How severe is CVE-2021-20267?

CVE-2021-20267 has a CVSS score of 7.1/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-20267?

CVE-2021-20267 affects the following packages: neutron (PyPI), neutron (PyPI), neutron (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2021-20267?

Update neutron to 16.3.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-20267 is resolved across your whole dependency graph.

How do I detect CVE-2021-20267 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for neutron. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

How do I mitigate CVE-2021-20267 if there is no patch (or I can't update yet)?

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 does O3 Security protect against CVE-2021-20267?

O3 pinpoints whether CVE-2021-20267 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is CVE-2021-20267 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-20267 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-20267?

CVE-2021-20267 has an EPSS (Exploit Prediction Scoring System) score of 1.0%, placing it in the 59th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

When was CVE-2021-20267 published, and has it been updated?

CVE-2021-20267 was published on May 28, 2021 and was last updated on April 10, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CVE-2021-20267: neutron Denial of Service (High 7.1)

EPSS Exploitation Probability

via FIRST.org ↗

1.0%probability of exploitation in next 30 days

Lower Risk59th percentile+0.89%

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.

Blast Radius

3 pkgs affected

🐍neutron🐍neutron🐍neutron

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

A flaw was found in openstack-neutron's default Open vSwitch firewall rules. By sending carefully crafted packets, anyone in control of a server instance connected to the virtual switch can impersonate the IPv6 addresses of other systems on the network, resulting in denial of service or in some cases possibly interception of traffic intended for other destinations. Only deployments using the Open vSwitch driver are affected. Source: OpenStack project. Versions before openstack-neutron 15.3.3, openstack-neutron 16.3.1 and openstack-neutron 17.1.1 are affected.

Affected Packages

3 total 3 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`neutron`	≥ 16.0.0&&< 16.3.1	16.3.1
🐍PyPI	`neutron`	all versions	15.3.3
🐍PyPI	`neutron`	≥ 17.0.0&&< 17.1.1	17.1.1

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for neutron. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update neutron to 16.3.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-20267 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 pinpoints whether CVE-2021-20267 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to CVE-2021-20267. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

A flaw was found in openstack-neutron's default Open vSwitch firewall rules. By sending carefully crafted packets, anyone in control of a server instance connected to the virtual switch can impersonate the IPv6 addresses of other systems on the network, resulting in denial of service or in some cases possibly interception of traffic intended for other destinations. Only deployments using the Open vSwitch driver are affected. Source: OpenStack project. Versions before openstack-neutron 15.3.3, openstack-neutron 16.3.1 and openstack-neutron 17.1.1 are affected.

O3 Security · Impact-Aware SCA

Is CVE-2021-20267 in your dependencies?

O3 detects CVE-2021-20267 across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

Scan my dependencies How O3 SCA works

CVE-2021-20267