CVE-2020-25739 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in gon. A fix is available for gon — see the affected versions and patch details below.
Gon gem lack of escaping certain input when outputting as JSON
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2020-25739 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
gonReal-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
An issue was discovered in the gon gem before gon-6.4.0 for Ruby. MultiJson does not honor the escape_mode parameter to escape fields as an XSS protection mechanism. To mitigate, json_dumper.rb in gon now does escaping for XSS by default without relying on MultiJson.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | gon | all versions | 6.4.0bundle update gon --conservative |
Affected Products
ubuntu linuxcanonicaldebian linuxdebiangongon_projectDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for gon, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update gon to 6.4.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-25739 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.
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.
Frequently Asked Questions
Is CVE-2020-25739 in your dependencies?
Find it across RubyGems, including transitive dependencies.