diff --git a/synapse/handlers/jwt.py b/synapse/handlers/jwt.py
new file mode 100644
index 0000000000..5fddc0e315
--- /dev/null
+++ b/synapse/handlers/jwt.py
@@ -0,0 +1,118 @@
+# Copyright 2023 Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+from typing import TYPE_CHECKING
+
+from authlib.jose import JsonWebToken, JWTClaims
+from authlib.jose.errors import BadSignatureError, InvalidClaimError, JoseError
+
+from synapse.api.errors import Codes, LoginError, StoreError, UserDeactivatedError
+from synapse.types import JsonDict, UserID
+
+if TYPE_CHECKING:
+ from synapse.server import HomeServer
+
+
+class JwtHandler:
+ def __init__(self, hs: "HomeServer"):
+ self.hs = hs
+ self._main_store = hs.get_datastores().main
+
+ self.jwt_secret = hs.config.jwt.jwt_secret
+ self.jwt_subject_claim = hs.config.jwt.jwt_subject_claim
+ self.jwt_algorithm = hs.config.jwt.jwt_algorithm
+ self.jwt_issuer = hs.config.jwt.jwt_issuer
+ self.jwt_audiences = hs.config.jwt.jwt_audiences
+
+ async def validate_login(self, login_submission: JsonDict) -> str:
+ """
+ Authenticates the user for the /login API
+
+ Args:
+ login_submission: the whole of the login submission
+ (including 'type' and other relevant fields)
+
+ Returns:
+ The user ID that is logging in.
+
+ Raises:
+ LoginError if there was an authentication problem.
+ """
+ token = login_submission.get("token", None)
+ if token is None:
+ raise LoginError(
+ 403, "Token field for JWT is missing", errcode=Codes.FORBIDDEN
+ )
+
+ jwt = JsonWebToken([self.jwt_algorithm])
+ claim_options = {}
+ if self.jwt_issuer is not None:
+ claim_options["iss"] = {"value": self.jwt_issuer, "essential": True}
+ if self.jwt_audiences is not None:
+ claim_options["aud"] = {"values": self.jwt_audiences, "essential": True}
+
+ try:
+ claims = jwt.decode(
+ token,
+ key=self.jwt_secret,
+ claims_cls=JWTClaims,
+ claims_options=claim_options,
+ )
+ except BadSignatureError:
+ # We handle this case separately to provide a better error message
+ raise LoginError(
+ 403,
+ "JWT validation failed: Signature verification failed",
+ errcode=Codes.FORBIDDEN,
+ )
+ except JoseError as e:
+ # A JWT error occurred, return some info back to the client.
+ raise LoginError(
+ 403,
+ "JWT validation failed: %s" % (str(e),),
+ errcode=Codes.FORBIDDEN,
+ )
+
+ try:
+ claims.validate(leeway=120) # allows 2 min of clock skew
+
+ # Enforce the old behavior which is rolled out in productive
+ # servers: if the JWT contains an 'aud' claim but none is
+ # configured, the login attempt will fail
+ if claims.get("aud") is not None:
+ if self.jwt_audiences is None or len(self.jwt_audiences) == 0:
+ raise InvalidClaimError("aud")
+ except JoseError as e:
+ raise LoginError(
+ 403,
+ "JWT validation failed: %s" % (str(e),),
+ errcode=Codes.FORBIDDEN,
+ )
+
+ user = claims.get(self.jwt_subject_claim, None)
+ if user is None:
+ raise LoginError(403, "Invalid JWT", errcode=Codes.FORBIDDEN)
+
+ user_id = UserID(user, self.hs.hostname).to_string()
+
+ # If the account has been deactivated, do not proceed with the login
+ # flow.
+ try:
+ deactivated = await self._main_store.get_user_deactivated_status(user_id)
+ except StoreError:
+ # JWT lazily creates users, so they may not exist in the database yet.
+ deactivated = False
+ if deactivated:
+ raise UserDeactivatedError("This account has been deactivated")
+
+ return user_id
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