CVE-2021-21409

MEDIUMNVD 5.95.9—
EchelonGraph scoreMEDIUM confidence

This medium-severity CVE scores 5.9 under NVD CVSS v3. EPSS exploit probability: 4.9% (91st 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
Trending — 3 sources updated this week
5.9EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 4.9%CVSS: 5.9Exploit: None knownExposed services: Not assessed

A fix is available — apply it.

Netty is an open-source, asynchronous event-driven network application framework for rapid development of maintainable high performance protocol servers & clients. In Netty (io.netty:netty-codec-http2) before version 4.1.61.Final there is a vulnerability that enables request smuggling. The content-length header is not correctly validated if the request only uses a single Http2HeaderFrame with the endStream set to to true. This could lead to request smuggling if the request is proxied to a remote peer and translated to HTTP/1.1. This is a followup of GHSA-wm47-8v5p-wjpj/CVE-2021-21295 which did miss to fix this one case. This was fixed as part of 4.1.61.Final.

CVSS v3
5.9
EG Score
5.9MEDIUMmedium confidence
EG Risk
44
EG Risk 44/100CISA 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
Severity59% × 45%
Exploitation5% × 40%
Automatability100% × 15%
CISA SSVC: Track at low or medium mission impact; 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
4.9%
EPSS %ILE
91st
KEV
Not listed

CISA SSVCTrack at low or medium mission impact; 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 (CISA Vulnrichment) · Automatable yes (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). 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

March 30, 2021

Last Modified

October 8, 2026

Advisory Details (10)

Auto-updated Oct 8, 2026
Patch available. Sources: github_commit, github.
generic

Pony Mail

https://lists.apache.org/thread.html/r31044fb995e894749cb821c6fe56f487c16a97028e6e360e59f09d58%40%3Cissues.zookeeper.apache.org%3E
generic

Pony Mail

https://lists.apache.org/thread.html/r2732aa3884cacfecac4c54cfaa77c279ba815cad44b464a567216f83%40%3Cissues.zookeeper.apache.org%3E
generic

Pony Mail

https://lists.apache.org/thread.html/r1b3cb056364794f919aaf26ceaf7423de64e7fdd05a914066e7d5219%40%3Cissues.zookeeper.apache.org%3E
generic

Pony Mail

https://lists.apache.org/thread.html/r101f82d8f3b5af0bf79aecbd5b2dd3b404f6bb51d1a54c2c3d29bed9%40%3Cnotifications.zookeeper.apache.org%3E
generic

Pony Mail

https://lists.apache.org/thread.html/r0ca82fec33334e571fe5b388272260778883e307e15415d7b1443de2%40%3Cissues.zookeeper.apache.org%3E
generic

Pony Mail

https://lists.apache.org/thread.html/r0b09f3e31e004fe583f677f7afa46bd30110904576c13c5ac818ac2c%40%3Cissues.flink.apache.org%3E
githubMentions a patch (unverified)

Possible request smuggling in HTTP/2 due missing validation · Advisory · netty/netty · GitHub

Affected: 4.1.60.Final

https://github.com/netty/netty/security/advisories/GHSA-wm47-8v5p-wjpj
githubMentions a patch (unverified)

Possible request smuggling in HTTP/2 due missing validation of content-length · Advisory · netty/netty · GitHub

https://github.com/netty/netty/security/advisories/GHSA-f256-j965-7f32
github_commit Patch Available

commit b0fa4d5aab42 (netty/netty)

Patch available: netty/netty netty-4.1.132.Final (contains commit b0fa4d5aab42)

https://github.com/netty/netty/commit/b0fa4d5aab4215f3c22ce6123dd8dd5f38dc0432
generic
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-21295

Vendor Advisories for CVE-2021-21409(14)

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

Patch Availability(14)

