CVE-2026-111056

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.

CVSS 4.0

6.9CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N

EPSS

0.3% chance of exploitation in the next 30 days; higher than 20% of CVEs (scores of 2026-10-26)

CWEs

CWE-200, CWE-204, CWE-307

Published

2026-10-19

Modified

2026-10-22

Record from

the NVD, synced 2026-10-22

References

• https://github.com/FOSSBilling/FOSSBilling/commit/19f41ff0a45874c7362b7732c8f68db84bc59c55
• https://github.com/FOSSBilling/FOSSBilling/pull/4234
• https://github.com/FOSSBilling/FOSSBilling/releases/tag/0.8.7
• https://github.com/FOSSBilling/FOSSBilling/security/advisories/GHSA-x4mh-g4pc-wfc5

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