CVE-2020-26237

Prototype Pollution in highlight.js

Workarounds

Patch your library

Manually patch your library to create null objects for both languages and aliases:

const HLJS = function(hljs) {
  // ...
  var languages = Object.create(null);
  var aliases = Object.create(null);

Filter out bad data from end users:

Filter the language names that users are allowed to inject into your HTML to guarantee they are valid.

References

• What is Prototype Pollution?
• https://github.com/highlightjs/highlight.js/pull/2636

For more information

If you have any questions or comments about this advisory:

• Please file an issue against highlight.js

Affected versions of this package are vulnerable to Prototype Pollution. A malicious HTML code block can be crafted that will result in prototype pollution of the base object's prototype during highlighting. If you allow users to insert custom HTML code blocks into your page/app via parsing Markdown code blocks (or similar) and do not filter the language names the user can provide you may be vulnerable.

The pollution should just be harmless data but this can cause problems for applications not expecting these properties to exist and can result in strange behavior or application crashes, i.e. a potential DOS vector.

If your website or application does not render user provided data it should be unaffected.

CVE-2020-26237 has a CVSS score of 5.8 (Medium). The vector is network-reachable, low privileges required, and user interaction required. 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 (9.18.2, 10.1.2); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

Versions 9.18.2 and 10.1.2 and newer include fixes for this vulnerability. If you are using version 7 or 8 you are encouraged to upgrade to a newer release.

1. What is CVE-2020-26237? CVE-2020-26237 is a medium-severity security vulnerability in highlight.js (npm), affecting versions < 9.18.2. It is fixed in 9.18.2, 10.1.2.
2. How severe is CVE-2020-26237? CVE-2020-26237 has a CVSS score of 5.8 (Medium). 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.
3. Which versions of highlight.js are affected by CVE-2020-26237? highlight.js (npm) versions < 9.18.2 is affected.
4. Is there a fix for CVE-2020-26237? Yes. CVE-2020-26237 is fixed in 9.18.2, 10.1.2. Upgrade to this version or later.
5. Is CVE-2020-26237 exploitable, and should I be worried? Whether CVE-2020-26237 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
6. What actually determines whether CVE-2020-26237 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.
7. How do I fix CVE-2020-26237?
   • Upgrade highlight.js to 9.18.2 or later
   • Upgrade highlight.js to 10.1.2 or later

Other vulnerabilities in highlight.js