---
id: CVE-2026-111056
title: >-
  FOSSBilling: Unauthenticated client account enumeration via guest signup
  endpoint
summary: >-
  FOSSBilling is an open source billing and client-management platform. From
  0.1.0 until 0.8.7, the unauthenticated POST /api/guest/client/create endpoint
  reveals whether a submitted email belongs to a registered client because guest
  signu…
severity: medium
cvss: 6.9
cvssVector: 'CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N'
cvssSource: cna
cwe:
  - CWE-200
  - CWE-204
  - CWE-307
vendor: FOSSBilling
product: FOSSBilling
affected:
  - 'FOSSBilling >= 0.1.0, < 0.8.7'
published: '2026-10-19'
updated: '2026-10-19'
sourceUpdated: '2026-10-19T16:00:57.576Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-111056'
references:
  - url: >-
      https://github.com/FOSSBilling/FOSSBilling/security/advisories/GHSA-x4mh-g4pc-wfc5
    label: >-
      https://github.com/FOSSBilling/FOSSBilling/security/advisories/GHSA-x4mh-g4pc-wfc5
  - url: 'https://github.com/FOSSBilling/FOSSBilling/pull/4234'
    label: 'https://github.com/FOSSBilling/FOSSBilling/pull/4234'
  - url: >-
      https://github.com/FOSSBilling/FOSSBilling/commit/19f41ff0a45874c7362b7732c8f68db84bc59c55
    label: >-
      https://github.com/FOSSBilling/FOSSBilling/commit/19f41ff0a45874c7362b7732c8f68db84bc59c55
  - url: 'https://github.com/FOSSBilling/FOSSBilling/releases/tag/0.8.7'
    label: 'https://github.com/FOSSBilling/FOSSBilling/releases/tag/0.8.7'
tags:
  - cve.org
ingestedAt: '2026-10-19T16:32:30.072Z'
---

## Overview

FOSSBilling is an open source billing and client-management platform. From 0.1.0 until 0.8.7, the unauthenticated POST /api/guest/client/create endpoint reveals whether a submitted email belongs to a registered client because guest signup returns a distinct unredacted error when the submitted email already exists. The trigger is an unauthenticated request to POST /api/guest/client/create with a target email address. An attacker can compare the guest-signup response for a target email to determine whether the email belongs to an existing FOSSBilling client. Box\Mod\Client\Api\Guest::create returns the error after Client\Service::clientAlreadyExists finds a matching record, and the API response exposes that exception message without redaction. This issue is fixed in version 0.8.7.

## Affected

- `FOSSBilling >= 0.1.0, < 0.8.7`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.