`CVE-2021-20220` Undertow

Affected software	Undertow
Severity	4.8 Medium CVSS 3.1 2 scores
Fixed in	`2.1.6`, `2.0.34`
Weakness	CWE-444
Published	23 Feb 2021
Updated	17 Jun 2026

Description

A flaw was found in Undertow. A regression in the fix for CVE-2020-10687 was found. HTTP request smuggling related to CVE-2017-2666 is possible against HTTP/1.x and HTTP/2 due to permitting invalid characters in an HTTP request. This flaw allows an attacker to poison a web-cache, perform an XSS attack, or obtain sensitive information from request other than their own. The highest threat from this vulnerability is to data confidentiality and integrity.

Exploitation status

No public exploit

No exploit has been reported in the sources we check. Published 5 years ago.

CISA KEV not listed
VulnCheck KEV not listed
CISA Vulnrichment no assessment published
Indexed PoC none indexed
EPSS 1.2% chance in 30 days

Waratek protection

No exploit published

Within Waratek scope. In the sources we check, nobody has published exploit code for this CVE or reported it being exploited.

A public code fix exists: undertow-io/undertow 9e797b2.

Protection none
Action unverified
Review automated Upstream data
Record active

Decided by	Medium or Low severity, no CISA or VulnCheck KEV listing, no PoC clearing the evidence bar, no CISA verdict, no Oracle advisory
Finding	This CVE is MEDIUM severity, is not listed in CISA KEV or VulnCheck KEV, has no public exploit or PoC in any indexed source, carries no CISA Vulnrichment finding of a demonstrated exploit, and is not part of an Oracle Critical Patch Update. None of the signals that would put it in front of an engineer is present, and the volume of CVEs below that bar is too great to take one at a time, so no rule is written for it. This is a decision about what we act on, not a finding that the CVE is harmless — a patch may well be derivable, and a low-severity defect can still be one link in a longer chain.
Candidate fix	https://bugzilla.redhat.com/show_bug.cgi?id=1923133 : a Red Hat bug: the triage, which links onward to the fix

Decided by

Medium or Low severity, no CISA or VulnCheck KEV listing, no PoC clearing the evidence bar, no CISA verdict, no Oracle advisory

Finding

This CVE is MEDIUM severity, is not listed in CISA KEV or VulnCheck KEV, has no public exploit or PoC in any indexed source, carries no CISA Vulnrichment finding of a demonstrated exploit, and is not part of an Oracle Critical Patch Update. None of the signals that would put it in front of an engineer is present, and the volume of CVEs below that bar is too great to take one at a time, so no rule is written for it. This is a decision about what we act on, not a finding that the CVE is harmless — a patch may well be derivable, and a low-severity defect can still be one link in a longer chain.

Candidate fix

https://bugzilla.redhat.com/show_bug.cgi?id=1923133 : a Red Hat bug: the triage, which links onward to the fix

CVSS

4.8 MEDIUM v3.1 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N 2 metrics

We show the highest score from the newest CVSS version. Scores from different versions can't be compared, and the first score listed is sometimes a 0.0 placeholder.

Version Score Band Vector Assigner Type

CVSS 3.1 4.8 MEDIUM CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N NVD Primary published

CVSS 2.0 5.8 none AV:N/AC:M/Au:N/C:P/I:P/A:N NVD Primary

Version	Score	Band	Vector	Assigner	Type
CVSS 3.1	4.8	MEDIUM	`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N`	NVD	Primary	published
CVSS 2.0	5.8	none	`AV:N/AC:M/Au:N/C:P/I:P/A:N`	NVD	Primary

Affected software and releases

Maven packages

Affected and fixed versions, from OSV.dev

Package Introduced Fixed Last affected

io.undertow:undertow-core 2.1.0 2.1.6 unbounded

io.undertow:undertow-core 0 2.0.34 unbounded

Package	Introduced	Fixed	Last affected
`io.undertow:undertow-core`	2.1.0	`2.1.6`	unbounded
`io.undertow:undertow-core`	0	`2.0.34`	unbounded

Products and versions

4 products, 6 version entries

Source	Vendor	Product	Scheme	Affected versions
osv	io.undertow	undertow-core	generic	＜ 2.0.34 · ≥ 2.1.0 and ＜ 2.1.6
nvd	netapp	active iq unified manager	generic	any version
nvd	netapp	oncommand workflow automation	generic	any version
nvd	redhat	undertow	generic	＜ 2.0.34 · ≥ 2.1.0 and ＜ 2.1.6

Source

Vendor

Product

Scheme

Affected versions

osv

io.undertow

undertow-core

generic

＜ 2.0.34 · ≥ 2.1.0 and ＜ 2.1.6

nvd

netapp

active iq unified manager

generic

any version

nvd

netapp

oncommand workflow automation

generic

any version

nvd

redhat

undertow

generic

＜ 2.0.34 · ≥ 2.1.0 and ＜ 2.1.6

References

URL	Tags
https://bugzilla.redhat.com/show_bug.cgi?id=1923133	bugzilla
https://github.com/undertow-io/undertow/commit/9e797b2f99617fdad0471eaa88c711ee7f44605f	WEB
https://security.netapp.com/advisory/ntap-20220210-0013	WEB
https://security.netapp.com/advisory/ntap-20220210-0013/	Third Party Advisory

URL

Timeline

Published	2021-02-23	By the CVE Program.
NVD record modified	2026-06-17	NVD's own last-modified date for this record.

