FOSSBilling up to 0.7.x API Endpoint /api/system special elements used in a template engine

Summaryinfo

A vulnerability categorized as problematic has been discovered in FOSSBilling up to 0.7.x. Affected by this issue is some unknown functionality of the file /api/system of the component API Endpoint. The manipulation results in improper neutralization of special elements used in a template engine. This vulnerability is reported as CVE-2026-28496. The attack can be launched remotely. No exploit exists. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability was found in FOSSBilling up to 0.7.x. It has been classified as problematic. Affected is an unknown functionality of the file /api/system of the component API Endpoint. The manipulation with an unknown input leads to a improper neutralization of special elements used in a template engine vulnerability. CWE is classifying the issue as CWE-1336. The product uses a template engine to insert or process externally-influenced input, but it does not neutralize or incorrectly neutralizes special elements or syntax that can be interpreted as template expressions or other code directives when processed by the engine. This is going to have an impact on confidentiality, integrity, and availability. CVE summarizes:

FOSSBilling is a free, open-source billing and client management system. Versions prior to 0.8.0 have a Server-Side Template Injection (SSTI) vulnerability in the 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. Version 0.8.0 patches the issue. Some workarounds are available. Audit existing email templates for suspicious Twig expressions, rotate all admin and client API tokens, and/or block external access to /api/system/* at reverse proxy/WAF to mitigate chaining with GHSA-78x5-c8gw-8279.

The advisory is shared for download at github.com. This vulnerability is traded as CVE-2026-28496 since 02/27/2026. The exploitability is told to be easy. It is possible to launch the attack remotely. Additional levels of successful authentication are required for exploitation. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1221.

Upgrading to version 0.8.0 eliminates this vulnerability.

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Productinfo

Type

• Billing Software

Name

• FOSSBilling

Version

• 0.1
• 0.2
• 0.3
• 0.4
• 0.5
• 0.6
• 0.7

Website

• Product: https://github.com/FOSSBilling/FOSSBilling/

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