents (si vous soupçonnez une compromission)

1. Isolate — place site in maintenance mode. If active shell access is suspected, isolate the instance from the network where possible.
2. Snapshot — take a full server snapshot (files + DB) for forensic analysis before making changes.
3. Disable plugin — immediately deactivate Modal Popup Box (or remove the plugin directory if deactivation is not possible).
4. Rotate secrets — change WordPress salts, admin passwords, FTP/SFTP accounts, database passwords, and API keys.
5. Recherchez des indicateurs :
   • Files: find recently modified files (e.g. find . -type f -mtime -N).
   • DB: inspect wp_options, wp_posts for suspicious serialized payloads or unauthorized admin accounts.
   • Logs: review web server access+error logs, PHP logs, and DB logs.
6. Remove webshells/backdoors — replace infected files from a known-good backup.
7. Restore from a clean backup where possible (from before the compromise).
8. Rebuild if necessary — in some incidents a full rebuild (reinstalling core/themes/plugins) is simpler and more reliable than attempted cleanup.
9. Check scheduled tasks — review WP-Cron and system cron jobs for injected tasks.
10. Review users and permissions — remove unauthorized accounts and confirm legitimate account integrity.
11. Harden — apply the plugin update (1.6.2), enforce strict file permissions and remove unused plugins/themes.
12. Monitor — after remediation, monitor traffic, logs and file changes for recurrence.
13. Report and review — perform root cause analysis and inform affected stakeholders if you operate hosting or manage multiple sites.

How to audit your site specifically for this vulnerability

1. Inventory — confirm whether Modal Popup Box is installed and note its version.
2. Endpoint discovery — identify plugin AJAX endpoints and admin actions used by the plugin; monitor these endpoints for serialized payloads.
3. Log analysis — search access logs for POST requests to relevant endpoints and for serialized object markers in request bodies.
4. Database scans — inspect wp_options and other plugin-stored settings for serialized content and recent modifications.
5. File integrity — compare installed plugin files with the official package checksums.
6. User audit — check recent contributor activity and newly created accounts for suspicious content or behaviour.
7. Automated scanning — use scanners that detect POI patterns and common gadget chains (vendor-agnostic).

Recommandations pour une posture de sécurité à long terme

• Apply least-privilege principles for WordPress roles; limit Contributor capabilities where possible.
• Enable timely plugin updates or automate updates for trusted releases after test validation.
• Adopt a secure development lifecycle: code reviews, static analysis, and security testing before releases.
• Maintain HTTP-layer protections and targeted virtual patches for emergency windows.
• Implement monitoring: file integrity checks, endpoint logging, and centralized log aggregation with alerting.
• Maintain regular backups and validate restoration procedures frequently.
• Educate contributors on credential hygiene and consider MFA for privileged accounts where feasible.

Technical appendix: sample detection regex & fingerprints

Adapt these patterns to your log-search or HTTP protection tools; test to reduce false positives.

• Detect serialized PHP object: O:\d+:"[A-Za-z0-9_\\]+";
• Detect serialized array/object beginnings: (a:\d+:{|O:\d+:"[A-Za-z0-9_\\]+"?:\d+:{)
• Detect suspicious Base64 strings (very long): ^[A-Za-z0-9+/]{200,}={0,2}$
• Detect magic-method or dangerous keywords in file content: __wakeup|__destruct|eval|base64_decode|system\(|passthru\(|exec\(|assert\(

Summary for operators

This POI issue is a sober reminder: plugins that unserialize user data without strict controls can introduce severe risk. Prioritise the following now:

1. Check if Modal Popup Box is installed and upgrade to 1.6.2 immediately where possible.
2. If you cannot update immediately, deactivate the plugin and apply targeted HTTP-layer protections while restricting Contributor accounts.
3. Perform a targeted integrity and compromise scan and follow the incident response checklist if indicators are found.
4. Implement the developer hardening steps to prevent similar issues in future releases.

Remain vigilant: treat plugin unserialization with suspicion, patch quickly, and monitor closely after remediation.