CVE-2021-21393

MEDIUMNVD 5.35.3—
EchelonGraph scoreMEDIUM confidence

This medium-severity CVE scores 5.3 under NVD CVSS v3. EPSS exploit probability: 1.6% (74th percentile of EPSS-scored CVEs). 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
5.3EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 1.6%CVSS: 5.3Exploit: None knownExposed services: Not assessed

A fix is available — apply it.

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
5.3MEDIUMmedium confidence
EG Risk
29
EG Risk 29/100CISA SSVC

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity53% × 45%
Exploitation2% × 40%
Automatability30% × 15%
CISA SSVC: Track at low or medium mission impact; Track or Attend at high (mission-essential systems).
Action: A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.
EPSS PROB
1.6%
EPSS %ILE
75th
KEV
Not listed

CISA SSVCTrack at low or medium mission impact; Track or Attend at high (mission-essential systems).

A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.

Exploitation none (no KEV listing, exploit record or EPSS ≥ 50%) · Automatable unknown (not published for this CVE) · Technical impact partial (CVSS below 9.0). Mission impact is CISA's Mission & Well-being decision point, and only you can judge it: high means the affected system is essential to your organisation's mission, or its compromise could cause irreversible harm to people. CISA's decision table

Published

April 12, 2021

Last Modified

November 21, 2024

References (10)

Affected Packages

(2 across 2 ecosystems)
Debian:14(1)
PackageVulnerable rangeFix by version rangeDependents
matrix-synapse0.19.2+dfsg-3 ... 1.9.1-1~bpo10+1 (152 versions)
  • every version up to 1.28.0-1: fixed in 1.28.0-1
—
PyPI(1)
PackageVulnerable rangeFix by version rangeDependents
matrix-synapse0.33.5 ... 1.28.0rc1 (151 versions)
  • every version up to 1.28.0: fixed in 1.28.0
—

Weakness Classification(1)

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

Data Freshness Timeline

(refreshed 9× in last 7d / 33× 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.

Showing the most recent 100 of 180 total refreshes for this CVE.

  1. 2026-10-09 20:04 UTCEPSS rescore
  2. 2026-10-09 19:58 UTCOSV refresh
  3. 2026-10-08 16:39 UTCEPSS rescore
  4. 2026-10-07 23:44 UTCEPSS rescore
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  11. 2026-10-01 19:46 UTCEPSS rescore
  12. 2026-09-30 15:29 UTCEG score recompute
  13. 2026-09-30 15:00 UTCEPSS rescore
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  21. 2026-09-23 13:44 UTCEPSS rescore
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  23. 2026-09-21 20:28 UTCEPSS rescore
  24. 2026-09-20 03:20 UTCOSV refresh
  25. 2026-09-19 15:37 UTCEPSS rescore
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  1. 2026-09-18 19:23 UTCEPSS rescore
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  3. 2026-09-16 14:05 UTCEPSS rescore
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  5. 2026-09-15 03:06 UTCEPSS rescore
  6. 2026-09-13 16:43 UTCEPSS rescore
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  18. 2026-09-01 13:50 UTCEPSS rescore
  19. 2026-09-01 06:53 UTCOSV refresh
  20. 2026-09-01 03:32 UTCEPSS rescore
  21. 2026-08-30 19:14 UTCEPSS rescore
  22. 2026-08-30 01:19 UTCEPSS rescore
  23. 2026-08-28 21:37 UTCEPSS rescore
  24. 2026-08-27 14:22 UTCEPSS rescore
  25. 2026-08-26 14:42 UTCEPSS rescore
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  27. 2026-08-23 00:16 UTCEPSS rescore
  28. 2026-08-21 23:46 UTCEPSS rescore
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  30. 2026-08-19 17:01 UTCEPSS rescore
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  34. 2026-08-16 02:11 UTCEPSS rescore
  35. 2026-08-15 06:37 UTCOSV refresh
  36. 2026-08-15 01:27 UTCEPSS rescore
  37. 2026-08-13 21:57 UTCEPSS rescore
  38. 2026-08-12 13:47 UTCEPSS rescore
  39. 2026-08-11 13:38 UTCEPSS rescore
  40. 2026-08-09 13:43 UTCEPSS rescore
  41. 2026-08-08 16:33 UTCEPSS rescore
  42. 2026-08-07 16:23 UTCEPSS rescore
  43. 2026-08-06 13:43 UTCEPSS rescore
  44. 2026-08-05 19:14 UTCEPSS rescore
  45. 2026-08-04 15:06 UTCEPSS rescore
  46. 2026-08-04 10:34 UTCEPSS rescore
  47. 2026-08-03 10:32 UTCEPSS rescore
  48. 2026-08-02 02:24 UTCEPSS rescore
  49. 2026-08-01 04:12 UTCEPSS rescore
  50. 2026-07-30 18:21 UTCOSV refresh
  51. 2026-07-30 16:25 UTCEPSS rescore
  52. 2026-07-30 01:27 UTCEPSS rescore
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  54. 2026-07-26 14:52 UTCEPSS rescore
  55. 2026-07-25 14:15 UTCEPSS rescore
  56. 2026-07-24 14:15 UTCEPSS rescore
  57. 2026-07-23 14:15 UTCEPSS rescore
  58. 2026-07-23 01:57 UTCEG score recompute
  59. 2026-07-22 22:43 UTCEG score recompute
  60. 2026-07-22 14:05 UTCEPSS rescore
  61. 2026-07-21 15:22 UTCEPSS rescore
  62. 2026-07-21 15:22 UTCEPSS rescore
  63. 2026-07-20 17:04 UTCEPSS rescore
  64. 2026-07-19 14:28 UTCEPSS rescore
  65. 2026-07-19 02:26 UTCEPSS rescore
  66. 2026-07-18 10:02 UTCEPSS rescore
  67. 2026-07-16 17:00 UTCEPSS rescore
  68. 2026-07-15 16:55 UTCEPSS rescore
  69. 2026-07-15 16:55 UTCEPSS rescore
  70. 2026-07-15 01:57 UTCEPSS rescore
  71. 2026-07-13 22:27 UTCEPSS rescore
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  73. 2026-07-13 06:10 UTCEPSS rescore
  74. 2026-07-13 06:10 UTCEPSS rescore
  75. 2026-07-13 03:37 UTCOSV refresh

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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 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 1.6% probability of exploitation in the next 30 days (75th percentile of EPSS-scored CVEs).
What is the CVSS score of CVE-2021-21393?
CVE-2021-21393 has a CVSS v3 base score of 5.3 (NVD).
How do I remediate CVE-2021-21393?
A fix for CVE-2021-21393 is available: update to the fixed version the vendor names in its advisory.

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