Fixing HTTP 401 Errors in Cross-Tenant Configuration for OCI Vault Authentication

Challenges in Cross-Tenant Vault Authentication Using OCI

Integrating HashiCorp Vault with Oracle Cloud Infrastructure (OCI) can be complex, especially when dealing with cross-tenant setups. When attempting to authenticate with Vault using the OCI auth method, users may encounter an HTTP 401 error during the login process.

This error typically arises when the instance and Vault are located in different OCI tenants. While authentication works seamlessly within the same tenant, cross-tenant setups present unique challenges that can complicate access permissions.

One such issue could be Vault's inability to properly access resources across tenants, even though policies allow instance listing from one tenant to another. Misconfigurations or overlooked permissions can also contribute to the 401 error.

This article explores the potential causes behind the 401 error and provides guidance on how to troubleshoot and resolve cross-tenant authentication issues in OCI Vault setups.

Understanding the Role of Cross-Tenant Vault Authentication Scripts

The scripts provided aim to solve a complex issue related to cross-tenant authentication between Oracle Cloud Infrastructure (OCI) and HashiCorp Vault. The primary problem arises when an instance in one OCI tenant (Tenant A) needs to authenticate with Vault in a different tenant (Tenant B). The Python script that uses OCI SDK and HashiCorp’s HVAC library is specifically crafted to authenticate an OCI instance to Vault via the OCI auth method. One of the key commands used is InstancePrincipalsSecurityTokenSigner, which allows the instance to authenticate itself without requiring pre-configured credentials, making it an essential solution for cross-tenant interactions.

This instance principal authentication method provides a secure and scalable way to authenticate OCI instances with Vault. The script connects to the Vault using the provided instance metadata and roles, attempting to verify permissions. The vault_client.auth.oci.login() method performs the actual login process by sending the role and instance metadata to Vault for verification. This login command is crucial for enabling OCI instances to communicate securely with Vault using instance-based authentication, especially in scenarios where tenants are separated.

In addition to Python scripts, a Terraform solution is included to configure the necessary OCI policies and dynamic groups for cross-tenant access. The oci_identity_policy resource defines policies that enable instances from Tenant A to access resources like Vault in Tenant B. This is achieved through the dynamic group matching_rule, which identifies instances that meet specific criteria, such as a compartment ID. These policies must be configured correctly to ensure that Vault can recognize and authenticate instances from a different tenant, which is key to resolving the HTTP 401 error in such setups.

Lastly, unit testing is implemented using Python's unittest.TestCase framework to ensure the authentication process works across different environments. The unit tests help verify both successful and failed login attempts, ensuring robustness in the cross-tenant authentication process. These tests simulate different scenarios, such as when Vault is unable to authenticate due to policy issues or if the instance principal is not recognized. By modularizing the scripts and testing them thoroughly, this solution provides a reliable framework for addressing cross-tenant authentication challenges in OCI and Vault environments.

import oci
import hvac
import os
# Initialize OCI config and vault client
config = oci.config.from_file()  # or config = oci.config.validate_config(oci.config.DEFAULT_LOCATION)
client = oci.identity.IdentityClient(config)
# Verify instance principal and get metadata
auth = oci.auth.signers.InstancePrincipalsSecurityTokenSigner()
metadata = client.list_instances(compartment_id='your_compartment_id')
# Connect to HashiCorp Vault
vault_client = hvac.Client(url=os.getenv('VAULT_ADDR'))
vault_login_path = 'v1/auth/oci/login'
response = vault_client.auth.oci.login(role='your_role', auth=auth, metadata=metadata)
if response['auth']:  # Successful authentication
    print("Vault login successful")
else:
    print("Vault login failed")

provider "oci" {
  tenancy_ocid       = var.tenant_A
  user_ocid          = var.user_ocid
  fingerprint        = var.fingerprint
  private_key_path   = var.private_key_path
  region             = var.region
}
resource "oci_identity_policy" "cross_tenant_policy" {
  compartment_id = var.compartment_id
  name           = "cross_tenant_policy"
  description    = "Policy for accessing Vault in tenant B from tenant A"
  statements     = [
    "Allow dynamic-group TenantBGroup to manage vaults in tenancy TenantA"
  ]
}
resource "oci_identity_dynamic_group" "tenant_b_group" {
  name        = "TenantBGroup"
  description = "Dynamic group for tenant B resources"
  matching_rule = "instance.compartment.id = 'tenant_A_compartment_id'"
}

import unittest
from vault_login_script import vault_login_function
# Test Vault login function
class TestVaultLogin(unittest.TestCase):
    def test_successful_login(self):
        self.assertTrue(vault_login_function())
    def test_failed_login(self):
        self.assertFalse(vault_login_function())
if __name__ == '__main__':
    unittest.main()

Addressing Cross-Tenant Challenges in OCI Vault Authentication

One often overlooked issue in cross-tenant setups is ensuring the correct configuration of dynamic groups and policies in OCI. When an instance from Tenant A attempts to authenticate with a Vault instance in Tenant B, proper policies must be configured on both sides to allow this communication. OCI’s security model is built around compartments, policies, and dynamic groups, which need to align perfectly across tenants. Without precise permissions, Vault may return a 401 error, signaling that authentication was denied.

A common solution involves setting up dynamic groups that include instances from Tenant A and allow them to authenticate with the resources in Tenant B. The dynamic group matching rule must be carefully crafted, usually by specifying the compartment ID or other unique identifiers. However, even with a correct dynamic group, the issue may arise if the policies in Tenant B don’t explicitly allow access from instances in Tenant A. This is why both policy configurations and dynamic groups must be reviewed meticulously to avoid authentication failures.

It’s also important to verify that the vault configuration itself allows cross-tenant access. HashiCorp Vault uses role-based access control (RBAC) to manage permissions. The role defined in the Vault auth method must be configured to recognize the dynamic groups and policies applied in OCI. Without proper role alignment, Vault will not be able to authenticate requests from instances in different tenants, leading to errors like the HTTP 401.