How severe is CVE-2020-8264?

CVE-2020-8264 has a CVSS score of 6.1/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2020-8264?

CVE-2020-8264 affects the following packages: actionpack (RubyGems). Ecosystems affected: RubyGems.

How do I fix CVE-2020-8264?

Update actionpack to 6.0.3.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-8264 is resolved across your whole dependency graph.

How do I detect CVE-2020-8264 in my RubyGems dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for actionpack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2020-8264 if there is no patch (or I can't update yet)?

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.

Is CVE-2020-8264 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2020-8264, including 1 in-the-wild exploitation observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2020-8264?

CVE-2020-8264 has an EPSS (Exploit Prediction Scoring System) score of 67.0%, placing it in the 99th 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. A score above 50% indicates significantly elevated exploitation risk — immediate patching is strongly recommended.

What type of vulnerability is CVE-2020-8264?

CVE-2020-8264 is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2020-8264 published, and has it been updated?

CVE-2020-8264 was published on January 6, 2021 and was last updated on September 9, 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-2020-8264 — actionpack

MEDIUM

CVE-2020-8264 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in actionpack. 1 public exploit reference exists, so weaponization risk is real. A fix is available for actionpack — see the affected versions and patch details below.

Cross-site scripting in actionpack

Also known asGHSA-35mm-cc6r-8fjp

Published

Jan 6, 2021

Updated

Sep 9, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

1 known

Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

67.0%probability of exploitation in next 30 days

High Risk0.00%

Lower risk than most CVEs99th percentile — riskier than 99% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2020-8264 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,386 CVEs with a current EPSS score, this one falls in the 50–90% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

💎actionpack

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

Workarounds

Until such time as the patch can be applied, application developers should disable the Actionable Exceptions middleware in their development environment via a line such as this one in their config/environment/development.rb: config.middleware.delete ActionDispatch::ActionableExceptions

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
💎RubyGems	`actionpack`	≥ 6.0.0&&< 6.0.3.4	6.0.3.4`bundle update actionpack --conservative`

Affected Products

1 product · 1 configurations

Application

railsrubyonrails

≥ 6.0.0 && < 6.0.3.4

range

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

In actionpack gem >= 6.0.0, a possible XSS vulnerability exists when an …

hackerone.comNVDJan 2021

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for actionpack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update actionpack to 6.0.3.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-8264 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.

Frequently Asked Questions

In actionpack gem >= 6.0.0, a possible XSS vulnerability exists when an application is running in development mode allowing an attacker to send or embed (in another page) a specially crafted URL which can allow the attacker to execute JavaScript in the context of the local application. This vulnerability is in the Actionable Exceptions middleware. Workarounds ----------- Until such time as the patch can be applied, application developers should disable the Actionable Exceptions middleware in their development environment via a line such as this one in their config/environment/development.rb: