CVE-2026-5199 is a security vulnerability classified under CWE-639 (Authorization Bypass Through User-Controlled Key) affecting Temporal Technologies, Inc.'s Temporal Server versions 1.29.0 and 1.30.0. The flaw arises from a logic error introduced in version 1.29.0 where the batch activity code incorrectly used a namespace name value controlled by the user rather than the server's trusted namespace name. This allowed a user with a writer role in an attacker-controlled namespace to perform privileged operations such as signaling, deleting, and resetting workflows or activities in victim namespaces residing on the same cluster. The vulnerability exploits the fact that while the batch activity validated the namespace ID, it failed to cross-check the namespace name against the worker's bound namespace, thereby granting the per-namespace worker's privileged credentials the ability to operate on arbitrary namespaces. Exploitation requires the attacker to know or guess specific victim workflow IDs and signal names, and depends on server configurations that permit cross-namespace authorization among internal components, such as the deployment of the internal-frontend service or equivalent TLS-based authorization for internal identities. This vulnerability also impacts Temporal Cloud when attacker and victim namespaces share the same cell under similar conditions. Although the CVSS 4.0 score is low (2.3), reflecting the need for some privileges and knowledge, the flaw represents a significant authorization bypass that could lead to unauthorized workflow manipulation. No public exploits have been reported to date, and no patches are linked in the provided data.

The primary impact of CVE-2026-5199 is unauthorized manipulation of workflows and activities across namespaces within the same Temporal cluster. An attacker with writer role privileges in one namespace can signal, delete, or reset workflows in other victim namespaces, potentially disrupting business processes, causing data loss, or triggering unintended operations. This could lead to integrity and availability issues within affected Temporal deployments. Organizations relying on Temporal for critical workflow orchestration may face operational disruptions, especially if sensitive or high-value workflows are targeted. The impact is somewhat mitigated by the requirement for the attacker to have writer role access in an attacker-controlled namespace and to know or guess victim workflow IDs and signal names. However, in multi-tenant or shared cluster environments, or where internal components allow cross-namespace authorization, the risk is elevated. Temporal Cloud customers sharing cells are also at risk under these conditions. While the CVSS score is low, the potential for unauthorized cross-namespace operations can have serious consequences for workflow integrity and availability.

To mitigate CVE-2026-5199, organizations should upgrade Temporal Server to versions beyond 1.30.0 once patches are released, as the vulnerability was introduced in 1.29.0 and persists in 1.30.0. Until patches are available, administrators should review and restrict server configurations that allow cross-namespace authorization among internal components, such as disabling or tightly controlling the internal-frontend service or equivalent TLS-based authorization mechanisms. Implement strict namespace isolation policies and limit writer role assignments to trusted users only. Monitor workflow operations for unusual signaling, deletion, or reset activities that could indicate exploitation attempts. Employ network segmentation and access controls to reduce the risk of unauthorized namespace access. For Temporal Cloud users, coordinate with the service provider to understand cell-sharing configurations and request mitigation or isolation if possible. Additionally, enforce strong workflow ID and signal name management to reduce the likelihood of guessing or discovery by attackers. Regularly audit namespace permissions and internal service configurations to ensure adherence to the principle of least privilege.