Merge pull request HackTricks-wiki#1250 from HackTricks-wiki/update_Critical_Vulnerability_Impacting_Over_100K_Sites_P_20250806_182934

Critical Vulnerability Impacting Over 100K Sites Patched in ...

Author: carlospolop

src/pentesting-web/deserialization/README.md

@@ -119,6 +119,64 @@ $o->param = "PARAM";
 $ser=serialize($o);
 ```
 
+### Preventing PHP Object Injection with `allowed_classes`
+
+> [!INFO]
+> Support for the **second argument** of `unserialize()` (the `$options` array) was added in **PHP 7.0**. On older versions the function only accepts the serialized string, making it impossible to restrict which classes may be instantiated.
+
+`unserialize()` will **instantiate every class** it finds inside the serialized stream unless told otherwise.  Since PHP 7 the behaviour can be restricted with the [`allowed_classes`](https://www.php.net/manual/en/function.unserialize.php) option:
+
+```php
+// NEVER DO THIS – full object instantiation
+$object = unserialize($userControlledData);
+
+// SAFER – disable object instantiation completely
+$object = unserialize($userControlledData, [
+    'allowed_classes' => false    // no classes may be created
+]);
+
+// Granular – only allow a strict white-list of models
+$object = unserialize($userControlledData, [
+    'allowed_classes' => [MyModel::class, DateTime::class]
+]);
+```
+
+If **`allowed_classes` is omitted _or_ the code runs on PHP < 7.0**, the call becomes **dangerous** as an attacker can craft a payload that abuses magic methods such as `__wakeup()` or `__destruct()` to achieve Remote Code Execution (RCE).
+
+#### Real-world example: Everest Forms (WordPress) CVE-2025-52709
+
+The WordPress plugin **Everest Forms ≤ 3.2.2** tried to be defensive with a helper wrapper but forgot about legacy PHP versions:
+
+```php
+function evf_maybe_unserialize($data, $options = array()) {
+    if (is_serialized($data)) {
+        if (version_compare(PHP_VERSION, '7.1.0', '>=')) {
+            // SAFE branch (PHP ≥ 7.1)
+            $options = wp_parse_args($options, array('allowed_classes' => false));
+            return @unserialize(trim($data), $options);
+        }
+        // DANGEROUS branch (PHP < 7.1)
+        return @unserialize(trim($data));
+    }
+    return $data;
+}
+```
+
+On servers that still ran **PHP ≤ 7.0** this second branch led to a classic **PHP Object Injection** when an administrator opened a malicious form submission. A minimal exploit payload could look like:
+
+```
+O:8:"SomeClass":1:{s:8:"property";s:28:"<?php system($_GET['cmd']); ?>";}
+```
+
+As soon as the admin viewed the entry, the object was instantiated and `SomeClass::__destruct()` got executed, resulting in arbitrary code execution.
+
+**Take-aways**
+1. Always pass `['allowed_classes' => false]` (or a strict white-list) when calling `unserialize()`.
+2. Audit defensive wrappers – they often forget about the legacy PHP branches.
+3. Upgrading to **PHP ≥ 7.x** alone is *not* sufficient: the option still needs to be supplied explicitly.
+
+---
+
 ### PHPGGC (ysoserial for PHP)
 
 [**PHPGGC**](https://github.com/ambionics/phpggc) can help you generating payloads to abuse PHP deserializations.\
@@ -663,6 +721,8 @@ The tool [JMET](https://github.com/matthiaskaiser/jmet) was created to **connect
 
 ### References
 
+- [Patchstack advisory – Everest Forms unauthenticated PHP Object Injection (CVE-2025-52709)](https://patchstack.com/articles/critical-vulnerability-impacting-over-100k-sites-patched-in-everest-forms-plugin/)
+
 - JMET talk: [https://www.youtube.com/watch?v=0h8DWiOWGGA](https://www.youtube.com/watch?v=0h8DWiOWGGA)
 - Slides: [https://www.blackhat.com/docs/us-16/materials/us-16-Kaiser-Pwning-Your-Java-Messaging-With-Deserialization-Vulnerabilities.pdf](https://www.blackhat.com/docs/us-16/materials/us-16-Kaiser-Pwning-Your-Java-Messaging-With-Deserialization-Vulnerabilities.pdf)