Synapse vulnerable to federation denial of service via malformed events

Exploitation

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

CVSS

7.1 · CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H

EPSS

• CVE-2025-30355: 1.23% estimated exploitation activity in the next 30 days; percentile 68.86 (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 server can craft events with a depth outside the integer range allowed by Canonical JSON. When such an event is received by Synapse version up to 1.127.0, it prevents it from federating with other servers. The vulnerability has been exploited in the wild. Patches Fixed in Synapse v1.127.1. Workarounds Closed federation environments of trusted servers or non-federating installations are not affected. For more information If you have any questions or comments about this advisory, please email us at security at element.io.

Published by the source

27 Mar 2025, 18:02 UTC

Last modified by the source

7 Jul 2026, 17:56 UTC

Source last checked

26 Oct 2026, 13:35 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-v56r-hwv5-mxg6

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-v56r-hwv5-mxg6

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# Patch Next advisory brief: CVE-2025-30355

Advisory: Synapse vulnerable to federation denial of service via malformed events
IDs: CVE-2025-30355, GHSA-v56r-hwv5-mxg6
Record: https://patchnext.com/advisories/cve-2025-30355 (revision 2, source checked 26 Oct 2026, 13:35 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: matrix-synapse 1.127.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI matrix-synapse: affected < 1.127.1; fixed in 1.127.1

## 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-v56r-hwv5-mxg6 (OSV record GHSA-v56r-hwv5-mxg6 lists affected ranges: PyPI matrix-synapse >=0 <1.127.1 (fixed))
- Verified: https://github.com/advisories/GHSA-v56r-hwv5-mxg6 (GitHub-reviewed advisory GHSA-v56r-hwv5-mxg6 lists affected ranges: PyPI matrix-synapse >=0 <1.127.1)

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