TYPO3 is a free and open source Content Management Framework. A vulnerability has been identified in the backend user interface functionality involving deep links. Specifically, this functionality is susceptible to Cross-Site Request Forgery (CSRF). Additionally, state-changing actions in downstream components incorrectly accepted submissions via HTTP GET and did not enforce the appropriate HTTP method. Successful exploitation of this vulnerability requires the victim to have an active session on the backend user interface and to be deceived into interacting with a malicious URL targeting the backend, which can occur under the following conditions: The user opens a malicious link, such as one sent via email. The user visits a compromised or manipulated website while the following settings are misconfigured: 1. `security.backend.enforceReferrer` feature is disabled, 2. `BE/cookieSameSite` configuration is set to lax or none. The vulnerability in the affected downstream component “Extension Manager Module” allows attackers to retrieve and install 3rd party extensions from the TYPO3 Extension Repository - which can lead to remote code execution in the worst case. Users are advised to update to TYPO3 versions 11.5.42 ELTS, 12.4.25 LTS, 13.4.3 LTS which fix the problem described.