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

Vulnerability summary

CVE-2026-111056 is a medium (CVSS 6.9) vulnerability in fossbilling. EPSS estimates a 0.29% chance of exploitation in the next 30 days, higher than 19.9% of scored vulnerabilities.

Timeline

1. 19-10-2026
   PublishedVulnerability first disclosed.
2. 22-10-2026
   Last modifiedVulnerability information updated.

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.

• CWE-200Exposure of Sensitive Information to an Unauthorized ActorThe product exposes sensitive information to an actor that is not explicitly authorized to have access to that information.
• CWE-204Observable Response DiscrepancyThe product provides different responses to incoming requests in a way that reveals internal state information to an unauthorized actor outside of the intended control sphere.
• CWE-307Improper Restriction of Excessive Authentication AttemptsThe product does not implement sufficient measures to prevent multiple failed authentication attempts within a short time frame.