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CVE Record

CVE-2020-26237: Prototype Pollution in highlight.js

Highlight.js is a syntax highlighter written in JavaScript. Highlight.js versions before 9.18.2 and 10.1.2 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. 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.

MediumCVSS 5.8Not KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

This vulnerability affects highlight.js when sites render user-submitted code blocks and allow uncontrolled language names. A crafted block can pollute JavaScript object prototypes during highlighting, causing unexpected behavior or crashes. Systems that do not render user-provided content are described as unaffected.

Executive priority

Treat as a targeted remediation for user-content rendering surfaces, not a broad emergency. Prioritize public-facing or multi-user applications where untrusted users can submit code blocks.

Technical view

CVE-2020-26237 is CWE-471 prototype pollution in highlight.js before 9.18.2 and in 10.0.0 before 10.1.2. The condition involves malicious HTML code blocks processed through highlighting, commonly via Markdown-like input, when language names are not filtered.

Likely exposure

Exposure is most likely in web applications, documentation portals, forums, CMS features, or internal tools that render user-supplied Markdown or HTML code blocks with affected highlight.js versions.

Exploitation context

The provided sources do not cite active exploitation, and KEV is false. Exploitation requires user-controlled content, a vulnerable highlight.js version, and a rendering path that processes unfiltered language names during highlighting.

Researcher notes

Impact evidence points mainly to unsafe prototype mutation causing application instability or crashes. The advisory notes pollution should be harmless data, but applications relying on clean object prototypes may fail unpredictably.

Mitigation direction

  • Upgrade highlight.js to 9.18.2, 10.1.2, or newer.
  • If using version 7 or 8, move to a newer supported release.
  • Filter or constrain user-provided language names before highlighting.
  • Review vendor or distribution advisories for packaged highlight.js updates.

Validation and detection

  • Check package manifests and lockfiles for affected highlight.js versions.
  • Identify pages that render user-submitted Markdown or HTML code blocks.
  • Verify user-controlled language names are filtered or allowlisted.
  • Confirm deployed assets use the upgraded highlight.js build.
Prepared
Confidence
high
Sources
5

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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ATT&CK lookup starting points

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cwe · low confidence lookup

CWE-471: Exact CWE lookup

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cve · low confidence lookup

CVE-2020-26237 mapping review

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Vulnerability profileCVE Program record
Severity
Medium
CVSS
5.8 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:C/C:N/I:H/A:N

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

1CVSS vectors
0Timeline events
0ADP providers
7Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
5.8CVSS 3.1MediumCVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:C/C:N/I:H/A:N1.34Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

5.8Medium
CVSS 3.1 vector shape for CVE-2020-26237Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:C/C:N/I:H/A:N

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
highlightjshighlight.js< 9.18.2, >= 10.0.0, < 10.1.2Listed
Weakness

CWE details

CWE-471

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.