Letta versions up to 0.16.8 contain an authentication bypass flaw that lets attackers set a self‑asserted user_id HTTP header to impersonate any tenant. Hijacking a tenant grants full read and write access to that tenant’s agents, memory, and configuration, effectively creating a complete compromise of the impersonated account. The weakness is classified as CWE‑639 – Insecure Direct Object Reference, reflecting the failure to enforce proper ownership checks on user‑supplied identifiers.

The flaw exists in the open‑source LETTA project, specifically in the LeTTA package whose APIs are exposed by the server component. All deployments using LeTTA up to version 0.16.8 are affected; later releases contain a fix.

The CVSS score of 8.6 marks this a high severity vulnerability, and there is no EPSS data currently available. The vulnerability is not listed in the CISA KEV catalog, but its impact warrants immediate action. Attackers can exploit the flaw by sending crafted HTTP requests to any LeTTA server endpoint that accepts the user_id header, regardless of LETTA_SERVER_SECURE being enabled. The attack can be executed over the network by anyone who can reach the API without additional authentication controls.