Vendor / EcosystemFix (by version range) / PatchReleasedSource
ubuntulibnetty-java (1:4.1.45-1ubuntu0.1~esm1) @ focal2026-10-08ubuntu
redhatcandlepin-0:4.1.13-1.el8sat2022-07-05redhat
redhatopenshift-logging/elasticsearch6-rhel8:v6.8.1-652021-12-14redhat
redhatnetty2021-12-14redhat
redhatopenshift-logging/elasticsearch6-rhel8:v6.8.1-662021-12-14redhat
redhatopenshift-logging/elasticsearch6-rhel8:v6.8.1-672021-12-14redhat
redhatpatch2021-09-30redhat
redhatio.netty/netty-all:4.1.63.Final-redhat-000012021-09-23redhat
redhateap7-netty-0:4.1.63-1.Final_redhat_00001.1.el8eap2021-09-23redhat
redhateap7-netty-0:4.1.63-1.Final_redhat_00001.1.el7eap2021-09-23redhat
redhateap7-wildfly-http-client-0:1.0.28-1.Final_redhat_00001.1.el6eap2021-07-13redhat
redhateap7-wildfly-http-client-0:1.0.28-1.Final_redhat_00001.1.el7eap2021-07-13redhat
redhateap7-wildfly-http-client-0:1.0.28-1.Final_redhat_00001.1.el8eap2021-07-13redhat
redhatqpid-proton-0:0.33.0-8.el82021-05-06redhat

Patches are aggregated from vendor advisories (Red Hat, Microsoft, Cisco, GitHub) and package ecosystems (OSV, GHSA). Multiple rows for the same upstream release have been deduplicated.

Affected Packages

(8 across 6 ecosystems)
Maven(3)
PackageVulnerable rangeFix by version rangeDependents
io.netty:netty3.10.0.Final ... 4.0.0.Alpha8 (71 versions)
  • every version on: no fix on record
—
io.netty:netty-codec-http24.1.0.Beta4 ... 4.1.9.Final (73 versions)
  • 4.0.0 up to 4.1.61.Final: fixed in 4.1.61.Final
—
org.jboss.netty:netty3.0.0.CR1 ... 3.2.9.Final (39 versions)
  • every version on: no fix on record
—
Debian:10(1)
PackageVulnerable rangeFix by version rangeDependents
netty1:4.1.33-1, 1:4.1.33-1+deb10u1
  • every version up to 1:4.1.33-1+deb10u2: fixed in 1:4.1.33-1+deb10u2
—
Debian:11(1)
PackageVulnerable rangeFix by version rangeDependents
netty—
  • every version up to 1:4.1.48-4: fixed in 1:4.1.48-4
—
Debian:12(1)
PackageVulnerable rangeFix by version rangeDependents
netty—
  • every version up to 1:4.1.48-4: fixed in 1:4.1.48-4
—
Debian:13(1)
PackageVulnerable rangeFix by version rangeDependents
netty—
  • every version up to 1:4.1.48-4: fixed in 1:4.1.48-4
—
Debian:14(1)
PackageVulnerable rangeFix by version rangeDependents
netty—
  • every version up to 1:4.1.48-4: fixed in 1:4.1.48-4
—

Weakness Classification(1)

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

Additional Vendor Advisories

(8)

Vendors that published advisories for this CVE beyond the curated set above. Broader coverage but minimal per-row detail — click through for the original advisory.

Data Freshness Timeline

(refreshed 9× in last 7d / 34× 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 182 total refreshes for this CVE.

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

Related CVEs(same product + same vendor + same CWE)

Frequently asked(5)

What is CVE-2021-21409?
CVE-2021-21409 is a medium vulnerability published on March 30, 2021. Netty is an open-source, asynchronous event-driven network application framework for rapid development of maintainable high performance protocol servers & clients. In Netty (io.netty:netty-codec-http2) before version 4.1.61.Final there is a vulnerability that enables request smuggling. The…
When was CVE-2021-21409 disclosed?
CVE-2021-21409 was first published on March 30, 2021, with the most recent update on October 8, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-21409 actively exploited?
CVE-2021-21409 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 4.9% probability of exploitation in the next 30 days (91st percentile of EPSS-scored CVEs).
What is the CVSS score of CVE-2021-21409?
CVE-2021-21409 has a CVSS v3 base score of 5.9 (NVD).
How do I remediate CVE-2021-21409?
A fix for CVE-2021-21409 is available: update to the fixed version the vendor names in its advisory. The vendor advisories EchelonGraph has for CVE-2021-21409 are linked in the Vendor Advisories panel on this page.

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