Move the "email unsubscribe" resource, refactor the macaroon generator & simplify the access token verification logic. (#12986)

This simplifies the access token verification logic by removing the `rights`
parameter which was only ever used for the unsubscribe link in email
notifications. The latter has been moved under the `/_synapse` namespace,
since it is not a standard API.

This also makes the email verification link more secure, by embedding the
app_id and pushkey in the macaroon and verifying it. This prevents the user
from tampering the query parameters of that unsubscribe link.

Macaroon generation is refactored:

- Centralised all macaroon generation and verification logic to the
  `MacaroonGenerator`
- Moved to `synapse.utils`
- Changed the constructor to require only a `Clock`, hostname, and a secret key
  (instead of a full `Homeserver`).
- Added tests for all methods.

1 files changed, 308 insertions, 0 deletions

diff --git a/synapse/util/macaroons.py b/synapse/util/macaroons.py
index 84e4f6ff55..df77edcce2 100644
--- a/synapse/util/macaroons.py
+++ b/synapse/util/macaroons.py
@@ -17,8 +17,14 @@
 
 from typing import Callable, Optional
 
+import attr
 import pymacaroons
 from pymacaroons.exceptions import MacaroonVerificationFailedException
+from typing_extensions import Literal
+
+from synapse.util import Clock, stringutils
+
+MacaroonType = Literal["access", "delete_pusher", "session", "login"]
 
 
 def get_value_from_macaroon(macaroon: pymacaroons.Macaroon, key: str) -> str:
@@ -86,3 +92,305 @@ def satisfy_expiry(v: pymacaroons.Verifier, get_time_ms: Callable[[], int]) -> N
         return time_msec < expiry
 
