How severe is CVE-2021-3518?

CVE-2021-3518 has a CVSS score of 8.8/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-3518?

CVE-2021-3518 affects the following packages: nokogiri (RubyGems). Ecosystems affected: RubyGems.

How do I fix CVE-2021-3518?

Update nokogiri to 1.11.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3518 is resolved across your whole dependency graph.

How do I detect CVE-2021-3518 in my RubyGems dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for nokogiri, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-3518 if there is no patch (or I can't update yet)?

Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.

Is CVE-2021-3518 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-3518 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-2021-3518?

CVE-2021-3518 has an EPSS (Exploit Prediction Scoring System) score of 21.9%, placing it in the 98th 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 warrants monitoring and prompt remediation.

What type of vulnerability is CVE-2021-3518?

CVE-2021-3518 is classified as Use After Free (CWE-416). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-3518 published, and has it been updated?

CVE-2021-3518 was published on May 18, 2021 and was last updated on October 24, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

💎

💎 RubyGems

Not in CISA KEV

HIGH severity

CVE-2021-3518 — nokogiri

HIGH

CVE-2021-3518 is a high-severity (CVSS 8.8) Use After Free vulnerability in nokogiri. EPSS puts its 30-day exploitation probability at 21.9% (98th percentile). A fix is available for nokogiri — see the affected versions and patch details below.

Nokogiri Implements libxml2 version vulnerable to use-after-free

Also known asGHSA-v4f8-2847-rwm7

Published

May 18, 2021

Updated

Oct 24, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 9, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

21.9%probability of exploitation in next 30 days

Moderate Risk0.00%

Lower risk than most CVEs98th percentile — riskier than 98% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected

💎nokogiri

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

Description

There's a flaw in libxml2 in versions before 2.9.11. An attacker who is able to submit a crafted file to be processed by an application linked with libxml2 could trigger a use-after-free. The greatest impact from this flaw is to confidentiality, integrity, and availability.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
💎RubyGems	`nokogiri`	all versions	1.11.4`bundle update nokogiri --conservative`

Affected Products

18 products · 21 configurations

debian linuxdebian

1 version

9.0

fedorafedoraproject

2 versions

3334

Application

active iq unified managernetapp

all

Application

clustered data ontapnetapp

all

Application

clustered data ontap antivirus connectornetapp

all

hci h410c firmwarenetapp

all

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for nokogiri, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update nokogiri to 1.11.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3518 is resolved across your whole dependency graph.
Workarounds
Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.

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.

Red HatModerate

This flaw is out of support scope for Red Hat Enterprise Linux 6 and 7. To learn more about Red Hat Enterprise Linux support life cycles, please see https://access.redhat.com/support/policy/updates/errata .

Product	Fixed in	Advisory
JBoss Core Services for RHEL 8	`jbcs-httpd24-apr-util-0:1.6.1-91.el8jbcs`	RHSA-2022:1389
Red Hat Enterprise Linux 8	`libxml2-0:2.9.7-9.el8_4.2`	RHSA-2021:2569
Red Hat JBoss Core Services	`libxml2`	RHSA-2022:1390
Red Hat Migration Toolkit for Containers 1.4	`rhmtc/openshift-migration-controller-rhel8:v1.4.6-4`	RHBA-2021:2854

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2021-3518 in your dependencies?

Find it across RubyGems, including transitive dependencies.

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CVE-2021-3518 — nokogiri

EPSS Exploitation Probability

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

Description

Affected Packages

Affected Products

Detection & mitigation playbook

Detect

Fix

Workarounds

Fixing This On Your OS

Frequently Asked Questions

Is CVE-2021-3518 in your dependencies?