High severityNVD Advisory· Published Dec 9, 2020· Updated Aug 4, 2024

Denial of service attack via incorrect parameters to federation APIs

CVE-2020-26257

Description

Matrix is an ecosystem for open federated Instant Messaging and VoIP. Synapse is a reference "homeserver" implementation of Matrix. A malicious or poorly-implemented homeserver can inject malformed events into a room by specifying a different room id in the path of a /send_join, /send_leave, /invite or /exchange_third_party_invite request. This can lead to a denial of service in which future events will not be correctly sent to other servers over federation. This affects any server which accepts federation requests from untrusted servers. The Matrix Synapse reference implementation before version 1.23.1 the implementation is vulnerable to this injection attack. Issue is fixed in version 1.23.1. As a workaround homeserver administrators could limit access to the federation API to trusted servers (for example via federation_domain_whitelist).

AI Insight

LLM-synthesized narrative grounded in this CVE's description and references.

Matrix Synapse before 1.23.1 allows federation request room ID injection, leading to denial of service via malformed events.

The vulnerability resides in how Synapse processes federation requests. Some endpoints (e.g., /send_join, /send_leave, /invite) include a room_id both in the URL path and in the request body. Synapse incorrectly used the path parameter for certain operations, allowing a malicious homeserver to inject a different room ID in the path than the one in the body. This causes the server to process events in the wrong room context [1][2][3].

To exploit this, an attacker controls a homeserver participating in federation and sends crafted requests with mismatched room IDs. No authentication is required beyond the typical federation trust model. The attack works against any Synapse server that accepts federation from untrusted servers, which is the default configuration [1][4].

The impact is a denial of service condition: future events from other servers are not correctly sent over federation for the affected room. The injected malformed events can cause persistent state inconsistencies, disrupting communication until the server is restarted or the room state is manually corrected [1][4].

Synapse version 1.23.1 fixes the issue by consistently using the room_id from the request body rather than the path parameter [2][3]. As a workaround, administrators can restrict federation to trusted servers using the federation_domain_whitelist setting [1][4]. No workarounds involving disabling the affected endpoints are available, as they are essential for federation.

References

AI Insight generated on May 21, 2026. Synthesized from this CVE's description and the cited reference URLs; citations are validated against the source bundle.

Affected packages

Versions sourced from the GitHub Security Advisory.

Package	Affected versions	Patched versions
matrix-synapsePyPI	< 1.23.1	1.23.1

Affected products

ghsa-coords2 versions
pkg:pypi/matrix-synapse pkg:rpm/opensuse/matrix-synapse&distro=openSUSE%20Tumbleweed
< 1.23.1+ 1 more
- (no CPE)range: < 1.23.1
- (no CPE)range: < 1.43.0-1.1
matrix-org/synapsev5
Range: < 1.23.1

Patches

3ce2f303f15f

Consistently use room_id from federation request body (#8776)

https://github.com/matrix-org/synapseRichard van der HoffNov 19, 2020via ghsa

commit

5 files changed · +49 −54

changelog.d/8776.bugfix+1 −0 added

@@ -0,0 +1 @@
+Fix a bug in some federation APIs which could lead to unexpected behaviour if different parameters were set in the URI and the request body.

synapse/federation/federation_server.py+10 −13 modified

@@ -49,6 +49,7 @@
 from synapse.federation.persistence import TransactionActions
 from synapse.federation.units import Edu, Transaction
 from synapse.http.endpoint import parse_server_name
+from synapse.http.servlet import assert_params_in_dict
 from synapse.logging.context import (
     make_deferred_yieldable,
     nested_logging_context,
@@ -391,7 +392,7 @@ async def _process_edu(edu_dict):
             TRANSACTION_CONCURRENCY_LIMIT,
         )
 
-    async def on_context_state_request(
+    async def on_room_state_request(
         self, origin: str, room_id: str, event_id: str
     ) -> Tuple[int, Dict[str, Any]]:
         origin_host, _ = parse_server_name(origin)
@@ -514,11 +515,12 @@ async def on_invite_request(
         return {"event": ret_pdu.get_pdu_json(time_now)}
 
     async def on_send_join_request(
-        self, origin: str, content: JsonDict, room_id: str
+        self, origin: str, content: JsonDict
     ) -> Dict[str, Any]:
         logger.debug("on_send_join_request: content: %s", content)
 
