FOSSBilling - Server-Side Template Injection

Description

A Server-Side Template Injection (SSTI) vulnerability exists in FOSSBilling's template rendering system. Administrators with access to features that render Twig templates (email templates, mass mail campaigns, custom payment adapters, and the string_render API endpoint) can inject arbitrary Twig expressions, leading to information disclosure and remote code execution. The vulnerability exists because Twig templates are rendered without a sandbox, allowing access to the full Twig environment, API context, and the application's dependency injection container.

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