Document mTLS PoP usage in MSAL (all pop features are on internal preview)

docs/msi_v2/how_to_mtls_pop_with_msi.md

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+---
+title: Use managed identities with mTLS proof-of-possession (preview)
+description: Learn how to use managed identity with mTLS proof-of-possession tokens in MSAL.NET.
+ms.service: entra-id
+ms.subservice: develop
+ms.topic: conceptual
+ms.date: 11/17/2025
+---
+
+# Use managed identities with mTLS proof-of-possession (internal microsoft only - preview)
+
+> [!IMPORTANT]
+> mTLS proof-of-possession (mTLS PoP) for managed identities is currently in internal preview.
+>
+> To use `WithMtlsProofOfPosession`, you must add the package
+> [`Microsoft.Identity.Client.MtlsPop`](https://www.nuget.org/packages/Microsoft.Identity.Client.MtlsPop) (for example, version `4.79.1-preview`).
+>
+> The resource (API) must be configured to accept mTLS PoP tokens and validate the certificate bound to the token.
+
+mTLS PoP builds directly on top of the existing managed identity experience:
+
+- You still use `ManagedIdentityApplicationBuilder`.
+- You still call `AcquireTokenForManagedIdentity`.
+
+The only changes are:
+
+- Build Managed Identity app [using MSAL](https://learn.microsoft.com/en-us/entra/msal/dotnet/advanced/managed-identity). 
+- Add the MtlsPoP package.
+- Add `.WithMtlsProofOfPosession()` when acquiring the token.
+- Use the returned binding certificate when calling the API over mTLS.
+
+Below we show the current (Bearer) code first, then the new (mTLS PoP) version, using Microsoft Graph as the example API.
+
+## 1. Add the MtlsPoP package
+
+Install the preview package alongside `Microsoft.Identity.Client`:
+
+```bash
+dotnet add package Microsoft.Identity.Client.MtlsPop --version 4.79.1-preview
+```
+
+This package:
+
+- exposes the `WithMtlsProofOfPosession()` extension, and
+- brings in a native dependency used to attest managed identity keys (for example KeyGuard keys) via Microsoft Azure Attestation (MAA).
+
+---
+
+## 2. System-assigned managed identity – from Bearer to mTLS PoP (Graph)
+
+### Current experience – Bearer (Graph)
+
+```csharp
+// System-assigned managed identity
+IManagedIdentityApplication mi =
+    ManagedIdentityApplicationBuilder
+        .Create(ManagedIdentityId.SystemAssigned)
+        .Build();
+
+// Microsoft Graph as the target API
+const string graphScope = "https://graph.microsoft.com/";
+
+AuthenticationResult result = await mi
+    .AcquireTokenForManagedIdentity(graphScope)
+    .ExecuteAsync()
+    .ConfigureAwait(false);
+
+// result.AccessToken is a Bearer token
+// result.TokenType == "Bearer"
+```
+
+### New experience – mTLS PoP (Graph)
+
+```csharp
+// System-assigned managed identity
+IManagedIdentityApplication mi =
+    ManagedIdentityApplicationBuilder
+        .Create(ManagedIdentityId.SystemAssigned)
+        .Build();
+
+// Microsoft Graph as the target API
+const string graphScope = "https://graph.microsoft.com/";
+
+AuthenticationResult result = await mi
+    .AcquireTokenForManagedIdentity(graphScope)
+    .WithMtlsProofOfPosession()   // <-- new API
+    .ExecuteAsync()
+    .ConfigureAwait(false);
+
+// result.TokenType == "mtls_pop"
+// result.BindingCertificate is the client cert to use for mTLS
+```
+
+---
+
+## 3. User-assigned managed identity – from Bearer to mTLS PoP (Graph)
+
+### Current experience – Bearer (Graph)
+
+```csharp
+// User-assigned managed identity
+IManagedIdentityApplication mi =
+    ManagedIdentityApplicationBuilder
+        .Create(ManagedIdentityId.WithUserAssignedClientId(userAssignedClientId))
+        .Build();
+
+// Microsoft Graph as the target API
+const string graphScope = "https://graph.microsoft.com/";
+
+AuthenticationResult result = await mi
+    .AcquireTokenForManagedIdentity(graphScope)
+    .ExecuteAsync()
+    .ConfigureAwait(false);
+
+// result.TokenType == "Bearer"
+```
+
+### New experience – mTLS PoP (Graph)
+
+```csharp
+// User-assigned managed identity
+IManagedIdentityApplication mi =
+    ManagedIdentityApplicationBuilder
+        .Create(ManagedIdentityId.WithUserAssignedClientId(userAssignedClientId))
+        .Build();
+
+// Microsoft Graph as the target API
+const string graphScope = "https://graph.microsoft.com/";
+
+AuthenticationResult result = await mi
+    .AcquireTokenForManagedIdentity(graphScope)
+    .WithMtlsProofOfPosession()   // <-- new API
+    .ExecuteAsync()
+    .ConfigureAwait(false);
+
+// result.TokenType == "mtls_pop"
+```
+
+---
+
+## 4. Call Microsoft Graph with an mTLS PoP token
+
+Once you have an `AuthenticationResult` from `WithMtlsProofOfPosession()`:
+
+- `result.TokenType` will be `"mtls_pop"`.
+- `result.BindingCertificate` is the certificate that the token is bound to.
+
+Use that certificate for the mTLS handshake, and send the token in the `Authorization` header:
+
+```csharp
+// Use the certificate returned by MSAL for the mTLS handshake
+using var handler = new HttpClientHandler();
+handler.ClientCertificates.Add(result.BindingCertificate);
+
+// Create an HttpClient that uses mTLS
+using var httpClient = new HttpClient(handler);
+
+// Example: call Microsoft Graph
+var request = new HttpRequestMessage(
+    HttpMethod.Get,
+    "https://graph.microsoft.com/v1.0/me");
+
+// Use the token type and token from MSAL
+request.Headers.Authorization =
+    new AuthenticationHeaderValue(result.TokenType, result.AccessToken);
+// result.TokenType == "mtls_pop"
+
+HttpResponseMessage response = await httpClient.SendAsync(request);
+response.EnsureSuccessStatusCode();
+```
+
+> In production, you should cache `HttpClient` instances per certificate to avoid socket exhaustion.