CVE-2021-42321

CRITICALNVD 8.89.0▲
EchelonGraph scoreHIGH confidence

Score elevated to 9.0 because this CVE is listed on the CISA Known Exploited Vulnerabilities catalog (added 2021-11-17), indicating real-world exploitation has been confirmed by US federal agencies. NVD baseline CVSS 8.8 retained for reference. Confidence: HIGH.

Triggered by: CISA KEV (actively exploited)
Sources: cisa_kev, epss, ghsa, nvd
Trending — 5 sources updated this weekExploited in the wild
9.0EG
EchelonGraph verdictPatch nowTreat as an emergency — this is being exploited.
  • Actively exploited in the wild (CISA-KEV)
CISA-KEV: ExploitedEPSS PROB: 92%CVSS: 8.8Exploit: cisa kev · ssvc active · kev ransomware · Metasploit · epss top5pct · epss high · public exploitExposed: 0

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

Microsoft Exchange Server Remote Code Execution Vulnerability

CVSS v3
8.8
EG Score
9.0(high)
EG Risk
81(Attend)
EG Risk 81/100SSVC: Attend

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
Severity90% × 45%
Exploitation100% × 40%
Automatability0% × 15%
Action: Remediate soon — notable exploitation risk.
EPSS PROB
92%
EPSS %ILE
100%
KEV
⚠ Exploited

Published

November 10, 2021

Last Modified

August 19, 2026

Advisory Details (4)

Auto-updated Jun 2, 2026
⚠️ Active exploitation confirmed. Patch available. Sources: microsoft, cisa.
cisa Patch Available🔴 Active Exploitation

Known Exploited Vulnerabilities Catalog | CISA

Known Exploited Vulnerabilities Catalog | CISA. Listed in CISA Known Exploited Vulnerabilities catalog.

https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-42321
microsoft Patch Available

Security Update Guide - Microsoft Security Response Center

Security Update Guide - Microsoft Security Response Center. Patch available via Microsoft Security Update

https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-42321
generic

Packet Storm

http://packetstormsecurity.com/files/168131/Microsoft-Exchange-Server-ChainedSerializationBinder-Remote-Code-Execution.html
generic

Packet Storm

http://packetstormsecurity.com/files/166153/Microsoft-Exchange-Server-Remote-Code-Execution.html

Vendor Advisories for CVE-2021-42321(1)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

All Vendor Advisories

(1)

Every vendor that published an advisory referencing this CVE — pulled from our cve_vendor_advisories aggregation. Click any row for the vendor's original advisory page.

Data Freshness Timeline

(refreshed 84× in last 7d / 369× 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 1,172 total refreshes for this CVE.

