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Vulnerability · Published Oct 19, 2026 · Assigned by GitHub_M

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

Medium 6.9

In plain English

A medium-severity information exposure in FOSSBilling from FOSSBilling. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 21% of all known vulnerabilities).

Severity (CVSS 4.0)6.9Medium
Exploit odds (30 days)0.3%top 78.8% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

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.

Affected products (as reported)

VendorProductVersions
FOSSBilling FOSSBilling >= 0.1.0, < 0.8.7

Weakness: CWE-200, CWE-204, CWE-307 · Vector: 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

References