GHSA-F3WC-3VXV-XMVR
Vulnerability from github – Published: 2023-05-24 17:28 – Updated: 2025-02-13 18:54Impact
A 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.
Synapse instances with federation disabled are not affected.
Details
The Matrix protocol allows homeservers to provide an invite_room_state field on a room invite containing a summary of room 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.
An 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.
Patches
Synapse 1.74 refuses to create oversized invite_room_state fields. Server operators should upgrade to Synapse 1.74 or newer urgently.
Workarounds
There are no robust workarounds.
This 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.
If 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.
References
- https://github.com/matrix-org/synapse/issues/14492 was caused by this issue.
- https://github.com/matrix-org/synapse/issues/14642 includes the patch described above.
For more information
If you have any questions or comments about this advisory, e-mail us at security@matrix.org.
{
"affected": [
{
"package": {
"ecosystem": "PyPI",
"name": "matrix-synapse"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "1.74.0"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2023-32323"
],
"database_specific": {
"cwe_ids": [
"CWE-20"
],
"github_reviewed": true,
"github_reviewed_at": "2023-05-24T17:28:26Z",
"nvd_published_at": "2023-05-26T14:15:10Z",
"severity": "MODERATE"
},
"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\u0027s 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\u0027s 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).",
"id": "GHSA-f3wc-3vxv-xmvr",
"modified": "2025-02-13T18:54:39Z",
"published": "2023-05-24T17:28:26Z",
"references": [
{
"type": "WEB",
"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": "WEB",
"url": "https://github.com/matrix-org/synapse/issues/14492"
},
{
"type": "WEB",
"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"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:L",
"type": "CVSS_V3"
},
{
"score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:N/SC:N/SI:N/SA:L",
"type": "CVSS_V4"
}
],
"summary": "Synapse Outgoing federation to specific hosts can be disabled by sending malicious invites"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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