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fix: verify MCP tokens against the MCP application's issuer

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Second failure on the same path. With the aud fix in place, tokens now carry
`aud: shared-memory` correctly but are still rejected — this time on `iss`.

The MCP endpoint is a separate application in the IdP from the Web UI, and
Authentik's default per_provider issuer mode stamps each token with its own
application slug. MCP tokens therefore carry
`.../application/o/shared-memory-mcp/` while OIDC_ISSUER points at
`.../application/o/shared-memory/`, so jwtVerify throws "claim invalid: iss".

Introduce OIDC_ISSUER_MCP (defaults to OIDC_ISSUER) and use it for both the
issuer check and the JWKS URL. The protected-resource metadata now advertises
that same issuer — previously it pointed clients at the Web UI's discovery
document while the tokens came from the MCP provider.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
This commit is contained in:
shadowdao
2026-07-27 06:10:35 -07:00
co-authored by Claude Opus 5
parent 7d4e8daaaf
commit 1fed65a187
5 changed files with 47 additions and 4 deletions
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README.md
+13
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@@ -144,6 +144,7 @@ Copy `.env.example` and fill in the values below.
| `OIDC_CLIENT_SECRET_WEB` | both | Client secret of the Web-UI client. |
| `OIDC_CLIENT_ID_MCP` | both | Client ID of the MCP resource-server client in your IdP. |
| `OIDC_AUDIENCE` | both | Audience string the MCP access token must carry in its `aud` claim. Recommended: `shared-memory`. |
| `OIDC_ISSUER_MCP` | optional | Issuer of MCP access tokens when the MCP endpoint is a separate IdP application (Authentik stamps each app's tokens with its own slug). Defaults to `OIDC_ISSUER`. |
| `OIDC_AUDIENCE_SCOPE` | optional | Name of the IdP scope whose mapping emits that `aud` claim. Advertised in `scopes_supported` so clients request it. Defaults to `aud-<OIDC_AUDIENCE>`. |
| `POSTGRES_USER` / `POSTGRES_PASSWORD` / `POSTGRES_DB` | both | Local Postgres credentials. |
| `NEXTAUTH_SECRET` | both | Session-cookie signing key. Generate with `openssl rand -base64 32`. |
@@ -322,6 +323,18 @@ reliable pattern:
> | jq .scopes_supported # must include aud-<your audience>
> ```
>
> **Second trap, same failure surface:** the MCP endpoint is a *separate
> application* from the Web UI, and Authentik's default `per_provider` issuer
> mode stamps each token with its own application slug. So MCP tokens carry
> `iss: .../application/o/shared-memory-mcp/` while `OIDC_ISSUER` points at
> `.../application/o/shared-memory/`, and verification fails with
> `claim invalid: iss` even once `aud` is correct. Set `OIDC_ISSUER_MCP` to the
> MCP application's issuer. Confirm which one your tokens actually carry:
>
> ```bash
> curl -s https://auth.example.com/application/o/shared-memory-mcp/.well-known/openid-configuration | jq .issuer
> ```
>
> Note also that Claude Code sends an RFC 8707 `resource` parameter on the
> authorize request; Authentik 2026.5 ignores it, so it cannot be relied on
> for audience binding. The scope mapping is what sets `aud`.