CVE-2026-28496

published 2026-06-23

CVE-2026-28496: 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…

Priority P1 83 critical 9.4 CVSS 4.0

AV N AC L AT N PR H UI N VC H VI H VA H SC H SI H SA H E X CR X IR X AR X MAV X MAC X MAT X MPR X MUI X MVC X MVI X MVA X MSC X MSI X MSA X S X AU X R X V X RE X U X

ITW EXPLOIT VulnCheck KEV Initial access

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.

Affected

1 ranges

Vendor	Product	Version range	Fixed in
fossbilling	fossbilling	< 0.8.0	0.8.0

Detection & IOCs extracted from sources · hover to see the quote

url /api/system/system/string_render

command {"_tpl":"{{ guest.getDi().db.getCell(\"SELECT @@version\") }}","_try":false}

other html:"FOSSBilling"

• → Detect POST requests to the string_render API endpoint with Twig expression payloads in the JSON body (_tpl field), which is the primary attack vector for this SSTI vulnerability.
• → Look for Twig expression syntax ({{ ... }}) in HTTP request bodies targeting FOSSBilling endpoints, particularly in email templates, mass mail campaigns, custom payment adapters, and the string_render API. ↗
• → Audit existing email templates and mass mail campaigns for suspicious Twig expressions that access getDi(), db, or other DI container objects. ↗
• → Block external access to /api/system/* at the reverse proxy or WAF level to mitigate chaining with GHSA-78x5-c8gw-8279. ↗

• · This vulnerability requires administrator-level access to exploit; it is not directly exploitable by unauthenticated users unless chained with another vulnerability (e.g., GHSA-78x5-c8gw-8279). ↗
• · The SSTI is possible because Twig templates are rendered without a sandbox; patched in version 0.8.0. Detections targeting versions >= 0.8.0 should not fire on patched instances. ↗

CVSS provenance

nvd v4.0 9.4 CRITICAL CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

vulncheck 9.4 CRITICAL

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