CVE-2021-21660 is a medium-severity (CVSS 5.4) Cross-site Scripting (XSS) vulnerability in io.jenkins.plugins:markdown-formatter. A fix is available for io.jenkins.plugins:markdown-formatter — see the affected versions and patch details below.
XSS vulnerability in Jenkins Markdown Formatter Plugin
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-2021-21660 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 377,333 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
io.jenkins.plugins:markdown-formatterReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
Description
Jenkins Markdown Formatter Plugin 0.1.0 and earlier uses a Markdown library to parse Markdown that does not escape crafted link target URLs.
This results in a stored cross-site scripting (XSS) vulnerability exploitable by attackers with the ability to edit any description rendered using the configured markup formatter.
Jenkins Markdown Formatter Plugin 0.2.0 uses a different Markdown library that is not affected by this problem.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | io.jenkins.plugins:markdown-formatter | all versions | 0.2.0io.jenkins.plugins:markdown-formatter:0.2.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for io.jenkins.plugins:markdown-formatter, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update io.jenkins.plugins:markdown-formatter to 0.2.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21660 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-2021-21660 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-21660. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2021-21660 in your dependencies?
O3 Security finds CVE-2021-21660 across Maven dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.