# CVE-2026-111056: FOSSBilling: Unauthenticated client account enumeration via guest signup endpoint

> Unauthenticated attackers can enumerate registered client emails by analyzing error responses from the guest signup endpoint.

- **Severity:** Medium 6.9 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 19 Oct 2026
- **Updated:** 22 Oct 2026
- **Also known as:** GHSA-x4mh-g4pc-wfc5

## What is CVE-2026-111056?

CVE-2026-111056 is a medium-severity vulnerability in FOSSBilling, published on 19 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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.

Weakness: CWE-307 (Improper Restriction of Excessive Authentication Attempts), CWE-204 (Observable Response Discrepancy), CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor)

## Which versions are affected by CVE-2026-111056?


## How do I fix CVE-2026-111056?

No fix has been published for CVE-2026-111056 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-111056 being exploited?

There is no known exploitation of CVE-2026-111056 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-111056 and fix it if it is affected.

What it is: CVE-2026-111056 is a medium-severity vulnerability in FOSSBilling, published on 19 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-111056 in the wild and no public exploit code.

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-2026-111056/ (Markdown: https://insights.sbomify.com/CVE-2026-111056/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-111056
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-111056
- OSV: https://osv.dev/vulnerability/CVE-2026-111056

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-111056/
Summaries are generated and may contain mistakes; the official description above is the record.