-        room_version = await self.store.get_room_version(room_id)
+        assert_params_in_dict(content, ["room_id"])
+        room_version = await self.store.get_room_version(content["room_id"])
         pdu = event_from_pdu_json(content, room_version)
 
         origin_host, _ = parse_server_name(origin)
@@ -547,12 +549,11 @@ async def on_make_leave_request(
         time_now = self._clock.time_msec()
         return {"event": pdu.get_pdu_json(time_now), "room_version": room_version}
 
-    async def on_send_leave_request(
-        self, origin: str, content: JsonDict, room_id: str
-    ) -> dict:
+    async def on_send_leave_request(self, origin: str, content: JsonDict) -> dict:
         logger.debug("on_send_leave_request: content: %s", content)
 
-        room_version = await self.store.get_room_version(room_id)
+        assert_params_in_dict(content, ["room_id"])
+        room_version = await self.store.get_room_version(content["room_id"])
         pdu = event_from_pdu_json(content, room_version)
 
         origin_host, _ = parse_server_name(origin)
@@ -748,12 +749,8 @@ async def exchange_third_party_invite(
         )
         return ret
 
-    async def on_exchange_third_party_invite_request(
-        self, room_id: str, event_dict: Dict
-    ):
-        ret = await self.handler.on_exchange_third_party_invite_request(
-            room_id, event_dict
-        )
+    async def on_exchange_third_party_invite_request(self, event_dict: Dict):
+        ret = await self.handler.on_exchange_third_party_invite_request(event_dict)
         return ret
 
     async def check_server_matches_acl(self, server_name: str, room_id: str):

synapse/federation/transport/server.py+33 −35 modified

@@ -440,13 +440,13 @@ async def on_GET(self, origin, content, query, event_id):
 
 
 class FederationStateV1Servlet(BaseFederationServlet):
-    PATH = "/state/(?P<context>[^/]*)/?"
+    PATH = "/state/(?P<room_id>[^/]*)/?"
 
-    # This is when someone asks for all data for a given context.
-    async def on_GET(self, origin, content, query, context):
-        return await self.handler.on_context_state_request(
+    # This is when someone asks for all data for a given room.
+    async def on_GET(self, origin, content, query, room_id):
+        return await self.handler.on_room_state_request(
             origin,
-            context,
+            room_id,
             parse_string_from_args(query, "event_id", None, required=False),
         )
 
@@ -463,16 +463,16 @@ async def on_GET(self, origin, content, query, room_id):
 
 
 class FederationBackfillServlet(BaseFederationServlet):
-    PATH = "/backfill/(?P<context>[^/]*)/?"
+    PATH = "/backfill/(?P<room_id>[^/]*)/?"
 
-    async def on_GET(self, origin, content, query, context):
+    async def on_GET(self, origin, content, query, room_id):
         versions = [x.decode("ascii") for x in query[b"v"]]
         limit = parse_integer_from_args(query, "limit", None)
 
         if not limit:
             return 400, {"error": "Did not include limit param"}
 
-        return await self.handler.on_backfill_request(origin, context, versions, limit)
+        return await self.handler.on_backfill_request(origin, room_id, versions, limit)
 
 
 class FederationQueryServlet(BaseFederationServlet):
@@ -487,9 +487,9 @@ async def on_GET(self, origin, content, query, query_type):
 
 
 class FederationMakeJoinServlet(BaseFederationServlet):
-    PATH = "/make_join/(?P<context>[^/]*)/(?P<user_id>[^/]*)"
+    PATH = "/make_join/(?P<room_id>[^/]*)/(?P<user_id>[^/]*)"
 
-    async def on_GET(self, origin, _content, query, context, user_id):
+    async def on_GET(self, origin, _content, query, room_id, user_id):
         """
         Args:
             origin (unicode): The authenticated server_name of the calling server
@@ -511,24 +511,24 @@ async def on_GET(self, origin, _content, query, context, user_id):
             supported_versions = ["1"]
 
         content = await self.handler.on_make_join_request(
-            origin, context, user_id, supported_versions=supported_versions
+            origin, room_id, user_id, supported_versions=supported_versions
         )
         return 200, content
 
 
 class FederationMakeLeaveServlet(BaseFederationServlet):
-    PATH = "/make_leave/(?P<context>[^/]*)/(?P<user_id>[^/]*)"
+    PATH = "/make_leave/(?P<room_id>[^/]*)/(?P<user_id>[^/]*)"
 
