FOSSBilling is a free, open-source billing and client management system. Prior to version 0.8.0, the PayPalEmail payment adapter accepts PayPal IPN callbacks and credits the IPN‑supplied amount (`mc_gross`) to the client's balance without validating it against the invoice total. Combined with a $0.05 floating‑point epsilon tolerance in the invoice credit‑payment logic, this allows a client to underpay an invoice by up to $0.04 and still have it marked as fully paid. Version 0.8.0 patches the issue. There is no effective workaround without modifying the source code. Merchants using the PayPalEmail adapter should monitor IPN transactions for amounts that do not match their corresponding invoice totals, and manually review and refund suspicious payments.

All versions of FOSSBilling prior to 0.8.0 that use the PayPalEmail payment adapter are affected. The issue lies in the core payment processing logic of the FOSSBilling community edition.

The vulnerability has a CVSS score of 2.3, classifying it as low severity, and it is not listed in the CISA KEV catalog. The EPSS score is less than 1%, indicating a very low probability of exploitation in the wild. While no public exploitation has been reported, it is inferred that an attacker would need to influence the PayPal IPN flow—such as forging or manipulating IPN messages—to cause an invoice to be credited with a lower amount than actually paid, allowing a client to underpay an invoice by up to $0.04 without triggering a payment discrepancy.