     v.satisfy_general(verify_expiry_caveat)
+
+
+@attr.s(frozen=True, slots=True, auto_attribs=True)
+class OidcSessionData:
+    """The attributes which are stored in a OIDC session cookie"""
+
+    idp_id: str
+    """The Identity Provider being used"""
+
+    nonce: str
+    """The `nonce` parameter passed to the OIDC provider."""
+
+    client_redirect_url: str
+    """The URL the client gave when it initiated the flow. ("" if this is a UI Auth)"""
+
+    ui_auth_session_id: str
+    """The session ID of the ongoing UI Auth ("" if this is a login)"""
+
+
+@attr.s(slots=True, frozen=True, auto_attribs=True)
+class LoginTokenAttributes:
+    """Data we store in a short-term login token"""
+
+    user_id: str
+
+    auth_provider_id: str
+    """The SSO Identity Provider that the user authenticated with, to get this token."""
+
+    auth_provider_session_id: Optional[str]
+    """The session ID advertised by the SSO Identity Provider."""
+
+
+class MacaroonGenerator:
+    def __init__(self, clock: Clock, location: str, secret_key: bytes):
+        self._clock = clock
+        self._location = location
+        self._secret_key = secret_key
+
+    def generate_guest_access_token(self, user_id: str) -> str:
+        """Generate a guest access token for the given user ID
+
+        Args:
+            user_id: The user ID for which the guest token should be generated.
+
+        Returns:
+            A signed access token for that guest user.
+        """
+        nonce = stringutils.random_string_with_symbols(16)
+        macaroon = self._generate_base_macaroon("access")
+        macaroon.add_first_party_caveat(f"user_id = {user_id}")
+        macaroon.add_first_party_caveat(f"nonce = {nonce}")
+        macaroon.add_first_party_caveat("guest = true")
+        return macaroon.serialize()
+
+    def generate_delete_pusher_token(
+        self, user_id: str, app_id: str, pushkey: str
+    ) -> str:
+        """Generate a signed token used for unsubscribing from email notifications
+
+        Args:
+            user_id: The user for which this token will be valid.
+            app_id: The app_id for this pusher.
+            pushkey: The unique identifier of this pusher.
+
+        Returns:
+            A signed token which can be used in unsubscribe links.
+        """
+        macaroon = self._generate_base_macaroon("delete_pusher")
+        macaroon.add_first_party_caveat(f"user_id = {user_id}")
+        macaroon.add_first_party_caveat(f"app_id = {app_id}")
+        macaroon.add_first_party_caveat(f"pushkey = {pushkey}")
+        return macaroon.serialize()
+
+    def generate_short_term_login_token(
+        self,
+        user_id: str,
+        auth_provider_id: str,
+        auth_provider_session_id: Optional[str] = None,
+        duration_in_ms: int = (2 * 60 * 1000),
+    ) -> str:
+        """Generate a short-term login token used during SSO logins
+
+        Args:
+            user_id: The user for which the token is valid.
+            auth_provider_id: The SSO IdP the user used.
+            auth_provider_session_id: The session ID got during login from the SSO IdP.
+
+        Returns:
+            A signed token valid for using as a ``m.login.token`` token.
+        """
+        now = self._clock.time_msec()
+        expiry = now + duration_in_ms
+        macaroon = self._generate_base_macaroon("login")
+        macaroon.add_first_party_caveat(f"user_id = {user_id}")
+        macaroon.add_first_party_caveat(f"time < {expiry}")
+        macaroon.add_first_party_caveat(f"auth_provider_id = {auth_provider_id}")
+        if auth_provider_session_id is not None:
+            macaroon.add_first_party_caveat(
+                f"auth_provider_session_id = {auth_provider_session_id}"
+            )
+        return macaroon.serialize()
+
+    def generate_oidc_session_token(
+        self,
+        state: str,
+        session_data: OidcSessionData,
+        duration_in_ms: int = (60 * 60 * 1000),
+    ) -> str:
+        """Generates a signed token storing data about an OIDC session.
+
+        When Synapse initiates an authorization flow, it creates a random state
+        and a random nonce. Those parameters are given to the provider and
+        should be verified when the client comes back from the provider.
+        It is also used to store the client_redirect_url, which is used to
+        complete the SSO login flow.
+
+        Args:
+            state: The ``state`` parameter passed to the OIDC provider.
+            session_data: data to include in the session token.
+            duration_in_ms: An optional duration for the token in milliseconds.
+                Defaults to an hour.
+
+        Returns:
+            A signed macaroon token with the session information.
+        """
+        now = self._clock.time_msec()
+        expiry = now + duration_in_ms
+        macaroon = self._generate_base_macaroon("session")
+        macaroon.add_first_party_caveat(f"state = {state}")
+        macaroon.add_first_party_caveat(f"idp_id = {session_data.idp_id}")
+        macaroon.add_first_party_caveat(f"nonce = {session_data.nonce}")
+        macaroon.add_first_party_caveat(
+            f"client_redirect_url = {session_data.client_redirect_url}"
+        )
+        macaroon.add_first_party_caveat(
+            f"ui_auth_session_id = {session_data.ui_auth_session_id}"
+        )
+        macaroon.add_first_party_caveat(f"time < {expiry}")
+
+        return macaroon.serialize()
+
+    def verify_short_term_login_token(self, token: str) -> LoginTokenAttributes:
+        """Verify a short-term-login macaroon
+
+        Checks that the given token is a valid, unexpired short-term-login token
+        minted by this server.
+
+        Args:
+            token: The login token to verify.
+
+        Returns:
+            A set of attributes carried by this token, including the
