CVE-2021-21409
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Possible request smuggling in HTTP/2 due missing validation of content-length
rank 27 = 1 + 15 + 0 + 11 + 0
rank@v1 · scheduled
Someone is already using it
CISA KEV catalog · public proof-of-concept · FIRST EPSS
EPSS 0.04935 · without this subject the record ranks 26
What it costs you if it fires
NVD CVSS base score, then the CNA's own
CVSS 5.9, from NVD · without this subject the record ranks 12
It is getting worse, not staying still
FIRST EPSS series, rescorings and bad loads removed
no persisting rise in 365 days · without this subject the record ranks 27
How much of the world it touches
NVD CPE configurations
26 products in the CPE ranges · without this subject the record ranks 16
Someone else already set the date
CISA KEV remediation due date
no CISA due date · without this subject the record ranks 27
Hindsight · the state
Foresight · where to check next
This record is one instance of CWE-444, inconsistent interpretation of http requests ('http request/response smuggling'). The index holds 449 prior instances of this class, 5 KEV-listed. The nearest of its KEV-listed records are drawn beside this. taxonomy@v1
Running netty? Check your version
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Harm, if it fires from the vector and the weakness
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CISA KEV & Federal Remediation Deadline from the record and CISA
The fix, and how long it's been waiting.
Show full BOD 26-04 deadline
CISA has not scored it, so the directive's own defaults apply: automatable no, technical impact total. Calendar days from the KEV listing or from when you find it on a system, whichever is first. Federal agencies are bound from 7 Dec 2026, and contracts often carry it to suppliers. The directive and its table
Exploit Status & Known PoCs a ladder, not a score
Current rung: Nothing observed in held sources.
- RUNG 1Nothing observed in held sources
- RUNG 2Proof-of-concept published
- RUNG 3Exploited in the wild · CISA KEV listed
- RUNG 4Ransomware campaign use known
Affected & Fixed Versions as published
Show all 26 published ranges
A rung lights from a row in the index, never from an assessment. "Held sources" is the true scope of the claim: this index reads the CVE record, NVD, CISA KEV, FIRST's EPSS and public proof-of-concept repositories, and it does not observe attacks.
$151,000 = $4.99M incident cost x 0.6121449559255632 exposure x 0.04935 likelihood
udv@v1 · unaudited · wrong for you until you say what you run
$4.99M is the cost of a breach that happened, from IBM Cost of a Data Breach Report 2026, global average. It is not the value of any asset of yours. That average already covers breaches of every severity, so scaling it by exposure understates a partial-impact record. That is an assumption, not a measurement. Your own asset value goes in the decision record.
Attack path
What it is stated by the CNA
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. receipt
NVD's analysis a second authority
- severity
- 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N
- weakness
- CWE-444
- affected versions
- netty < 4.1.61; debian linux 10.0; oracle banking corporate lending process management 14.2.0; and 23 more, listed in full below
- also scored by
- security-advisories@github.com 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N
check it yourself github.com github.com cve.mitre.org github.com debian.org lists.apache.org +14 morereceipt
Dated events
Movement the material steps
This score more than doubled inside 72 hours. Both observations are in the full history below.
Show all 63 observations
63 observations held. A highlighted step crossed a promotion threshold. A step marked with a scoring model moved because the instrument was replaced, not because the vulnerability changed. receipt
The full biography
Exploit probability more than doubled between 2021-09-08 and 2021-09-09 (0.2358 → 0.58062) and the record had been public for 5 years, 2 months when this index was last written.
The pattern, elsewhere
This record is one instance of CWE-444, inconsistent interpretation of http requests ('http request/response smuggling'). The index holds 449 prior instances of this class, 5 KEV-listed. The nearest of its KEV-listed records are drawn beside this. cwe-consequences@4.20
"Matches n of m" counts the attributes the two records can both be compared on: the CVSS metrics they share a version of, any weakness class beyond this one, and the vendor. fit-rules@v1
449 records in this class held. The six above are drawn from the ones CISA has listed as exploited, ranked by how much of the record matches this one. That is why every one of them is KEV-listed: the pool is, so the colour on those spokes says nothing the pool has not already said.
No curated product neighbourhood is held for this product, so the records above are the nearest this index can name. The neighbourhood table covers a small set of products and is extended by hand.
Siblings and hubs
Records related to this one
Its defect family
netty
Of the 10 records in this family that state one, this holds the highest EPSS percentile: 0.91544.
Grouped by shared product and weakness, not by shared vulnerability. Each record's own state is on its own page.
Family membership: held values
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Browse the index from here
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What usually comes next
Everything above needs no account. What is below is work: assembling it, scoping it to your estate, watching it, and handing it to someone else.