# CVE-2026-43928: FOSSBilling: Payment amount not validated in PayPalEmail adapter allows invoice underpayment

> Attackers can underpay invoices by exploiting missing validation in the PayPalEmail adapter, leading to financial loss for merchants.

- **Severity:** Low 2.3 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 6 Jul 2026
- **Updated:** 7 Jul 2026
- **Also known as:** GHSA-xjc6-g382-h942

## What is CVE-2026-43928?

CVE-2026-43928 is a low-severity vulnerability in FOSSBilling, published on 6 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

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.

Weakness: CWE-754 (Improper Check for Unusual or Exceptional Conditions), CWE-1284 (Improper Validation of Specified Quantity in Input)

## Which versions are affected by CVE-2026-43928?


## How do I fix CVE-2026-43928?

No fix has been published for CVE-2026-43928 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-43928 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-43928 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-43928 and fix it if it is affected.

What it is: CVE-2026-43928 is a low-severity vulnerability in FOSSBilling, published on 6 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-43928 exists.

Affected product: FOSSBilling. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-43928/ (Markdown: https://insights.sbomify.com/CVE-2026-43928/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-43928
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-43928
- OSV: https://osv.dev/vulnerability/CVE-2026-43928

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-43928/
Summaries are generated and may contain mistakes; the official description above is the record.