feat(m7.3): MCP tenant-scoped bearer tokens (#92)

MCP server validates per-tenant bearer tokens on incoming calls and routes each tool to the caller's tenant DB. Closes the cross-tenant data leak in the MCP path identified in M7.3.

compliance-agent/src/api/handlers/mcp_tokens.rs

@@ -0,0 +1,186 @@
+//! `/api/v1/mcp-tokens` — per-tenant API tokens for the MCP server.
+//!
+//! These are opaque static bearers issued via the dashboard (or a
+//! direct curl with a KC JWT) and copied into LLM clients (Claude
+//! Desktop / Cursor / ChatGPT). The MCP server hashes incoming bearers
+//! and looks them up in the cross-tenant `<prefix>__admin.mcp_tokens`
+//! collection to derive the tenant_id for routing.
+//!
+//! The raw token is shown to the caller exactly once at creation; the
+//! database only ever stores the SHA-256 hash. Revocation is a soft
+//! delete (sets `revoked: true`) so the audit log keeps the record.
+
+use axum::extract::{Extension, Path};
+use axum::http::StatusCode;
+use axum::Json;
+use base64::{engine::general_purpose::URL_SAFE_NO_PAD, Engine as _};
+use compliance_core::models::{McpToken, McpTokenView};
+use compliance_core::tenant_ctx::TenantCtx;
+use mongodb::bson::doc;
+use rand::RngCore;
+use sha2::{Digest, Sha256};
+
+use super::dto::{AgentExt, ApiResponse};
+
+/// Mongo collection name inside the admin DB.
+const COLLECTION: &str = "mcp_tokens";
+
+/// Token prefix the MCP server expects on every bearer.
+const TOKEN_PREFIX: &str = "mcpt_";
+
+/// Bytes of randomness behind each token. 32 → ~256 bits.
+/// Encoded as URL-safe base64 without padding → 43 chars.
+/// Combined with `mcpt_` → 48-char tokens.
+const TOKEN_RAND_BYTES: usize = 32;
+
+#[derive(serde::Deserialize)]
+pub struct CreateMcpTokenRequest {
+    pub name: String,
+}
+
+/// Returned exactly once at creation. The `token` field is gone from
+/// the listing endpoint — the user must save it now.
+#[derive(serde::Serialize)]
+pub struct CreateMcpTokenResponse {
+    pub token: String,
+    pub view: McpTokenView,
+}
+
+/// `POST /api/v1/mcp-tokens` — mint a new token for the caller's tenant.
+#[tracing::instrument(skip_all)]
+pub async fn create_mcp_token(
+    Extension(agent): AgentExt,
+    tenant: TenantCtx,
+    Json(req): Json<CreateMcpTokenRequest>,
+) -> Result<Json<CreateMcpTokenResponse>, StatusCode> {
+    if req.name.trim().is_empty() {
+        return Err(StatusCode::BAD_REQUEST);
+    }
+    let raw = generate_token();
+    let token_hash = sha256_hex(&raw);
+    let token_prefix: String = raw.chars().take(12).collect();
+
+    let mut token = McpToken {
+        id: None,
+        token_hash,
+        token_prefix,
+        tenant_id: tenant.0.tenant_id.clone(),
+        name: req.name.trim().to_string(),
+        created_by: tenant.0.user_id.clone(),
+        created_at: chrono::Utc::now(),
+        last_used_at: None,
+        revoked: false,
+    };
+
+    let col = agent.db_pool.admin_db().collection::<McpToken>(COLLECTION);
+    let res = col.insert_one(&token).await.map_err(|e| {
+        tracing::error!("Failed to insert MCP token: {e}");
+        StatusCode::INTERNAL_SERVER_ERROR
+    })?;
+    token.id = res.inserted_id.as_object_id();
+
+    Ok(Json(CreateMcpTokenResponse {
+        view: McpTokenView::from(&token),
+        token: raw,
+    }))
+}
+
+/// `GET /api/v1/mcp-tokens` — list tokens for the caller's tenant.
