CVE-2026-81905

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CVE-2026-81905

Summary: Concrete CMS below 9.5.3 does not enforce validation-hash type on redemption, allowing a hash issued for one purpose to be redeemed for another.

Details: Concrete CMS below 9.5.3 stores user validation hashes for multiple purposes (email/registration validation, password reset, and persistent login) in a single table with a type column, but the redemption path resolves a hash by value alone and does not verify its type. As a result, a hash issued for one purpose can be redeemed for another: a long-lived registration hash (60-day expiry) can be submitted to the password-change endpoint to set a new password on the target account, and a password-reset hash can be submitted to the email-validation endpoint to activate an account. Exploitation requires the attacker to first obtain a valid hash through a separate channel (for example email interception, log exposure, or SSRF against an internal mail relay), so the flaw amplifies the impact of any hash disclosure rather than creating a standalone entry point. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 6.3 with vector CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N. Thanks Yonatan Drori from Tenzai for reporting.

References: https://documentation.concretecms.org/developers/introduction/version-history/953-release-notes, https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/81xxx/CVE-2026-81905.json, https://nvd.nist.gov/vuln/detail/CVE-2026-81905, https://github.com/concretecms/concretecms