Compliance Impact

The vulnerability in FOSSBilling allows administrators to inject arbitrary Twig expressions, potentially leading to information disclosure and remote code execution. Such unauthorized access and data exposure could compromise the confidentiality and integrity of personal and sensitive data managed by the system.

This could negatively impact compliance with common standards and regulations like GDPR and HIPAA, which require strict controls to protect personal data and ensure system security. The risk of data breaches or unauthorized access due to this vulnerability may lead to violations of these regulations.

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Executive Summary

CVE-2026-28496 is a Server-Side Template Injection (SSTI) vulnerability found in FOSSBilling versions prior to 0.8.0. It occurs because the Twig templates used for rendering emails, mass mail campaigns, custom payment adapters, and the string_render API endpoint are processed without sandboxing. This allows administrators with access to these features to inject arbitrary Twig expressions.

Due to the lack of sandboxing, the injected expressions can access the full Twig environment, the API context, and the application's dependency injection container, which can lead to information disclosure and remote code execution.

Version 0.8.0 of FOSSBilling patches this issue. Workarounds include auditing email templates for suspicious Twig expressions, rotating all admin and client API tokens, and blocking external access to certain API endpoints via reverse proxy or WAF.

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Impact Analysis

This vulnerability can have severe impacts including unauthorized information disclosure and remote code execution on the affected system.

An attacker with administrative access to template rendering features can exploit this vulnerability to execute arbitrary code on the server, potentially gaining full control over the application and its environment.

This can lead to data breaches, service disruption, and compromise of sensitive client and billing information managed by FOSSBilling.

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Detection Guidance

This vulnerability can be detected by auditing existing email templates and other Twig template usages for suspicious Twig expressions that could indicate Server-Side Template Injection (SSTI).

Since the vulnerability involves administrators injecting arbitrary Twig expressions in features like email templates, mass mail campaigns, custom payment adapters, and the `string_render` API endpoint, reviewing these areas for unexpected or malicious template code is essential.

No specific commands are provided in the available information to detect this vulnerability on your network or system.

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Mitigation Strategies

Immediate mitigation steps include upgrading FOSSBilling to version 0.8.0 or later, which patches the vulnerability.

• Audit existing email templates for suspicious Twig expressions.
• Rotate all admin and client API tokens.
• Block external access to the /api/system/* endpoint at the reverse proxy or Web Application Firewall (WAF) to prevent exploitation, especially to mitigate chaining with related vulnerabilities.

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