+            ``user_id`` and informations about the SSO IDP used during that
+            login.
+
+        Raises:
+            MacaroonVerificationFailedException if the verification failed
+        """
+        macaroon = pymacaroons.Macaroon.deserialize(token)
+
+        v = self._base_verifier("login")
+        v.satisfy_general(lambda c: c.startswith("user_id = "))
+        v.satisfy_general(lambda c: c.startswith("auth_provider_id = "))
+        v.satisfy_general(lambda c: c.startswith("auth_provider_session_id = "))
+        satisfy_expiry(v, self._clock.time_msec)
+        v.verify(macaroon, self._secret_key)
+
+        user_id = get_value_from_macaroon(macaroon, "user_id")
+        auth_provider_id = get_value_from_macaroon(macaroon, "auth_provider_id")
+
+        auth_provider_session_id: Optional[str] = None
+        try:
+            auth_provider_session_id = get_value_from_macaroon(
+                macaroon, "auth_provider_session_id"
+            )
+        except MacaroonVerificationFailedException:
+            pass
+
+        return LoginTokenAttributes(
+            user_id=user_id,
+            auth_provider_id=auth_provider_id,
+            auth_provider_session_id=auth_provider_session_id,
+        )
+
+    def verify_guest_token(self, token: str) -> str:
+        """Verify a guest access token macaroon
+
+        Checks that the given token is a valid, unexpired guest access token
+        minted by this server.
+
+        Args:
+            token: The access token to verify.
+
+        Returns:
+            The ``user_id`` that this token is valid for.
+
+        Raises:
+            MacaroonVerificationFailedException if the verification failed
+        """
+        macaroon = pymacaroons.Macaroon.deserialize(token)
+        user_id = get_value_from_macaroon(macaroon, "user_id")
+
+        # At some point, Synapse would generate macaroons without the "guest"
+        # caveat for regular users. Because of how macaroon verification works,
+        # to avoid validating those as guest tokens, we explicitely verify if
+        # the macaroon includes the "guest = true" caveat.
+        is_guest = any(
+            (caveat.caveat_id == "guest = true" for caveat in macaroon.caveats)
+        )
+
+        if not is_guest:
+            raise MacaroonVerificationFailedException("Macaroon is not a guest token")
+
+        v = self._base_verifier("access")
+        v.satisfy_exact("guest = true")
+        v.satisfy_general(lambda c: c.startswith("user_id = "))
+        v.satisfy_general(lambda c: c.startswith("nonce = "))
+        satisfy_expiry(v, self._clock.time_msec)
+        v.verify(macaroon, self._secret_key)
+
+        return user_id
+
+    def verify_delete_pusher_token(self, token: str, app_id: str, pushkey: str) -> str:
+        """Verify a token from an email unsubscribe link
+
+        Args:
+            token: The token to verify.
+            app_id: The app_id of the pusher to delete.
+            pushkey: The unique identifier of the pusher to delete.
+
+        Return:
+            The ``user_id`` for which this token is valid.
+
+        Raises:
+            MacaroonVerificationFailedException if the verification failed
+        """
+        macaroon = pymacaroons.Macaroon.deserialize(token)
+        user_id = get_value_from_macaroon(macaroon, "user_id")
+
+        v = self._base_verifier("delete_pusher")
+        v.satisfy_exact(f"app_id = {app_id}")
+        v.satisfy_exact(f"pushkey = {pushkey}")
+        v.satisfy_general(lambda c: c.startswith("user_id = "))
+        v.verify(macaroon, self._secret_key)
+
+        return user_id
+
+    def verify_oidc_session_token(self, session: bytes, state: str) -> OidcSessionData:
+        """Verifies and extract an OIDC session token.
+
+        This verifies that a given session token was issued by this homeserver
+        and extract the nonce and client_redirect_url caveats.
+
+        Args:
+            session: The session token to verify
+            state: The state the OIDC provider gave back
+
+        Returns:
+            The data extracted from the session cookie
+
+        Raises:
+            KeyError if an expected caveat is missing from the macaroon.
+        """
+        macaroon = pymacaroons.Macaroon.deserialize(session)
+
+        v = self._base_verifier("session")
+        v.satisfy_exact(f"state = {state}")
+        v.satisfy_general(lambda c: c.startswith("nonce = "))
+        v.satisfy_general(lambda c: c.startswith("idp_id = "))
+        v.satisfy_general(lambda c: c.startswith("client_redirect_url = "))
+        v.satisfy_general(lambda c: c.startswith("ui_auth_session_id = "))
+        satisfy_expiry(v, self._clock.time_msec)
+
+        v.verify(macaroon, self._secret_key)
+
+        # Extract the session data from the token.
+        nonce = get_value_from_macaroon(macaroon, "nonce")
+        idp_id = get_value_from_macaroon(macaroon, "idp_id")
+        client_redirect_url = get_value_from_macaroon(macaroon, "client_redirect_url")
+        ui_auth_session_id = get_value_from_macaroon(macaroon, "ui_auth_session_id")
+        return OidcSessionData(
+            nonce=nonce,
+            idp_id=idp_id,
+            client_redirect_url=client_redirect_url,
+            ui_auth_session_id=ui_auth_session_id,
+        )
+
+    def _generate_base_macaroon(self, type: MacaroonType) -> pymacaroons.Macaroon:
+        macaroon = pymacaroons.Macaroon(
+            location=self._location,
+            identifier="key",
+            key=self._secret_key,
+        )
+        macaroon.add_first_party_caveat("gen = 1")
+        macaroon.add_first_party_caveat(f"type = {type}")
+        return macaroon
+
+    def _base_verifier(self, type: MacaroonType) -> pymacaroons.Verifier:
+        v = pymacaroons.Verifier()
+        v.satisfy_exact("gen = 1")
+        v.satisfy_exact(f"type = {type}")
+        return v