# CVE-2026-43928: FOSSBilling invoice underpayment in PayPalEmail adapter

Source: https://junglewise.ai/threats/cve-2026-43928-fossbilling-invoice-underpayment-in-paypalemail-adapter  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-06. Updated: 2026-07-06  
License: CC BY 4.0 with a link back

CVE-2026-43928 · Severity: info · CVSS 2.3 · Published 2026-07-06

FOSSBilling, an open-source billing and client management platform, contains a flaw in how it processes PayPal payments. A customer can intentionally underpay an invoice by a small amount (up to $0.04), and the system will still mark the invoice as fully paid. While the loss per transaction is minor, this could lead to measurable financial losses if exploited repeatedly across many automated subscriptions or invoices.

Technologies: [FOSSBilling](https://junglewise.ai/threats/technologies/fossbilling). Vendors: [FOSSBilling](https://junglewise.ai/threats/vendors/fossbilling).

## Quick answers

**Is CVE-2026-43928 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-43928 being exploited in the wild.

**How severe is CVE-2026-43928?** Informational severity, with a CVSS score of 2.3, published 6 July 2026.

**Which products and versions are affected?** FOSSBilling < 0.8.0.

**Is there a fix for CVE-2026-43928?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-43928 attacked?** It is attacked over the network, in a network appliance.

**Am I affected by CVE-2026-43928?** Check whether you run FOSSBilling at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The vulnerability exists in the PayPalEmail payment adapter (src/library/Payment/Adapter/PayPalEmail.php) and the invoice credit-payment logic (src/modules/Invoice/Service.php). The adapter accepts PayPal Instant Payment Notification (IPN) callbacks and credits the 'mc_gross' amount to a user's balance without verifying it against the actual invoice total. When combined with a $0.05 floating-point epsilon tolerance used in the 'tryPayWithCredits' function, a user can pay $0.04 less than required and the system will treat the invoice as settled. This is exploitable by any registered user making a payment via PayPal. The issue is resolved in version 0.8.0.

## Affected products

- FOSSBilling FOSSBilling < 0.8.0

## Timeline

- 2026-06-04: advisory: GitHub Security Advisory published
- 2026-07-06: disclosed: CVE published to NVD

## References

- [https://github.com/FOSSBilling/FOSSBilling/security/advisories/GHSA-xjc6-g382-h942](https://github.com/FOSSBilling/FOSSBilling/security/advisories/GHSA-xjc6-g382-h942)

## Related threats

- [FOSSBilling stored XSS in support ticket notes](https://junglewise.ai/threats/cve-2026-111048-fossbilling-is-an-open-source-billing-and-client-management)
- [FOSSBilling CSRF vulnerability in staff guest login endpoint](https://junglewise.ai/threats/cve-2026-112374-fossbilling-before-0-8-9-contains-a-cross-site-request-forgery)
- [FOSSBilling email enumeration via guest signup endpoint](https://junglewise.ai/threats/cve-2026-111056-fossbilling-is-an-open-source-billing-and-client-management)
- [FOSSBilling path traversal in template_exists endpoint](https://junglewise.ai/threats/cve-2026-111053-fossbilling-is-an-open-source-billing-and-client-management)
- [FOSSBilling authorization bypass in extension configuration management](https://junglewise.ai/threats/cve-2026-111046-fossbilling-is-an-open-source-billing-and-client-management)