CVE-2021-23017: A security issue in nginx resolver was identified, which might allow an attacker who is able to forge UDP pack

This week

HIGH 7.7REMOTEWATCHLISTUPGRADEUPGRADEMITIGATE

CVE-2021-23017 is a high-severity flaw (CVSS 7.7) affecting nginx, openresty, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A security issue in nginx resolver was identified, which might allow an attacker who is able to forge UDP packets from the DNS server to cause 1-byte memory overwrite, resulting in worker process crash or potential other impact.

Why this priority

High severity
EPSS top 1%: very likely to be exploited
Public exploit reference
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	7.7 (High)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	53.5% (top 1%)
CISA exploited list	No
Weakness	CWE-193
Published	2021-06-01
Last modified	2026-06-17

Publisher (CNA): F5 Networks · Vendor: f5 · fedoraproject · netapp · openresty · oracle

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-06-01 13:15 UTCPublished CVE record published
2026-06-17 03:38 UTCUpdated Record last modified

f5 nginx UPGRADE

Affected: >= 0.6.18 and < 1.20.1

Fixed in: 1.20.1

Safe to downgrade to: a release before 0.6.18

1. Check installed version

apt-cache policy nginx || rpm -q nginx || nginx --version

2. UPGRADE to 1.20.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade nginx

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade nginx

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 0.6.18 (Debian / Ubuntu)

apt-cache madison nginx sudo apt install nginx=<VERSION_BEFORE_0.6.18> sudo apt-mark hold nginx

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list nginx sudo dnf downgrade nginx-<VERSION_BEFORE_0.6.18> sudo dnf versionlock add nginx

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

openresty openresty UPGRADE

Affected: < 1.19.3.2

Fixed in: 1.19.3.2

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy openresty || rpm -q openresty || openresty --version

2. UPGRADE to 1.19.3.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade openresty

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade openresty

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

fedoraproject fedora MITIGATE

Affected: 33 · 34

Fixed in: no fixed release known

Safe to downgrade to: a release before 33

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

netapp ontap select deploy administration utility MITIGATE

Affected: all versions

Fixed in: no fixed release known

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy ontap-select-deploy-administration-utility || rpm -q ontap-select-deploy-administration-utility || ontap-select-deploy-administration-utility --version

No fixed or unaffected release known

Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.

oracle blockchain platform UPGRADE

Affected: < 21.1.2

Fixed in: 21.1.2

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy blockchain-platform || rpm -q blockchain-platform || blockchain-platform --version

2. UPGRADE to 21.1.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade blockchain-platform

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade blockchain-platform

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

oracle communications control plane monitor DOWNGRADE

Affected: 3.4 · 4.2 · 4.3 · 4.4

Fixed in: no fixed release known

Safe to downgrade to: a release before 3.4

1. Check installed version

apt-cache policy communications-control-plane-monitor || rpm -q communications-control-plane-monitor || communications-control-plane-monitor --version

2. No fixed release: DOWNGRADE to a release before 3.4 (Debian / Ubuntu)

apt-cache madison communications-control-plane-monitor sudo apt install communications-control-plane-monitor=<VERSION_BEFORE_3.4> sudo apt-mark hold communications-control-plane-monitor

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list communications-control-plane-monitor sudo dnf downgrade communications-control-plane-monitor-<VERSION_BEFORE_3.4> sudo dnf versionlock add communications-control-plane-monitor

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

Exploit-DB: Nginx 1.20.0 - Denial of Service (DOS)

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-23017?

nginx: upgrade to 1.20.1; openresty: upgrade to 1.19.3.2; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-23017 being exploited?

CVE-2021-23017 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-23017 affect?

CVE-2021-23017 affects nginx, openresty, fedora. Severity is high (CVSS 7.7). A security issue in nginx resolver was identified, which might allow an attacker who is able to forge UDP packets from the DNS server to cause 1-byte memory overwrite, resulting in worker process crash or potential other impact.

How urgent is CVE-2021-23017?

Priority 67/100. High severity; EPSS top 1%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist

Software pages

nginx vulnerabilities · F5 BIG-IP vulnerabilities

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References

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