# Boundcredentials > Guide for configuring mTLS Proof-of-Possession (PoP) using Microsoft.Identity.Web. Covers all credential types: certificate, pure managed identity, and federated identity credentials (FIC). Use this when asked about mTLS PoP, MTLS_POP protocol scheme, token-bound authentication, or calling downstream APIs with proof-of-possession tokens. - Skill: `azuread/boundcredentials` (Agent Skill, multi-file: 4 files) - Install (CLI): `npx skillmds@latest add azuread/boundcredentials` - Raw SKILL.md: https://api.skillmd.com/api/skills/azuread/boundcredentials/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - License: MIT - Author: AzureAD (https://skillmd.com/u/azuread) - Updated: 2026-09-10 - Page: https://skillmd.com/skills/azuread/boundcredentials --- # mTLS Proof-of-Possession (PoP) This skill helps you configure **mTLS Proof-of-Possession** in applications that use **Microsoft.Identity.Web**. mTLS PoP cryptographically binds the access token to a client certificate, proving possession of the private key at the transport layer. ## When to Use This Skill - Configuring `ProtocolScheme = "MTLS_POP"` in DownstreamApi options - Calling downstream APIs with proof-of-possession tokens - Troubleshooting 401 errors related to mTLS PoP - Adding `x-ms-tokenboundauth` header for Azure Key Vault - Understanding which credential type to use for mTLS PoP --- ## Credential Types mTLS PoP supports three credential types. Each binds the token to a certificate differently: | Credential Type | Certificate Source | Use Case | |----------------|-------------------|----------| | **Certificate** | App's own cert (from Key Vault, store, or file) | Confidential client apps with their own cert | | **Pure MSI** | Binding cert from IMDS v2 | Azure VM/VMSS workloads using managed identity | | **FIC** | MSI signed assertion + app cert | Apps using federated identity credentials | --- ## 1. Certificate Credential The app uses its own certificate for both client authentication and PoP token binding. ```json { "AzureAd": { "Instance": "https://login.microsoftonline.com/", "TenantId": "", "ClientId": "", "ClientCredentials": [ { "SourceType": "KeyVault", "KeyVaultUrl": "https://myvault.vault.azure.net", "KeyVaultCertificateName": "my-app-cert" } ] }, "DownstreamApi": { "BaseUrl": "https://myapi.contoso.com/", "RequestAppToken": true, "ProtocolScheme": "MTLS_POP", "Scopes": [ "api:///.default" ] } } ``` --- ## 2. Pure Managed Identity (MSI) The binding certificate comes from the **IMDS v2 credential endpoint** on the VM/VMSS. No app-owned cert needed. **Requirements:** Azure VM or VMSS with Managed Identity v2 credential endpoint. ### System-Assigned MSI ```json { "DownstreamApi": { "BaseUrl": "https://myapi.contoso.com/", "RequestAppToken": true, "ProtocolScheme": "MTLS_POP", "Scopes": [ "api:///.default" ], "AcquireTokenOptions": { "ManagedIdentity": { } } } } ``` ### User-Assigned MSI ```json { "DownstreamApi": { "BaseUrl": "https://myapi.contoso.com/", "RequestAppToken": true, "ProtocolScheme": "MTLS_POP", "Scopes": [ "api:///.default" ], "AcquireTokenOptions": { "ManagedIdentity": { "UserAssignedClientId": "" } } } } ``` --- ## 3. Federated Identity Credential (FIC) Uses a managed identity's signed assertion as a client credential for a regular Entra ID app. The downstream call configuration is the same as other types. ```json { "AzureAd": { "Instance": "https://login.microsoftonline.com/", "TenantId": "", "ClientId": "", "ClientCredentials": [ { "SourceType": "SignedAssertionFromManagedIdentity", "ManagedIdentityClientId": "" } ] }, "DownstreamApi": { "BaseUrl": "https://myapi.contoso.com/", "RequestAppToken": true, "ProtocolScheme": "MTLS_POP", "Scopes": [ "api:///.default" ] } } ``` --- ## Azure Key Vault: Extra Header Required Azure Key Vault uses TLS renegotiation for client certificate presentation. It requires `x-ms-tokenboundauth: true` to trigger renegotiation. Without it, AKV returns 401: *"Client certificate required for using MTLS_POP token."* **Other resources (ARM, Storage, Graph) do NOT need this header** — they bind the cert on the initial TLS handshake. Add `ExtraHeaderParameters` to your AKV service config: ```json { "AzureKeyVault": { "BaseUrl": "https://myvault.vault.azure.net/", "RelativePath": "secrets/mysecret?api-version=7.4", "RequestAppToken": true, "ProtocolScheme": "MTLS_POP", "Scopes": [ "https://vault.azure.net/.default" ], "AcquireTokenOptions": { "ManagedIdentity": { "UserAssignedClientId": "" } }, "ExtraHeaderParameters": { "x-ms-tokenboundauth": "true" } } } ``` ### Sovereign Clouds | Cloud | Vault Suffix | Scope | |-------|-------------|-------| | Commercial | `.vault.azure.net` | `https://vault.azure.net/.default` | | China (21Vianet) | `.vault.azure.cn` | `https://vault.azure.cn/.default` | | US Government | `.vault.usgovcloudapi.net` | `https://vault.usgovcloudapi.net/.default` | All require `"x-ms-tokenboundauth": "true"`. --- ## How It Works 1. **Token Acquisition:** `ProtocolScheme = "MTLS_POP"` tells MSAL to request a PoP token. For MSI, IMDS v2 returns both the token and a binding certificate. 2. **HTTP Client:** `MsalMtlsHttpClientFactory` creates an `HttpClient` with the binding certificate in `HttpClientHandler.ClientCertificates`. 3. **Request:** The outbound request carries: - `Authorization: MTLS_POP ` — the proof-of-possession token - Client certificate via TLS — proves possession of the private key - `ExtraHeaderParameters` — any additional headers (e.g., `x-ms-tokenboundauth` for AKV) 4. **Validation:** The resource verifies the token's `cnf` claim matches the presented certificate thumbprint. --- ## Troubleshooting | Issue | Cause | Solution | |-------|-------|----------| | 401 "Client certificate required for using MTLS_POP token" | Missing `x-ms-tokenboundauth` header | Add `ExtraHeaderParameters` with the header (AKV only) | | 401 from AKV with valid token | Cert not bound to token | Verify VM has v2 IMDS endpoint returning binding cert | | `MsalServiceException` "MTLS_POP not supported" | Platform doesn't support mTLS PoP | Only Azure VM/VMSS support MSI v2 with binding cert | | Works on ARM but not AKV | Different cert binding mechanisms | AKV needs extra header; ARM doesn't | | `ExtraHeaderParameters` silently dropped | Header is in reserved list | `x-ms-tokenboundauth` is NOT reserved — check spelling | | Token works locally but not in prod | Local lacks MSI | mTLS PoP with MSI requires actual VM/VMSS; use cert credential locally | --- ## References - [mTLS PoP support (PR #3839)](https://github.com/AzureAD/microsoft-identity-web/pull/3839) - [ExtraHeaderParameters tests (PR #3864)](https://github.com/AzureAD/microsoft-identity-web/pull/3864) - [Sample: daemon-app-msi-mtls](../../tests/DevApps/daemon-app/daemon-app-msi-mtls/) - [Managed Identity docs](https://learn.microsoft.com/entra/identity/managed-identities-azure-resources/) - [DownstreamApi options](https://learn.microsoft.com/entra/msal/dotnet/microsoft-identity-web/downstream-api)