+/// Hash is never returned; only metadata + the 12-char prefix so the
+/// user can identify which row is which.
+#[tracing::instrument(skip_all)]
+pub async fn list_mcp_tokens(
+    Extension(agent): AgentExt,
+    tenant: TenantCtx,
+) -> Result<Json<ApiResponse<Vec<McpTokenView>>>, StatusCode> {
+    let col = agent.db_pool.admin_db().collection::<McpToken>(COLLECTION);
+    let mut cursor = col
+        .find(doc! { "tenant_id": &tenant.0.tenant_id })
+        .sort(doc! { "created_at": -1 })
+        .await
+        .map_err(|e| {
+            tracing::error!("Failed to list MCP tokens: {e}");
+            StatusCode::INTERNAL_SERVER_ERROR
+        })?;
+    let mut out = Vec::new();
+    while cursor.advance().await.map_err(|e| {
+        tracing::warn!("MCP tokens cursor advance failed: {e}");
+        StatusCode::INTERNAL_SERVER_ERROR
+    })? {
+        match cursor.deserialize_current() {
+            Ok(t) => out.push(McpTokenView::from(&t)),
+            Err(e) => tracing::warn!("Failed to deserialize MCP token: {e}"),
+        }
+    }
+    Ok(Json(ApiResponse {
+        data: out,
+        total: None,
+        page: None,
+    }))
+}
+
+/// `DELETE /api/v1/mcp-tokens/{id}` — revoke (soft delete).
+/// Scoped to the caller's tenant: a user can't revoke another tenant's
+/// token even if they guess its id.
+#[tracing::instrument(skip_all, fields(id = %id))]
+pub async fn revoke_mcp_token(
+    Extension(agent): AgentExt,
+    tenant: TenantCtx,
+    Path(id): Path<String>,
+) -> Result<Json<serde_json::Value>, StatusCode> {
+    let oid = mongodb::bson::oid::ObjectId::parse_str(&id).map_err(|_| StatusCode::BAD_REQUEST)?;
+    let col = agent.db_pool.admin_db().collection::<McpToken>(COLLECTION);
+    let result = col
+        .update_one(
+            doc! { "_id": oid, "tenant_id": &tenant.0.tenant_id },
+            doc! { "$set": { "revoked": true } },
+        )
+        .await
+        .map_err(|e| {
+            tracing::error!("Failed to revoke MCP token: {e}");
+            StatusCode::INTERNAL_SERVER_ERROR
+        })?;
+    if result.matched_count == 0 {
+        return Err(StatusCode::NOT_FOUND);
+    }
+    Ok(Json(serde_json::json!({ "status": "revoked" })))
+}
+
+/// 32 bytes random → URL-safe base64 → 43 chars, no padding.
+/// Prefixed with `mcpt_` so the MCP server can sniff the format
+/// before bothering with the DB lookup.
+fn generate_token() -> String {
+    let mut bytes = [0u8; TOKEN_RAND_BYTES];
+    rand::rng().fill_bytes(&mut bytes);
+    format!("{TOKEN_PREFIX}{}", URL_SAFE_NO_PAD.encode(bytes))
+}
+
+fn sha256_hex(s: &str) -> String {
+    let mut h = Sha256::new();
+    h.update(s.as_bytes());
+    hex::encode(h.finalize())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn generated_tokens_are_unique_and_prefixed() {
+        let a = generate_token();
+        let b = generate_token();
+        assert_ne!(a, b);
+        assert!(a.starts_with(TOKEN_PREFIX));
+        assert!(b.starts_with(TOKEN_PREFIX));
+        // 5 + 43 = 48 chars
+        assert_eq!(a.len(), 5 + 43);
+    }
+
+    #[test]
+    fn sha256_is_stable_and_64_hex() {
+        let h = sha256_hex("mcpt_abc");
+        assert_eq!(h.len(), 64);
+        assert!(h.chars().all(|c| c.is_ascii_hexdigit()));
+        assert_eq!(sha256_hex("mcpt_abc"), h);
+    }
+}