CVE-2021-3537 — nokogiri
MEDIUMCVE-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
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-2021-3537 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 378,156 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
nokogiriReal-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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | nokogiri | all versions | 1.11.4bundle update nokogiri --conservative |
Detection & mitigation playbook
Open-source dependencyDetect
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.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-3537 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-3537. 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.
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 Migration Toolkit for Containers 1.4 | rhmtc/openshift-migration-controller-rhel8:v1.4.6-4 | RHBA-2021:2854 |
Frequently Asked Questions
Is CVE-2021-3537 in your dependencies?
O3 Security finds CVE-2021-3537 across RubyGems dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.