CVE-2021-21393

MEDIUMNVD 5.35.3
EchelonGraph scoreLOW confidence

This medium-severity CVE scores 5.3 under NVD CVSS v3. EPSS exploit-prediction score not yet available (the EPSS model rescores nightly; freshly-published CVEs typically appear within 48 hours). 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: nvd
5.3
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS: 2%CVSS: 5.3Exploit: 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 1.28.0 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.

CVSS v3
5.3
EG Score
5.3(low)
EPSS
72.9%
KEV
Not listed

Published

April 12, 2021

Last Modified

November 21, 2024

References (10)

Affected Packages

(1 across 1 ecosystem)
PyPI(1)
PackageVulnerable rangeFixed inDependents
matrix-synapse0.33.5 ... 1.28.0rc1 (151 versions)1.28.0—

Weakness Classification(1)

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

Data Freshness Timeline

(refreshed 7× in last 7d / 42× 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
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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
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  14. 2026-06-30 23:20 UTCEPSS rescore
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  16. 2026-06-28 14:05 UTCEPSS rescore
  17. 2026-06-28 04:54 UTCEPSS rescore
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  19. 2026-06-27 03:06 UTCEPSS rescore
  20. 2026-06-27 03:06 UTCEPSS rescore
  21. 2026-06-25 13:48 UTCEPSS rescore
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  23. 2026-06-24 14:03 UTCEPSS rescore
  24. 2026-06-24 14:03 UTCEPSS rescore
  25. 2026-06-24 13:02 UTCOSV refresh
Show 53 more
  1. 2026-06-23 21:31 UTCEPSS rescore
  2. 2026-06-23 21:31 UTCEPSS rescore
  3. 2026-06-21 14:54 UTCEPSS rescore
  4. 2026-06-21 14:54 UTCEPSS rescore
  5. 2026-06-21 01:58 UTCEPSS rescore
  6. 2026-06-21 01:57 UTCEPSS rescore
  7. 2026-06-19 19:24 UTCEPSS rescore
  8. 2026-06-19 19:24 UTCEPSS rescore
  9. 2026-06-18 17:51 UTCEPSS rescore
  10. 2026-06-18 17:51 UTCEPSS rescore
  11. 2026-06-17 17:51 UTCEPSS rescore
  12. 2026-06-16 17:50 UTCEPSS rescore
  13. 2026-06-15 17:46 UTCEPSS rescore
  14. 2026-06-14 23:15 UTCEPSS rescore
  15. 2026-06-13 22:58 UTCEPSS rescore
  16. 2026-06-12 23:10 UTCEPSS rescore
  17. 2026-06-12 23:10 UTCEPSS rescore
  18. 2026-06-11 13:58 UTCEPSS rescore
  19. 2026-06-10 22:16 UTCEPSS rescore
  20. 2026-06-10 22:16 UTCEPSS rescore
  21. 2026-06-10 13:20 UTCEPSS rescore
  22. 2026-06-08 14:15 UTCEPSS rescore
  23. 2026-06-08 14:15 UTCEPSS rescore
  24. 2026-06-07 17:46 UTCOSV refresh
  25. 2026-06-07 15:23 UTCEPSS rescore
  26. 2026-06-07 15:23 UTCEPSS rescore
  27. 2026-06-06 13:46 UTCEPSS rescore
  28. 2026-06-06 13:46 UTCEPSS rescore
  29. 2026-06-06 13:46 UTCEPSS rescore
  30. 2026-06-05 22:45 UTCEPSS rescore
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  32. 2026-06-05 06:09 UTCEPSS rescore
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  35. 2026-06-04 13:10 UTCEPSS rescore
  36. 2026-06-02 20:11 UTCEPSS rescore
  37. 2026-06-02 20:11 UTCEPSS rescore
  38. 2026-06-01 13:50 UTCEPSS rescore
  39. 2026-06-01 13:50 UTCEPSS rescore
  40. 2026-05-31 22:29 UTCEPSS rescore
  41. 2026-05-31 22:29 UTCEPSS rescore
  42. 2026-05-31 00:15 UTCEPSS rescore
  43. 2026-05-31 00:15 UTCEPSS rescore
  44. 2026-05-29 13:42 UTCEPSS rescore
  45. 2026-05-28 13:43 UTCEPSS rescore
  46. 2026-05-28 13:43 UTCEPSS rescore
  47. 2026-05-27 13:39 UTCEPSS rescore
  48. 2026-05-26 13:42 UTCEPSS rescore
  49. 2026-05-26 13:42 UTCEPSS rescore
  50. 2026-05-26 07:17 UTCEPSS rescore
  51. 2026-05-26 07:17 UTCEPSS rescore
  52. 2026-05-24 16:49 UTCEPSS rescore
  53. 2026-05-23 08:05 UTCEG score recompute

Related CVEs(same CWE)

Frequently asked(5)

What is CVE-2021-21393?
CVE-2021-21393 is a medium vulnerability published on April 12, 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 1.28.0 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party…
When was CVE-2021-21393 disclosed?
CVE-2021-21393 was first published in the National Vulnerability Database on April 12, 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-2021-21393 actively exploited?
CVE-2021-21393 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 72.9% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2021-21393?
CVE-2021-21393 has a CVSS v3 base score of 5.3 (NVD). The EG score is currently aggregating — additional source signals are being incorporated as they become available..
How do I remediate CVE-2021-21393?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-21393, 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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