CVE-2021-3690 is a high-severity (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in io.undertow:undertow-core. 1 public exploit reference exists, so weaponization risk is real. A fix is available for io.undertow:undertow-core — see the affected versions and patch details below.
Undertow vulnerable to memory exhaustion due to buffer leak
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2021-3690 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,238 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
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 buffer leak on the incoming WebSocket PONG message may lead to memory exhaustion. This flaw allows an attacker to cause a denial of service. The highest threat from this vulnerability is availability.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | io.undertow:undertow-core | all versions | 2.0.40io.undertow:undertow-core:2.0.40 |
| ☕Maven | io.undertow:undertow-core | ≥ 2.2.0&&< 2.2.10 | 2.2.10io.undertow:undertow-core:2.2.10 |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & 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 2.0.40 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3690 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-3690 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-3690. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
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.
Although Red Hat OpenStack Platform packages the vulnerable code in Opendaylight, it does not use or support the undertow-encapsulating features. The security impact for RHOSP is therefore rated as Low and no update will be provided at this time.
| Product | Fixed in | Advisory |
|---|---|---|
| EAP 7.3.9 release | undertow | RHSA-2021:3471 |
| EAP 7.3 async | see advisory | RHSA-2021:3216 |
| Red Hat EAP-XP 2.0.0 via EAP 7.3.x base | undertow | RHSA-2021:3516 |
| Red Hat Fuse 7.10 | undertow | RHSA-2021:5134 |
| Red Hat Integration Camel Quarkus 2 | see advisory | RHSA-2021:4767 |
| Red Hat JBoss Enterprise Application Platform | io.undertow/undertow-core:2.2.5.Final-redhat-00001 | RHSA-2021:3218 |
| Red Hat JBoss Enterprise Application Platform | io.undertow/undertow-core:2.2.9.SP1-redhat-00001 | RHSA-2021:3660 |
| Red Hat JBoss Enterprise Application Platform 7.1 EUS for RHEL 7 | eap7-activemq-artemis-0:1.5.5.016-1.redhat_00001.1.ep7.el7 | RHSA-2025:4226 |
Frequently Asked Questions
Is CVE-2021-3690 in your dependencies?
O3 Security finds CVE-2021-3690 across Maven dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.