Denial of service attack via .well-known lookups

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:N/UI:P/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2021-21274: 2.16% estimated exploitation activity in the next 30 days; percentile 82.25 (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 homeserver could redirect requests to their .well-known file to a large file. This can lead to a denial of service attack where homeservers will consume significantly more resources when requesting the .well-known file of a malicious homeserver. This affects any server which accepts federation requests from untrusted servers. Patches Issue is resolved by #8950. A bug not affecting the security aspects of this was fixed in #9108. Workarounds The federationdomainwhitelist setting can be used to restrict the homeservers communicated with over federation.

Published by the source

1 Mar 2021, 19:34 UTC

Last modified by the source

10 Sep 2026, 03:49 UTC

Source last checked

26 Oct 2026, 13:21 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-2hwx-mjrm-v3g8

2. Revision 2 · amended · 25 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-2hwx-mjrm-v3g8

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# Patch Next advisory brief: CVE-2021-21274

Advisory: Denial of service attack via .well-known lookups
IDs: CVE-2021-21274, GHSA-2hwx-mjrm-v3g8
Record: https://patchnext.com/advisories/cve-2021-21274 (revision 3, source checked 26 Oct 2026, 13:21 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 1.25.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI matrix-synapse: affected >= 0.99.0, < 1.25.0; fixed in 1.25.0

## 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-2hwx-mjrm-v3g8 (OSV record GHSA-2hwx-mjrm-v3g8 lists affected ranges: PyPI matrix-synapse >=0.99.0 <1.25.0 (fixed))
- Verified: https://github.com/advisories/GHSA-2hwx-mjrm-v3g8 (GitHub-reviewed advisory GHSA-2hwx-mjrm-v3g8 lists affected ranges: PyPI matrix-synapse >=0.99.0 <1.25.0)

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