CVE-2026-87857

Temporal Server includes a Worker Controller Instance module (go.temporal.io/auto-scaled-workers) that registers a 'k8s' compute provider which builds its Kubernetes client from the caller's request configuration rather than operator configuration. An authenticated caller with only read access in a single namespace can invoke ValidateWorkerDeploymentVersionComputeConfig, leading to arbitrary command execution on the Worker Service host under the server process account. Since the process holds persistence credentials for all namespaces and cluster TLS material, the impact extends beyond the caller's namespace to the entire cluster.

Temporal Server does not bind internal completion callbacks to the namespace that attached them, which allows an authenticated caller with write permission in one namespace to alter Scheduler V2 schedule state in another namespace. The callback address temporal://internal is reserved for callbacks that Scheduler V2 attaches to the Workflows it starts. In affected releases, any authenticated caller could attach a callback with that address to a Workflow start request, the address was exempt from the component.callbacks.allowedAddresses allowlist, and the History service delivered the callback to the component its token named, in whatever namespace that was, without authorizing the original caller there. When the forged completion matches an action a target schedule is running, the schedule records the action as completed while its Workflow still runs, which can defeat an overlap policy such as SKIP, and passes caller-controlled data to the next scheduled Workflow as its last completion result. A forged failure may also pause a schedule configured to pause on failure. A forgery requires the target namespace identifier, the schedule identifier, and the identity and timing of a running action. No disclosure of Workflow payloads or history was identified. Only namespaces that hold Scheduler V2 schedules can be targeted; Scheduler V2 is off by default, and existing Scheduler V2 schedules remain targets after the settings that create them are turned off.

A vulnerability in Temporal Server allows a connected peer cluster to modify local namespaces in the receiving cluster, which should not be replicated. The receiving cluster does not check if the namespace is global and overwrites its state, description, owner, retention, archival settings, cluster list, and active cluster if the task carries a higher failover version. An attacker controlling a registered peer's replies can change any local namespace without credentials on the receiving cluster, e.g., marking it Deprecated, causing new Workflow starts to fail.

Temporal Server vulnerability allows an authenticated user with write access to a single namespace to execute arbitrary commands on the host running the Worker Service. The command is executed immediately, and since the Worker Service process holds credentials and TLS material for the entire cluster, the impact escalates to full cluster compromise. Affected versions are 1.31.0 through 1.31.2 (before 1.31.3), and the issue is present in default binaries and container images.

Temporal Server in releases 1.30.0 and later decided whether a Workflow completion callback was internal by reading a caller-supplied HTTP header. An authenticated caller with write permission in a single namespace could attach a callback whose 'source' header caused the request to be re-targeted at the internal frontend and executed as a system administrator against the administrative API. Releases 1.25.0 through 1.29.7 are affected by a narrower form of the same defect requiring the header to match a configured cluster ID.

Temporal Server did not bound the work performed while searching for a Schedule's next action time. An authenticated caller with namespace write permission could create or update a Schedule combining a fine-grained cadence with an exclusion calendar that rejects every candidate time, causing excessive CPU consumption in Frontend and Schedule worker components.

The frontend gRPC server's streaming interceptor chain did not include the authorization interceptor. Configured ClaimMapper and Authorizer were bypassed for the AdminService/StreamWorkflowReplicationMessages endpoint, allowing unauthenticated access.

Insecure defaults in open-source Temporal Server before version 1.20 on all platforms allow an attacker to craft a task token with access to a namespace other than the one specified in the request. This requires the namespace UUID and information from the workflow history for the target namespace.