-    async def on_GET(self, origin, content, query, context, user_id):
-        content = await self.handler.on_make_leave_request(origin, context, user_id)
+    async def on_GET(self, origin, content, query, room_id, user_id):
+        content = await self.handler.on_make_leave_request(origin, room_id, user_id)
         return 200, content
 
 
 class FederationV1SendLeaveServlet(BaseFederationServlet):
     PATH = "/send_leave/(?P<room_id>[^/]*)/(?P<event_id>[^/]*)"
 
     async def on_PUT(self, origin, content, query, room_id, event_id):
-        content = await self.handler.on_send_leave_request(origin, content, room_id)
+        content = await self.handler.on_send_leave_request(origin, content)
         return 200, (200, content)
 
 
@@ -538,43 +538,43 @@ class FederationV2SendLeaveServlet(BaseFederationServlet):
     PREFIX = FEDERATION_V2_PREFIX
 
     async def on_PUT(self, origin, content, query, room_id, event_id):
-        content = await self.handler.on_send_leave_request(origin, content, room_id)
+        content = await self.handler.on_send_leave_request(origin, content)
         return 200, content
 
 
 class FederationEventAuthServlet(BaseFederationServlet):
-    PATH = "/event_auth/(?P<context>[^/]*)/(?P<event_id>[^/]*)"
+    PATH = "/event_auth/(?P<room_id>[^/]*)/(?P<event_id>[^/]*)"
 
-    async def on_GET(self, origin, content, query, context, event_id):
-        return await self.handler.on_event_auth(origin, context, event_id)
+    async def on_GET(self, origin, content, query, room_id, event_id):
+        return await self.handler.on_event_auth(origin, room_id, event_id)
 
 
 class FederationV1SendJoinServlet(BaseFederationServlet):
-    PATH = "/send_join/(?P<context>[^/]*)/(?P<event_id>[^/]*)"
+    PATH = "/send_join/(?P<room_id>[^/]*)/(?P<event_id>[^/]*)"
 
-    async def on_PUT(self, origin, content, query, context, event_id):
-        # TODO(paul): assert that context/event_id parsed from path actually
+    async def on_PUT(self, origin, content, query, room_id, event_id):
+        # TODO(paul): assert that room_id/event_id parsed from path actually
         #   match those given in content
-        content = await self.handler.on_send_join_request(origin, content, context)
+        content = await self.handler.on_send_join_request(origin, content)
         return 200, (200, content)
 
 
 class FederationV2SendJoinServlet(BaseFederationServlet):
-    PATH = "/send_join/(?P<context>[^/]*)/(?P<event_id>[^/]*)"
+    PATH = "/send_join/(?P<room_id>[^/]*)/(?P<event_id>[^/]*)"
 
     PREFIX = FEDERATION_V2_PREFIX
 
-    async def on_PUT(self, origin, content, query, context, event_id):
-        # TODO(paul): assert that context/event_id parsed from path actually
+    async def on_PUT(self, origin, content, query, room_id, event_id):
+        # TODO(paul): assert that room_id/event_id parsed from path actually
         #   match those given in content
-        content = await self.handler.on_send_join_request(origin, content, context)
+        content = await self.handler.on_send_join_request(origin, content)
         return 200, content
 
 
 class FederationV1InviteServlet(BaseFederationServlet):
-    PATH = "/invite/(?P<context>[^/]*)/(?P<event_id>[^/]*)"
+    PATH = "/invite/(?P<room_id>[^/]*)/(?P<event_id>[^/]*)"
 
-    async def on_PUT(self, origin, content, query, context, event_id):
+    async def on_PUT(self, origin, content, query, room_id, event_id):
         # We don't get a room version, so we have to assume its EITHER v1 or
         # v2. This is "fine" as the only difference between V1 and V2 is the
         # state resolution algorithm, and we don't use that for processing
@@ -589,12 +589,12 @@ async def on_PUT(self, origin, content, query, context, event_id):
 
 
 class FederationV2InviteServlet(BaseFederationServlet):
-    PATH = "/invite/(?P<context>[^/]*)/(?P<event_id>[^/]*)"
+    PATH = "/invite/(?P<room_id>[^/]*)/(?P<event_id>[^/]*)"
 
