How severe is CVE-2021-3537?

CVE-2021-3537 has a CVSS score of 5.9/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-3537?

CVE-2021-3537 affects the following packages: nokogiri (RubyGems). Ecosystems affected: RubyGems.

How do I fix CVE-2021-3537?

Update nokogiri to 1.11.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3537 is resolved across your whole dependency graph.

How do I detect CVE-2021-3537 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-2021-3537 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.

Is CVE-2021-3537 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-3537 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-2021-3537?

CVE-2021-3537 has an EPSS (Exploit Prediction Scoring System) score of 3.6%, placing it in the 90th 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.

What type of vulnerability is CVE-2021-3537?

CVE-2021-3537 is classified as NULL Pointer Dereference (CWE-476). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-3537 published, and has it been updated?

CVE-2021-3537 was published on May 14, 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

MEDIUM severity

CVE-2021-3537 — nokogiri

MEDIUM

CVE-2021-3537 is a medium-severity (CVSS 5.9) NULL Pointer Dereference vulnerability in nokogiri. A fix is available for nokogiri — see the affected versions and patch details below.

Nokogiri Implements libxml2 version vulnerable to null pointer dereferencing

Also known asGHSA-286v-pcf5-25rc

Published

May 14, 2021

Updated

Oct 24, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 8, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

3.6%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs90th percentile — riskier than 90% 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

A vulnerability found in libxml2 in versions before 2.9.11 shows that it did not propagate errors while parsing XML mixed content, causing a NULL dereference. If an untrusted XML document was parsed in recovery mode and post-validated, the flaw could be used to crash the application. The highest threat from this vulnerability is to system 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

19 products · 24 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-2021-3537 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.

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

For the flaw to be exploitable, the document must be parsed in recovery mode and post validated (e.g.: when used in the `xmlling` tool, both `-recover` and `-postvalid` options must be used for the flaw to be exploitable)

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-2021-3537 in your dependencies?

Find it across RubyGems, including transitive dependencies.

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CVE-2021-3537 — 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-2021-3537 in your dependencies?