CVE-2021-26539 is a medium-severity (CVSS 5.3) vulnerability in sanitize-html. 1 public exploit reference exists, so weaponization risk is real. A fix is available for sanitize-html — see the affected versions and patch details below.
Improper Input Validation in sanitize-html
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-2021-26539 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 377,166 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
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
sanitize-htmlnpmDescription
Apostrophe Technologies sanitize-html before 2.3.1 does not properly handle internationalized domain name (IDN) which could allow an attacker to bypass hostname whitelist validation set by the "allowedIframeHostnames" option.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | sanitize-html | all versions | 2.3.1npm install sanitize-html@2.3.1 |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for sanitize-html, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update sanitize-html to 2.3.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-26539 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-2021-26539 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-26539. 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.
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat OpenShift Container Platform 4.7 | openshift4/ose-console:v4.7.0-202102130115.p0 | RHSA-2020:5633 |
| Red Hat OpenShift Container Platform 4.8 | openshift4/ose-thanos-rhel8:v4.8.0-202106291913.p0.git.c358e96.assembly.stream | RHSA-2021:2438 |
| Red Hat OpenShift Container Platform 4.9 | openshift4/ose-prometheus:v4.9.0-202109302016.p0.git.3197fa7.assembly.stream | RHSA-2021:3759 |
Frequently Asked Questions
Is CVE-2021-26539 in your dependencies?
O3 Security finds CVE-2021-26539 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.