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

Quick assessment

Affected
FOSSBilling FOSSBilling
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

fossbilling是fossbilling团队开源的一种高效计费和客户管理方案。 FOSSBilling 0.8.0之前版本存在安全漏洞，该漏洞源于PayPalEmail支付适配器接受IPN回调时，未将IPN提供的金额与发票总额进行验证，并结合发票信用支付逻辑中的0.05美元浮点容差，导致用户可少付发票款而系统仍标记为已全额支付。

AI Predicted 6.5 Difficulty: Easy EPSS 0.42% · P34

Possible ATT&CK Techniques 1 AI

T1589 · Gather Victim Identity Information

Affected Version Matrix 1

VendorProduct Version RangeStatus
FOSSBilling FOSSBilling < 0.8.0 affected
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I. Basic Information for CVE-2026-43928

Vulnerability Information
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Vulnerability Title
FOSSBilling: Payment amount not validated in PayPalEmail adapter allows invoice underpayment
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
对因果或异常条件的不恰当检查
Source: CVE Program / CVE List V5
Vulnerability Title
FOSSBilling 输入验证错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
fossbilling是fossbilling团队开源的一种高效计费和客户管理方案。 FOSSBilling 0.8.0之前版本存在安全漏洞，该漏洞源于PayPalEmail支付适配器接受IPN回调时，未将IPN提供的金额与发票总额进行验证，并结合发票信用支付逻辑中的0.05美元浮点容差，导致用户可少付发票款而系统仍标记为已全额支付。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
FOSSBilling FOSSBilling < 0.8.0 -

II. Public POCs for CVE-2026-43928

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III. Intelligence Information for CVE-2026-43928

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Vendor Advisories for CVE-2026-43928 (1)

Same Patch Batch · FOSSBilling · 2026-07-06 · 17 CVEs total

CVE-2026-42341 FOSSBilling has an unauthenticated payment bypass via IPN callback forgery
CVE-2026-42331 FOSSBilling missing authorization in guest Invoice API endpoints
CVE-2026-33734 FOSSBilling has improper SQL neutralization in `Massmailer` recipient filters
CVE-2026-43921 FOSSBilling vulnerable to arbitrary PHP code injection via unescaped config serialization
CVE-2026-43918 Suspended or inactive FOSSBilling accounts can retain or regain access through existing se
CVE-2026-43927 FOSSBilling has race condition in cart checkout that bypasses promo code usage limits
CVE-2026-43925 FOSSBilling: Mass assignment of group_id in guest client registration allows unauthorized
CVE-2026-53640 FOSSBilling missing authorization checks on read-only admin API endpoints expose sensitive
CVE-2026-53641 FOSSBilling has stored XSS in client email views via unescaped content in JavaScript templ
CVE-2026-53645 FOSSBilling's missing self-edit prevention in staff permission management allows persisten
CVE-2026-53646 FOSSBilling: Client password reset token reuse allows persistent account takeover
CVE-2026-53643 FOSSBilling allows low-privileged staff accounts to perform unauthorized actions via admin
CVE-2026-53644 FOSSBilling's missing order-state validation allows clients to read and reset API key secr
CVE-2026-53642 FOSSBilling: Unverified clients can access client-area pages when email confirmation is re
CVE-2026-53647 FOSSBilling vulnerable to unauthenticated API key configuration disclosure via guest Servi
CVE-2026-53648 FOSSBilling: Downloadable product files can be overwritten through filename collisions

IV. Related Vulnerabilities

Same product: FOSSBilling
Same vendor: FOSSBilling
Same weakness: CWE-754

V. Comments for CVE-2026-43928

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