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CVE-2026-42341 – Unauthenticated Payment Bypass – FOSSBilling 0.6.0 through 0.7.2

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Summary

CVE-2026-42341 is a critical unauthenticated payment bypass vulnerability in FOSSBilling, a free open-source web-based billing and client management system. Affecting versions 0.6.0 through 0.7.2, the flaw allows an unauthenticated remote attacker to fraudulently mark any unpaid invoice as paid and credit the associated client account without making a real payment, by sending a single crafted HTTP request to the IPN callback endpoint. The vulnerability carries a CVSS 4.0 score of 9.2 (Critical) and is patched in version 0.8.0.

Technical details

  • Root cause: The IPN (Instant Payment Notification) callback endpoint (/ipn.php) fails to properly authenticate or validate the origin of incoming callback requests when the Custom payment adapter is enabled. The flaw is classified under CWE-306 (Missing Authentication for Critical Function) and CWE-346 (Origin Validation Error).
  • Trigger conditions: The Custom payment gateway must be enabled on the affected FOSSBilling instance. No credentials, session tokens, or user interaction are required for exploitation.
  • Attack vector: A remote, unauthenticated attacker sends a single crafted HTTP request to /ipn.php. The server processes the request as a legitimate payment callback and marks the targeted invoice as paid, crediting the associated client account.
  • Impact: An attacker can fraudulently obtain services, subscriptions, or account credits without completing any actual payment. This constitutes a critical business logic and financial integrity violation. The integrity impact affects both the vulnerable system (VI:H) and downstream systems (SI:H).

Affected software

  • FOSSBilling versions 0.6.0 through 0.7.2 (when the Custom payment adapter is enabled)

Severity

  • CVSS 4.0 Base Score: 9.2 (Critical)
  • Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:H/SA:N

Mitigation and recommended actions

  • Immediate – Apply the patch: Upgrade to FOSSBilling 0.8.0 or later. Version 0.8.0 includes hardened validation of payment IPN handling, transaction creation, and invoice payment processing, and directly resolves this vulnerability.
  • Workaround (if immediate patching is not feasible):
    • Disable the Custom payment gateway if it is not actively required. This eliminates the vulnerable code path entirely.
    • Restrict access to /ipn.php at the web server level using IP allowlisting to limit which IP addresses (e.g., known payment processor IPs) can reach the endpoint. Note that this may interfere with legitimate payment callback processing if not configured carefully.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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