Synapse Outgoing federation to specific hosts can be disabled by sending malicious invites

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

5.3 · 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 · score source

EPSS

• CVE-2023-32323: 0.98% estimated exploitation activity in the next 30 days; percentile 62.32 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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 inviteroomstate 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 inviteroomstate, 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…

Published by the source

24 May 2023, 17:28 UTC

Last modified by the source

13 Feb 2025, 19:13 UTC

Source last checked

26 Oct 2026, 13:17 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-f3wc-3vxv-xmvr

2. Revision 2 · amended · 24 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-f3wc-3vxv-xmvr

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# Patch Next advisory brief: CVE-2023-32323

Advisory: Synapse Outgoing federation to specific hosts can be disabled by sending malicious invites
IDs: CVE-2023-32323, GHSA-f3wc-3vxv-xmvr
Record: https://patchnext.com/advisories/cve-2023-32323 (revision 3, source checked 26 Oct 2026, 13:17 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: matrix-synapse 1.74.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI matrix-synapse: affected < 1.74.0; fixed in 1.74.0

## Suggested inspection (read-only)
- `python -m pip show matrix-synapse`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on matrix-synapse.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-f3wc-3vxv-xmvr (OSV record GHSA-f3wc-3vxv-xmvr lists affected ranges: PyPI matrix-synapse >=0 <1.74.0 (fixed))
- Verified: https://github.com/advisories/GHSA-f3wc-3vxv-xmvr (GitHub-reviewed advisory GHSA-f3wc-3vxv-xmvr lists affected ranges: PyPI matrix-synapse >=0 <1.74.0)

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