     PREFIX = FEDERATION_V2_PREFIX
 
-    async def on_PUT(self, origin, content, query, context, event_id):
-        # TODO(paul): assert that context/event_id parsed from path actually
+    async def on_PUT(self, origin, content, query, room_id, event_id):
+        # TODO(paul): assert that room_id/event_id parsed from path actually
         #   match those given in content
 
         room_version = content["room_version"]
@@ -616,9 +616,7 @@ class FederationThirdPartyInviteExchangeServlet(BaseFederationServlet):
     PATH = "/exchange_third_party_invite/(?P<room_id>[^/]*)"
 
     async def on_PUT(self, origin, content, query, room_id):
-        content = await self.handler.on_exchange_third_party_invite_request(
-            room_id, content
-        )
+        content = await self.handler.on_exchange_third_party_invite_request(content)
         return 200, content

synapse/handlers/federation.py+5 −5 modified

@@ -55,6 +55,7 @@
 from synapse.events.snapshot import EventContext
 from synapse.events.validator import EventValidator
 from synapse.handlers._base import BaseHandler
+from synapse.http.servlet import assert_params_in_dict
 from synapse.logging.context import (
     make_deferred_yieldable,
     nested_logging_context,
@@ -2686,20 +2687,19 @@ async def exchange_third_party_invite(
             )
 
     async def on_exchange_third_party_invite_request(
-        self, room_id: str, event_dict: JsonDict
+        self, event_dict: JsonDict
     ) -> None:
         """Handle an exchange_third_party_invite request from a remote server
 
         The remote server will call this when it wants to turn a 3pid invite
         into a normal m.room.member invite.
 
         Args:
-            room_id: The ID of the room.
-
-            event_dict (dict[str, Any]): Dictionary containing the event body.
+            event_dict: Dictionary containing the event body.
 
         """
-        room_version = await self.store.get_room_version_id(room_id)
+        assert_params_in_dict(event_dict, ["room_id"])
+        room_version = await self.store.get_room_version_id(event_dict["room_id"])
 
         # NB: event_dict has a particular specced format we might need to fudge
         # if we change event formats too much.

tests/handlers/test_federation.py+0 −1 modified

@@ -59,7 +59,6 @@ def test_exchange_revoked_invite(self):
         )
 
         d = self.handler.on_exchange_third_party_invite_request(
-            room_id=room_id,
             event_dict={
                 "type": EventTypes.Member,
                 "room_id": room_id,

Vulnerability mechanics

Generated on May 9, 2026. Inputs: CWE entries + fix-commit diffs from this CVE's patches. Citations validated against bundle.

References

github.com/advisories/GHSA-hxmp-pqch-c8mmghsaADVISORY
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/DBTIU3ZNBFWZ56V4X7JIAD33V5H2GOMC/mitrevendor-advisoryx_refsource_FEDORA
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/QR4MMYZKX5N5GYGH4H5LBUUC5TLAFHI7/mitrevendor-advisoryx_refsource_FEDORA
nvd.nist.gov/vuln/detail/CVE-2020-26257ghsaADVISORY
github.com/matrix-org/synapse/blob/develop/CHANGES.mdghsax_refsource_MISCWEB
github.com/matrix-org/synapse/commit/3ce2f303f15f6ac3dc352298972dc6e04d9b7a8bghsax_refsource_MISCWEB
github.com/matrix-org/synapse/pull/8776ghsax_refsource_MISCWEB
github.com/matrix-org/synapse/security/advisories/GHSA-hxmp-pqch-c8mmghsax_refsource_CONFIRMWEB
github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2020-236.yamlghsaWEB
lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/DBTIU3ZNBFWZ56V4X7JIAD33V5H2GOMCghsaWEB
lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/QR4MMYZKX5N5GYGH4H5LBUUC5TLAFHI7ghsaWEB

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cvss	0.455
epss	0.000
exploit	0.000
kev	0.000
patch	-0.070
ransomware	0.000