An upsert operation targeting a record’s primary key can ignore the tenant attribute, allowing a user from one tenant to read, overwrite, and take ownership of another tenant’s data. The vulnerability exists because the upsert logic does not enforce tenant scoping, and the resulting attributes—including the tenant identifier—are written to the target record. This gives the attacker full access to the victim’s data and the ability to permanently replace it with their own records. The weakness is classified as CWE‑639, highlighting an authorization bypass that results in data tampering and loss of integrity.

The issue affects the Ash framework version range from 1.22.0 up to but not including 3.37.2. Users running any of those releases on multi‑tenant deployments are vulnerable if they employ create actions with the upsert option enabled.

The CVSS score of 7.6 indicates a high severity level, and the lack of an EPSS score does not diminish the risk, especially since the defect is not yet listed in the CISA KEV catalog. The flaw can be exploited remotely by calling the exposed create API endpoint with an upsert payload that includes another tenant’s record ID, thereby violating isolation boundaries. An attacker does not need elevated privileges beyond legitimate tenant access, making the vulnerability highly practical for malicious tenants.