GHSA-7c4c-749j-pfp2

### Summary
An unsafe deserialization vulnerability allows any unauthenticated user to execute arbitrary code on the server.

### PoC
1. Go to
https://www.admidio.org/demo_en/adm_program/modules/messages/messages.php
2. Click on Send Private Message
3. In the `Message` field, enter the following payload
`Testing<br><h1>HTML</h1><br><h2>Injection</h2>`

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4. Send the message
5. Open the message again

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### Impact
1. Data Theft: Stealing sensitive information like cookies, session tokens, and user credentials.
2. Session Hijacking: Gaining unauthorized access to user accounts.
3. Phishing: Tricking users into revealing sensitive information.
4. Website Defacement: Altering the appearance or content of the website.
5. Malware Distribution: Spreading malware to users' devices.
6. Denial of Service (DoS): Overloading the server with malicious requests.