diff --git a/app/deps.py b/app/deps.py
index f0eeeba..7e93072 100644
--- a/app/deps.py
+++ b/app/deps.py
@@ -311,6 +311,10 @@ def require_permission(permission: str):
         current_user: dict[str, Any] = Depends(get_current_user),
     ) -> dict[str, Any]:
         # API token scope enforcement (Problem 2 fix)
+        # F10 (Astra): token scopes are an UPPER BOUND, not a replacement —
+        # the user's own permissions must ALSO grant the permission. A token
+        # can never grant more than its owner has; revoking the user's
+        # permission takes effect on existing tokens.
         token_scopes = current_user.get("_token_scopes")
         if token_scopes is not None:
             from app.core.permissions import _permission_matches_any
@@ -322,7 +326,7 @@ def require_permission(permission: str):
                         "code": "insufficient_scope",
                     },
                 )
-            return current_user
+            # fall through: the normal user-permission check applies too
 
         if current_user.get("is_system_admin"):
             return current_user
diff --git a/tests/test_s1_security_guards.py b/tests/test_s1_security_guards.py
new file mode 100644
index 0000000..6f4c4f0
--- /dev/null
+++ b/tests/test_s1_security_guards.py
@@ -0,0 +1,113 @@
+"""S1 security guard tests (Astra audit findings — Phase S Welle 1).
+
+F10: token scopes must be an UPPER BOUND, not a replacement for user
+permissions. A token can never grant more than its owner has.
+
+Covers require_permission directly (unit level, no HTTP) plus the
+acceptance criteria from docs/audits/astra-audit-2026-09-17.md.
+"""
+
+from __future__ import annotations
+
+import uuid
+
+import pytest
+from fastapi import HTTPException
+
+from app.deps import require_permission
+
+
+def _user_ctx(
+    permissions: list[str] | None = None,
+    denied: list[str] | None = None,
+    token_scopes: list[str] | None = None,
+    is_system_admin: bool = False,
+) -> dict:
+    """Build a current_user dict in the session/bearer context shape."""
+    ctx: dict = {
+        "user_id": str(uuid.uuid4()),
+        "tenant_id": str(uuid.uuid4()),
+        "permissions": permissions or [],
+        "denied_permissions": denied or [],
+        "field_permissions": {},
+        "is_system_admin": is_system_admin,
+        "role": "user",
+    }
+    if token_scopes is not None:
+        ctx["_token_scopes"] = token_scopes
+    return ctx
+
+
+@pytest.mark.asyncio
+class TestF10TokenScopesAreUpperBound:
+    """F10 (Astra P1): token scopes restrict, they never grant."""
+
+    async def test_token_scope_does_not_replace_user_permission(self):
+        """Token with mail:write scope, user WITHOUT mail:write → 403.
+
+        Astra repro: this exact combination was allowed before the fix.
+        """
+        checker = require_permission("mail:write")
+        ctx = _user_ctx(permissions=[], token_scopes=["mail:write"])
+        with pytest.raises(HTTPException) as exc_info:
+            await checker(current_user=ctx)
+        assert exc_info.value.status_code == 403
+        assert exc_info.value.detail.get("code") != "insufficient_scope"
+
+    async def test_token_scope_with_user_permission_passes(self):
+        """Token scope AND user permission both present → allowed."""
+        checker = require_permission("mail:write")
+        ctx = _user_ctx(permissions=["mail:write"], token_scopes=["mail:write"])
+        result = await checker(current_user=ctx)
+        assert result is ctx
+
+    async def test_missing_scope_rejected_even_with_user_permission(self):
+        """User HAS the permission but the token lacks the scope → 403
+        insufficient_scope (the token may do less than its owner)."""
+        checker = require_permission("mail:write")
+        ctx = _user_ctx(permissions=["mail:write"], token_scopes=["mail:read"])
+        with pytest.raises(HTTPException) as exc_info:
+            await checker(current_user=ctx)
+        assert exc_info.value.status_code == 403
+        assert exc_info.value.detail.get("code") == "insufficient_scope"
+
+    async def test_permission_revocation_applies_to_existing_tokens(self):
+        """User permission revoked (moved to deny) → token rejected.
+
+        Astra acceptance: later permission revocation must take effect on
+        existing tokens.
+        """
+        checker = require_permission("mail:write")
+        ctx = _user_ctx(
+            permissions=["mail:write"],
+            denied=["mail:write"],
+            token_scopes=["mail:write"],
+        )
+        with pytest.raises(HTTPException) as exc_info:
+            await checker(current_user=ctx)
+        assert exc_info.value.status_code == 403
+
+    async def test_session_path_unchanged(self):
+        """Without token scopes the normal permission check applies."""
+        checker = require_permission("mail:write")
+        ok_ctx = _user_ctx(permissions=["mail:write"])
+        assert await checker(current_user=ok_ctx) is ok_ctx
+
+        bad_ctx = _user_ctx(permissions=[])
+        with pytest.raises(HTTPException) as exc_info:
+            await checker(current_user=bad_ctx)
+        assert exc_info.value.status_code == 403
+
+    async def test_wildcard_scope_still_bound_to_user_permissions(self):
+        """A *:* token scope cannot bypass missing user permissions."""
+        checker = require_permission("mail:write")
+        ctx = _user_ctx(permissions=[], token_scopes=['*:*'])
+        with pytest.raises(HTTPException) as exc_info:
+            await checker(current_user=ctx)
+        assert exc_info.value.status_code == 403
+
+    async def test_system_admin_bypass_unchanged(self):
+        """System admins keep their bypass (both session and token path)."""
+        checker = require_permission("mail:write")
+        ctx = _user_ctx(token_scopes=["mail:write"], is_system_admin=True)
+        assert await checker(current_user=ctx) is ctx