CVE-2021-21274

MEDIUMNVD 4.34.3—
EchelonGraph scoreMEDIUM confidence

This medium-severity CVE scores 4.3 under NVD CVSS v3. EPSS exploit probability: 0.4%, top 36% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, nvd
4.3
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS: 2%CVSS: 4.3Exploit: NoneExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.25.0, 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. Issue is resolved in version 1.25.0. As a workaround the federation_domain_whitelist setting can be used to restrict the homeservers communicated with over federation.

CVSS v3
4.3
EG Score
4.3(medium)
EPSS
80.1%
KEV
Not listed

Published

February 26, 2021

Last Modified

November 21, 2024

References (10)

Affected Packages

(1 across 1 ecosystem)
PyPI(1)
PackageVulnerable rangeFixed inDependents
matrix-synapse0.99.0 ... 1.9.1 (124 versions)1.25.0—

Weakness Classification(2)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 7× in last 7d / 45× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-07-11 08:25 UTCEPSS rescore
  2. 2026-07-09 19:07 UTCEPSS rescore
  3. 2026-07-08 15:12 UTCEPSS rescore
  4. 2026-07-07 13:43 UTCEPSS rescore
  5. 2026-07-06 16:25 UTCEPSS rescore
  6. 2026-07-06 02:21 UTCEPSS rescore
  7. 2026-07-06 02:21 UTCEPSS rescore
  8. 2026-07-05 02:28 UTCEPSS rescore
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  10. 2026-07-04 06:29 UTCEPSS rescore
  11. 2026-07-04 06:29 UTCEPSS rescore
  12. 2026-07-02 16:57 UTCEPSS rescore
  13. 2026-07-01 15:03 UTCEPSS rescore
  14. 2026-07-01 07:48 UTCOSV refresh
  15. 2026-06-30 23:20 UTCEPSS rescore
  16. 2026-06-30 23:20 UTCEPSS rescore
  17. 2026-06-29 14:04 UTCEPSS rescore
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  19. 2026-06-28 14:05 UTCEPSS rescore
  20. 2026-06-28 04:54 UTCEPSS rescore
  21. 2026-06-28 04:54 UTCEPSS rescore
  22. 2026-06-27 03:06 UTCEPSS rescore
  23. 2026-06-27 03:06 UTCEPSS rescore
  24. 2026-06-25 13:48 UTCEPSS rescore
  25. 2026-06-25 13:48 UTCEPSS rescore
Show 55 more
  1. 2026-06-24 14:03 UTCEPSS rescore
  2. 2026-06-24 14:03 UTCEPSS rescore
  3. 2026-06-23 21:31 UTCEPSS rescore
  4. 2026-06-23 21:31 UTCEPSS rescore
  5. 2026-06-21 14:54 UTCEPSS rescore
  6. 2026-06-21 14:54 UTCEPSS rescore
  7. 2026-06-21 01:58 UTCEPSS rescore
  8. 2026-06-21 01:57 UTCEPSS rescore
  9. 2026-06-19 19:24 UTCEPSS rescore
  10. 2026-06-19 19:24 UTCEPSS rescore
  11. 2026-06-18 17:51 UTCEPSS rescore
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  13. 2026-06-17 17:51 UTCEPSS rescore
  14. 2026-06-16 17:50 UTCEPSS rescore
  15. 2026-06-15 17:46 UTCEPSS rescore
  16. 2026-06-14 23:15 UTCEPSS rescore
  17. 2026-06-13 22:58 UTCEPSS rescore
  18. 2026-06-13 13:13 UTCOSV refresh
  19. 2026-06-12 23:10 UTCEPSS rescore
  20. 2026-06-12 23:10 UTCEPSS rescore
  21. 2026-06-11 13:58 UTCEPSS rescore
  22. 2026-06-10 22:16 UTCEPSS rescore
  23. 2026-06-10 22:16 UTCEPSS rescore
  24. 2026-06-10 13:20 UTCEPSS rescore
  25. 2026-06-08 14:15 UTCEPSS rescore
  26. 2026-06-08 14:15 UTCEPSS rescore
  27. 2026-06-07 15:23 UTCEPSS rescore
  28. 2026-06-07 15:23 UTCEPSS rescore
  29. 2026-06-06 13:46 UTCEPSS rescore
  30. 2026-06-06 13:46 UTCEPSS rescore
  31. 2026-06-06 13:46 UTCEPSS rescore
  32. 2026-06-05 22:45 UTCEPSS rescore
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  34. 2026-06-05 06:09 UTCEPSS rescore
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  36. 2026-06-05 06:08 UTCEPSS rescore
  37. 2026-06-04 13:10 UTCEPSS rescore
  38. 2026-06-02 20:11 UTCEPSS rescore
  39. 2026-06-02 20:11 UTCEPSS rescore
  40. 2026-06-01 13:50 UTCEPSS rescore
  41. 2026-06-01 13:50 UTCEPSS rescore
  42. 2026-05-31 22:29 UTCEPSS rescore
  43. 2026-05-31 22:29 UTCEPSS rescore
  44. 2026-05-31 00:15 UTCEPSS rescore
  45. 2026-05-31 00:15 UTCEPSS rescore
  46. 2026-05-29 13:42 UTCEPSS rescore
  47. 2026-05-28 13:43 UTCEPSS rescore
  48. 2026-05-28 13:43 UTCEPSS rescore
  49. 2026-05-27 13:39 UTCEPSS rescore
  50. 2026-05-26 20:20 UTCEG score recompute
  51. 2026-05-26 13:42 UTCEPSS rescore
  52. 2026-05-26 13:42 UTCEPSS rescore
  53. 2026-05-26 07:17 UTCEPSS rescore
  54. 2026-05-26 07:17 UTCEPSS rescore
  55. 2026-05-24 16:49 UTCEPSS rescore

Related CVEs(same CWE)

Frequently asked(5)

What is CVE-2021-21274?
CVE-2021-21274 is a medium vulnerability published on February 26, 2021. Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.25.0, a malicious homeserver could redirect requests to their .well-known file to a large file. This can lead…
When was CVE-2021-21274 disclosed?
CVE-2021-21274 was first published in the National Vulnerability Database on February 26, 2021, with the most recent update on November 21, 2024. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-21274 actively exploited?
CVE-2021-21274 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 80.1% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2021-21274?
CVE-2021-21274 has a CVSS v3 base score of 4.3 (NVD).
How do I remediate CVE-2021-21274?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-21274, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

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