Summary
Cross-site Scripting in Sanitize
When HTML is sanitized using Sanitize's "relaxed" config or a custom config that allows certain elements, some content in a <math> or <svg> element may not be sanitized correctly even if math and svg are not in the allowlist.
You are likely to be vulnerable to this issue if you use Sanitize's relaxed config or a custom config that allows one or more of the following HTML elements:
iframemathnoembednoframesnoscriptplaintextscriptstylesvgxmp
Releases
This problem has been fixed in Sanitize 5.2.1.
Workarounds
If upgrading is not possible, a workaround is to override the default value of Sanitize's :remove_contents config option with the following value, which ensures that the contents of math and svg elements (among others) are removed entirely when those elements are not in the allowlist:
%w[iframe math noembed noframes noscript plaintext script style svg xmp]
For example, if you currently use Sanitize's relaxed config, you can create a custom config object that overrides the default value of :remove_contents like this:
custom_config = Sanitize::Config.merge(
Sanitize::Config::RELAXED,
:remove_contents => %w[iframe math noembed noframes noscript plaintext script style svg xmp]
)
You would then pass this custom config to Sanitize when sanitizing HTML.
For more information
If you have any questions or comments about this advisory:
- Open an issue in the Sanitize repo.
- See Sanitize's security policy.
Credits
Many thanks to Michal Bentkowski of Securitum for reporting this bug and helping to verify the fix.
References
Impact
Using carefully crafted input, an attacker may be able to sneak arbitrary HTML through Sanitize, potentially resulting in XSS (cross-site scripting) or other undesired behavior when that HTML is rendered in a browser.
Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.
CVE-2020-4054 has a CVSS score of 7.3 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (5.2.1); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2020-4054? CVE-2020-4054 is a high-severity cross-site scripting (XSS) vulnerability in sanitize (rubygems), affecting versions >= 3.0.0, < 5.2.1. It is fixed in 5.2.1. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- How severe is CVE-2020-4054? CVE-2020-4054 has a CVSS score of 7.3 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of sanitize are affected by CVE-2020-4054? sanitize (rubygems) versions >= 3.0.0, < 5.2.1 is affected.
- Is there a fix for CVE-2020-4054? Yes. CVE-2020-4054 is fixed in 5.2.1. Upgrade to this version or later.
- Is CVE-2020-4054 exploitable, and should I be worried? Whether CVE-2020-4054 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2020-4054 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2020-4054? Upgrade
sanitizeto 5.2.1 or later.