  1. 2026-09-08 09:37 UTCVendor advisory
  2. 2026-09-08 09:36 UTCGHSA enrichment
  3. 2026-09-08 09:35 UTCGHSA enrichment
  4. 2026-09-08 05:14 UTCVendor advisory
  5. 2026-09-08 05:14 UTCGHSA enrichment
  6. 2026-09-08 00:58 UTCVendor advisory
  7. 2026-09-08 00:58 UTCGHSA enrichment
  8. 2026-09-07 20:42 UTCVendor advisory
  9. 2026-09-07 20:42 UTCGHSA enrichment
  10. 2026-09-07 16:26 UTCVendor advisory
  11. 2026-09-07 16:26 UTCGHSA enrichment
  12. 2026-09-07 12:10 UTCVendor advisory
  13. 2026-09-07 12:10 UTCGHSA enrichment
  14. 2026-09-07 07:54 UTCVendor advisory
  15. 2026-09-07 07:54 UTCGHSA enrichment
  16. 2026-09-07 03:34 UTCVendor advisory
  17. 2026-09-07 03:34 UTCGHSA enrichment
  18. 2026-09-06 23:16 UTCVendor advisory
  19. 2026-09-06 23:16 UTCGHSA enrichment
  20. 2026-09-06 19:01 UTCVendor advisory
  21. 2026-09-06 19:01 UTCGHSA enrichment
  22. 2026-09-06 14:45 UTCEG score recompute
  23. 2026-09-06 14:45 UTCVendor advisory
  24. 2026-09-06 14:45 UTCGHSA enrichment
  25. 2026-09-06 13:45 UTCEPSS rescore
Show 75 more
  1. 2026-09-06 10:29 UTCVendor advisory
  2. 2026-09-06 10:29 UTCGHSA enrichment
  3. 2026-09-06 06:13 UTCVendor advisory
  4. 2026-09-06 06:13 UTCGHSA enrichment
  5. 2026-09-06 01:57 UTCVendor advisory
  6. 2026-09-06 01:57 UTCGHSA enrichment
  7. 2026-09-05 21:41 UTCVendor advisory
  8. 2026-09-05 21:41 UTCGHSA enrichment
  9. 2026-09-05 17:26 UTCEG score recompute
  10. 2026-09-05 17:26 UTCVendor advisory
  11. 2026-09-05 17:25 UTCGHSA enrichment
  12. 2026-09-05 15:25 UTCEPSS rescore
  13. 2026-09-05 13:10 UTCVendor advisory
  14. 2026-09-05 13:10 UTCGHSA enrichment
  15. 2026-09-05 08:53 UTCVendor advisory
  16. 2026-09-05 08:53 UTCGHSA enrichment
  17. 2026-09-05 04:37 UTCVendor advisory
  18. 2026-09-05 04:37 UTCGHSA enrichment
  19. 2026-09-05 00:21 UTCVendor advisory
  20. 2026-09-05 00:21 UTCGHSA enrichment
  21. 2026-09-04 20:05 UTCVendor advisory
  22. 2026-09-04 20:05 UTCGHSA enrichment
  23. 2026-09-04 17:38 UTCCISA KEV update
  24. 2026-09-04 15:49 UTCVendor advisory
  25. 2026-09-04 15:49 UTCGHSA enrichment
  26. 2026-09-04 11:33 UTCVendor advisory
  27. 2026-09-04 11:33 UTCGHSA enrichment
  28. 2026-09-04 07:17 UTCVendor advisory
  29. 2026-09-04 07:17 UTCGHSA enrichment
  30. 2026-09-04 03:01 UTCVendor advisory
  31. 2026-09-04 03:01 UTCGHSA enrichment
  32. 2026-09-03 22:44 UTCVendor advisory
  33. 2026-09-03 22:44 UTCGHSA enrichment
  34. 2026-09-03 18:25 UTCVendor advisory
  35. 2026-09-03 18:25 UTCGHSA enrichment
  36. 2026-09-03 14:06 UTCVendor advisory
  37. 2026-09-03 14:05 UTCGHSA enrichment
  38. 2026-09-03 09:46 UTCVendor advisory
  39. 2026-09-03 09:46 UTCGHSA enrichment
  40. 2026-09-03 05:30 UTCVendor advisory
  41. 2026-09-03 05:30 UTCGHSA enrichment
  42. 2026-09-03 01:14 UTCVendor advisory
  43. 2026-09-03 01:14 UTCGHSA enrichment
  44. 2026-09-02 20:58 UTCVendor advisory
  45. 2026-09-02 20:57 UTCGHSA enrichment
  46. 2026-09-02 17:33 UTCCISA KEV update
  47. 2026-09-02 16:41 UTCVendor advisory
  48. 2026-09-02 16:41 UTCGHSA enrichment
  49. 2026-09-02 12:25 UTCVendor advisory
  50. 2026-09-02 12:25 UTCGHSA enrichment
  51. 2026-09-02 08:09 UTCGHSA enrichment
  52. 2026-09-02 03:53 UTCVendor advisory
  53. 2026-09-02 03:52 UTCGHSA enrichment
  54. 2026-09-01 23:36 UTCVendor advisory
  55. 2026-09-01 23:36 UTCGHSA enrichment
  56. 2026-09-01 19:21 UTCVendor advisory
  57. 2026-09-01 19:20 UTCGHSA enrichment
  58. 2026-09-01 15:00 UTCVendor advisory
  59. 2026-09-01 14:59 UTCGHSA enrichment
  60. 2026-09-01 10:40 UTCVendor advisory
  61. 2026-09-01 10:40 UTCGHSA enrichment
  62. 2026-09-01 06:25 UTCVendor advisory
  63. 2026-09-01 06:24 UTCGHSA enrichment
  64. 2026-09-01 02:08 UTCVendor advisory
  65. 2026-09-01 02:08 UTCGHSA enrichment
  66. 2026-08-31 21:49 UTCVendor advisory
  67. 2026-08-31 21:49 UTCGHSA enrichment
  68. 2026-08-31 17:34 UTCVendor advisory
  69. 2026-08-31 17:34 UTCGHSA enrichment
  70. 2026-08-31 14:19 UTCCISA KEV update
  71. 2026-08-31 13:16 UTCVendor advisory
  72. 2026-08-31 13:15 UTCGHSA enrichment
  73. 2026-08-31 08:56 UTCVendor advisory
  74. 2026-08-31 08:56 UTCGHSA enrichment
  75. 2026-08-31 04:41 UTCVendor advisory

Publicly available exploits

(3 references)

Working exploit code is in the public domain (1 Metasploit module) (2 GitHub PoCs). Defenders should treat patch urgency accordingly — public PoCs typically lead to mass-exploitation within 24-72 hours.

  • GitHub PoC7BitsTeam/exch_CVE-2021-42321
    First seen Oct 8, 2022
    Open source ↗
  • Metasploitexploit/windows/http/exchange_chainedserializationbinder_rce✓ verified
    First seen Dec 9, 2021

    Microsoft Exchange Server ChainedSerializationBinder RCE

    Open source ↗
  • GitHub PoCDarkSprings/CVE-2021-42321
    First seen Nov 23, 2021

    Microsoft Exchange Server Poc

    Open source ↗

Frequently asked(6)

What is CVE-2021-42321?
CVE-2021-42321 is a critical vulnerability published on November 10, 2021. Microsoft Exchange Server Remote Code Execution Vulnerability
When was CVE-2021-42321 disclosed?
CVE-2021-42321 was first published in the National Vulnerability Database on November 10, 2021, with the most recent update on August 19, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-42321 actively exploited?
Yes. CISA added CVE-2021-42321 to the Known Exploited Vulnerabilities catalog on November 17, 2021, affecting Microsoft Exchange. KEV listing indicates confirmed exploitation in the wild; this CVE warrants immediate patching attention.
What is the CVSS score of CVE-2021-42321?
CVE-2021-42321 has a CVSS v3 base score of 8.8 (NVD). EchelonGraph synthesises NVD + CISA KEV + FIRST EPSS + GHSA into a combined EG score of 9.0.
Which products are affected by CVE-2021-42321?
CVE-2021-42321 affects Microsoft Exchange. The full affected-products list, including version ranges and fixed versions, is shown in the Affected Packages section of this page.
How do I remediate CVE-2021-42321?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-42321, 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

Explore the affected products and dependency analysis for CVE-2021-42321

Explore →

Is Your Infrastructure Affected by CVE-2021-42321?

EchelonGraph automatically scans your cloud infrastructure and maps CVE exposure using blast radius analysis.

Start Free Scan →Browse All CVEs