CVE-2026-39315Public exploitNetwork attack vector

Cross-site scripting via padded entity sanitizer bypass

Published Apr 9, 2026 · Updated Apr 9, 2026

Cross-site scripting in UnJS Unhead before 2.1.13 allows remote attackers to execute JavaScript through crafted head-tag URLs. The hasDangerousProtocol() decoder skips numeric HTML entities whose leading-zero padding exceeds its digit caps, and makeTagSafe() emits the raw value into SSR output. Exploitation requires an application to pass attacker-controlled content to useHeadSafe() and downstream code to load the decoded URL in a browsing context.

CVSS severity6.1
Medium
EPSS probability0.29%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Cross-site scripting in UnJS Unhead before 2.1.13 allows remote attackers to execute JavaScript through crafted head-tag URLs. The hasDangerousProtocol() decoder skips numeric HTML entities whose leading-zero padding exceeds its digit caps, and makeTagSafe() emits the raw value into SSR output. Exploitation requires an application to pass attacker-controlled content to useHeadSafe() and downstream code to load the decoded URL in a browsing context.

The record

CVE
CVE-2026-39315
Published
Apr 9, 2026
Updated
Apr 9, 2026
Vendor
UnJS
Product
Unhead
Classifications
CWE-184, T1059.007
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 9, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 9, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 2.1.13

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

UnJS · Unheadversion=< 2.1.13

Published CVSS scores

6.1GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-184Incomplete List of Disallowed Inputs
T1059.007JavaScript/JScript

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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