{"id":"CVE-2023-32323","aliases":["GHSA-f3wc-3vxv-xmvr","PYSEC-2023-67"],"url":"https://o3.security/vulnerability/CVE-2023-32323","summary":"Synapse Outgoing federation to specific hosts can be disabled by sending malicious invites","details":"### Impact\n\nA malicious user on a Synapse homeserver X with permission to create certain state events can disable outbound federation from X to an arbitrary homeserver Y.\n\nSynapse instances with federation disabled are not affected.\n\n#### Details\n\nThe Matrix protocol allows homeservers to provide an `invite_room_state` field on a [room invite](https://spec.matrix.org/v1.5/client-server-api/#mroommember) containing a [summary of room state](https://spec.matrix.org/v1.5/client-server-api/#stripped-state).  In versions of Synapse up to and including v1.73.0, Synapse did not limit the size of `invite_room_state`, meaning that it was possible to create an arbitrarily large invite event.\n\nAn attacker with an account on a vulnerable Synapse homeserver X could exploit this by having X create an over-sized invite event in a room with a user from another homeserver Y. Once acknowledged by the invitee's homeserver, the invite event would be sent in a batch of events to Y. If the malicious invite is so large that the entire batch is rejected as too large, X's outgoing traffic to Y would become \"stuck\", meaning that messages and state events created by X would remain unseen by Y.\n\n### Patches\n\nSynapse 1.74 refuses to create oversized `invite_room_state` fields. Server operators should upgrade to Synapse 1.74 or newer urgently.\n\n### Workarounds\n\nThere are no robust workarounds.\n\nThis attack needs an account on Synapse homeserver X to deny federation from X to another homeserver Y. As a partial mitigation, Synapse operators can disable open registration to limit the ability of attackers to create new accounts on homeserver X.\n\nIf homeserver X has been attacked in this way, restarting it will resume outgoing federation by entering \"catchup mode\". For catchup mode to ignore the oversized invites, every attacked room must have a correctly-sized event sent by X which is newer than any oversized invite. This is difficult to arrange, and does not prevent the attacker from repeating their attack.\n\n### References\n\n- https://github.com/matrix-org/synapse/issues/14492 was caused by this issue.\n- https://github.com/matrix-org/synapse/issues/14642  includes the patch described above.\n\n### For more information\n\nIf you have any questions or comments about this advisory, e-mail us at [security@matrix.org](mailto:security@matrix.org).","published":"2023-05-26T13:32:01.632Z","modified":"2026-10-28T02:30:44.128485063Z","cvss":{"score":5,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:L"},"epss":null,"cisaKev":null,"exploitsKnown":1,"affectedPackages":[{"ecosystem":"PyPI","name":"matrix-synapse","fixedVersion":"1.74.0"}],"fix":{"url":"https://github.com/matrix-org/synapse/pull/14642","label":"matrix-org/synapse#14642"},"references":[{"type":"WEB","url":"https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/UJIJRP5ZH6B3KGFLHCAKR2IX2Y4Z25QD/"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2023/32xxx/CVE-2023-32323.json"},{"type":"ADVISORY","url":"https://github.com/matrix-org/synapse/security/advisories/GHSA-f3wc-3vxv-xmvr"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2023-32323"},{"type":"REPORT","url":"https://github.com/matrix-org/synapse/issues/14492"},{"type":"FIX","url":"https://github.com/matrix-org/synapse/pull/14642"},{"type":"PACKAGE","url":"https://github.com/matrix-org/synapse"},{"type":"WEB","url":"https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2023-67.yaml"},{"type":"WEB","url":"https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/UJIJRP5ZH6B3KGFLHCAKR2IX2Y4Z25QD"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:30:44.128485063Z"}}