CVE-2021-20220 is a medium-severity (CVSS 4.8) CWE-444 vulnerability in io.undertow:undertow-core. A fix is available for io.undertow:undertow-core — see the affected versions and patch details below.
HTTP request smuggling in Undertow
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-20220 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
io.undertow:undertow-core☕io.undertow:undertow-coreReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | io.undertow:undertow-core | ≥ 2.1.0&&< 2.1.6 | 2.1.6io.undertow:undertow-core:2.1.6 |
| ☕Maven | io.undertow:undertow-core | all versions | 2.0.34io.undertow:undertow-core:2.0.34 |
Affected Products
active iq unified managernetapponcommand workflow automationnetappundertowredhatDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for io.undertow:undertow-core, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update io.undertow:undertow-core to 2.1.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-20220 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat EAP-XP 2.0.0 via EAP 7.3.x base | wildfly-undertow | RHSA-2021:2755 |
| Red Hat EAP-XP via EAP 7.3.x base | wildfly-undertow | RHSA-2021:2210 |
| Red Hat JBoss Enterprise Application Platform 7 | wildfly-undertow | RHSA-2021:0885 |
| Red Hat JBoss Enterprise Application Platform 7.3 for RHEL 6 | eap7-activemq-artemis-0:2.9.0-9.redhat_00019.1.el6eap | RHSA-2021:0872 |
| Red Hat JBoss Enterprise Application Platform 7.3 for RHEL 7 | eap7-activemq-artemis-0:2.9.0-9.redhat_00019.1.el7eap | RHSA-2021:0873 |
| Red Hat JBoss Enterprise Application Platform 7.3 for RHEL 8 | eap7-activemq-artemis-0:2.9.0-9.redhat_00019.1.el8eap | RHSA-2021:0874 |
| Red Hat Single Sign-On 7.4.6 | wildfly-undertow | RHSA-2021:0974 |
Frequently Asked Questions
Is CVE-2021-20220 in your dependencies?
Find it across Maven, including transitive dependencies.