GHSA-F3WC-3VXV-XMVR

Vulnerability from github – Published: 2023-05-24 17:28 – Updated: 2025-02-13 18:54
VLAI?
Summary
Synapse Outgoing federation to specific hosts can be disabled by sending malicious invites
Details

Impact

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.

Severity ?
5.0 (Medium)
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:L
5.3 (Medium)
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
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{
  "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"
}


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Sightings

Author Source Type Date

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.



Detection rules are retrieved from Rulezet.