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CVE-2026-33543: FOSSBilling: Authentication bypass allows unauthenticated administrator creation

• Severity: critical (CVSS 9.3 v4.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-06-24 · updated 2026-06-25
• Weakness: CWE-288

Description

FOSSBilling is a free, open-source billing and client management system. Versions 0.7.2 and prior expose a guest API endpoint, /api/guest/staff/create, intended for initial administrator bootstrap. Due to a flawed admin-existence check, the endpoint remains usable after an administrator already exists. The flawed guard check uses is_countable() on a value that returns a Model_Admin object or null rather than a countable type, causing the expression to always evaluate as true and bypass the intended protection. As a result, an attacker can reach the unprotected endpoint to create a new administrator account and immediately authenticate, gaining a fully privileged admin session even when an admin already exists. This issue has been fixed in version 0.8.0.

Affected products

• FOSSBilling: before 0.8.0

References

• https://github.com/FOSSBilling/FOSSBilling/releases/tag/0.8.0
• https://github.com/FOSSBilling/FOSSBilling/security/advisories/GHSA-28mh-j262-q49w

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