Published

2021-02-23

By the CVE Program.

NVD record modified

2026-06-17

NVD's own last-modified date for this record.

How VRT decided (17 of 24 rules ran)

17 of 24 rules ran. The rest didn't run because an earlier rule had already settled the CVE.

# Rule Outcome Status Detail

identity: Does this identifier describe a vulnerability at all? the first rule that matches decides

1 rejected no match CVE Program withdrawal of this identifier

scope: Is it a vulnerability Waratek protects? the first rule that matches decides

2 armr-patch-file no match An ARMR patch file on disk for this CVE

3 armr-secure-rule-file no match An ARMR security-rule template on disk for this CVE

4 manual-classification no match A researcher's recorded decision for this specific CVE

5 manual classification no match Manual classification of this CVE in the legacy dataset

6 non-java-upstream no match A root cause in non-Java code: an OS kernel, a C library, or a bundled runtime

7 oracle-component no match The defect being in a component of a tracked product the agent does not reach: JavaFX, the Java SE installer, the JInitiator browser plug-in, the native WebLogic proxy plug-in, or a native part of Oracle Application Server or Collaboration Suite (HTTP Server, Web Cache, Internet Directory, mail and calendar servers). Oracle gives these no CPE of their own and names them only in the description.

8 java-client-side no match Oracle's own risk-matrix note saying the CVE is reached only through untrusted code in the Java sandbox (applets, Web Start) or only through a client installation of Java, so a server running its own trusted code is not exposed

9 osv-maven matched In scope Maven packages: io.undertow:undertow-core

10 cna-maven-package not reached An affected Maven artifact named by the CNA itself

11 known-cpe-product not reached An NVD CPE naming a product ARMR tracks

12 cna-cpe-product not reached A CPE the CNA published naming a tracked product. Oracle publishes these on advisory day, months before NVD.

13 oracle-advisory-family not reached A tracked product family named in Oracle's own risk matrix. CPE cannot carry a suite name, only a module.

14 cna-named-product not reached A tracked product named in words by the CVE's own CNA, on a record with no CPE yet. Apache and Eclipse publish these on advisory day, before NVD adds CPE data.

15 non-deployable-product not reached The only affected product being a sealed appliance image, where Java runs but no -javaagent can be attached

16 unmatched-product not reached No tracked product matched above, and none present at all

disposition: What do we do about one that is? every rule runs

17 patch-hint matched Mitigation candidate References include candidate fix URL (bugzilla tier): https://bugzilla.redhat.com/show_bug.cgi?id=1923133

18 open-source-maven skipped The affected package being open source on Maven, so the fix can be read

19 poc-derivable skipped An indexed exploit clearing the evidence bar to reverse-engineer, where there is no source to read

20 no-exploit skipped Nothing to build a rule from yet: no published exploit, no CISA record of exploitation and no source-level change to read

21 secure-rule-match no match A vulnerability class already blocked by an ARMR security rule

22 exploited-no-fix-or-poc skipped Somebody exploiting this CVE (a CISA or VulnCheck KEV listing, or a CISA verdict of active exploitation), with no fix, source code or PoC to build a rule from

23 below-action-threshold decided it matched No exploit published MEDIUM severity, no CISA or VulnCheck KEV listing, no public PoC clearing the evidence bar, no CISA exploitation finding, and not in an Oracle advisory — none of the signals we act on is present.

24 no-exploit-published skipped Exploit code for this candidate, published in a reviewed catalogue or a GitHub repository with 5 or more stars, or in use per CISA or VulnCheck; not only a CISA verdict, a forecast, or nothing at all

Products the scope rules checked

The products this CVE's record names, and whether VRT tracks each one. The scope rules decided on these.

Vendor Product Tracked CPE Source

redhat undertow tracked cpe:2.3:a:redhat:undertow:*:*:*:*:*:*:*:* yaml

netapp active_iq_unified_manager not tracked cpe:2.3:a:netapp:active_iq_unified_manager:-:*:*:*:*:linux:*:*

netapp oncommand_workflow_automation not tracked cpe:2.3:a:netapp:oncommand_workflow_automation:-:*:*:*:*:*:*:*

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