The langgraph-api prior to version 0.10.0 allows webhook targets to specify relative URLs delivered via an in-process loopback transport. The authentication middleware treats this loopback transport as internal and does not apply the authentication context used for external requests. In deployments relying on per-user authorization boundaries, this flaw enables an authenticated user to direct a webhook to the server's own thread and run routes, thereby creating or modifying runs on other users' threads and incorporating metadata from those threads. The vulnerability requires webhook targets and per-user authorization to be in use. The issue is resolved in version 0.10.0.

An authenticated user can bypass intended per-user authorization boundaries by exploiting the loopback transport webhook delivery mechanism. This allows unauthorized creation or modification of runs on other users' threads and limited incorporation of their metadata. The impact includes unauthorized modification of data (integrity) and limited availability impact. Confidentiality is not affected.

Upgrade langgraph-api to version 0.10.0 or later where this issue is fixed. Deployments that deliberately enable loopback delivery should restrict it strictly to controlled same-process routes to avoid unauthenticated webhook requests. No other mitigations are indicated.