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Vulnerabilities/CVE-2026-5388Public Intelligence Dossier
Source: Source type: NVD · Limited verification•Last verified: •Collection Degraded
CVE-2026-5388Unknown / EvaluatingCRITICAL (raw)CVSS 9.8CWE-20

justhtml before 1.15.0 contains multiple security issues in URL sanitization helpers (clean_url_value/clean_url_in_js_string), HTML serialization, Markdown passthrough (html_passthrough=True), and several custom sanitization-policy edge cases

Published: 23 Aug 2026•Last updated: 12 Oct 2026, 09:40 PM IST•Source identifier: disclosure@vulncheck.com
Dossier completeness: source-provided sections 1 / 3 · rule guidance available. Missing source facts count as incomplete.
Official source
CISA KEV catalog
Unknown
KEV status was not determined.
CVSS score
9.8CRITICAL (raw)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Metric sourcedisclosure@vulncheck.com
EPSS probability
0.6%0.006
Percentile47th
Weakness type
CWE-20
No further CWE recorded
Remediation
Patch status unknown
No fixed version or KB recorded

Why It Matters

Analysis dossier

Why It Matters

justhtml before 1.15.0 contains multiple security issues in URL sanitization helpers (clean_url_value/clean_url_in_js_string), HTML serialization, Markdown passthrough (html_passthrough=True), and several custom sanitization-policy edge cases. Depending on configuration, an attacker can bypass sanitization to inject active HTML and JavaScript — for example via encoded javascript: URLs, backslash-based relative URLs resolved as remote hosts, markup-breaking programmatic element/attribute names or HTML comments, raw reintroduction through Markdown passthrough, or preserved / / tags in custom policies. Most custom-policy issues do not affect the default sanitize=True configuration; they primarily affect helper APIs, programmatic DOM construction, html_passthrough=True, and custom policies/transform pipelines.

SOC priority

High Severity / Review

Actively exploited

Unknown

Publicly disclosed

Unknown

Risk basis: Microsoft or Microsoft-managed product relevance; CRITICAL severity; Critical CVSS without confirmed KEV exploitation

Affected Products & Remediation

0 source-backed products
Affected products and versions (source)Not provided by source
Recorded patch / KBNot provided by source

Vendor or catalog guidance (source)

Not provided by source

Known exploitation (source facts)

Only what a source states. Unknown means no source has said either way; it is not "not exploited".

CISA KEV
Unknown
Exploited (source statement)
Unknown
Publicly disclosed (source)
Unknown

Suggested hunting (rule-based inference)

SecOps Signal recommendation (rule-based inference). These steps are hunting hypotheses from the vulnerability class; they do not state or imply that exploitation occurred here or anywhere.

Matched network-unauthenticated (CVSS AV:N, CVSS PR:N, CVSS UI:N)

Exposure

  1. Identify devices reporting this CVE and note which expose the affected service to untrusted networks.Telemetry: DeviceTvmSoftwareVulnerabilities · A network-reachable, no-privilege vector makes internet-facing instances the first exposure question. · Rule network-unauthenticated

Exploitation evidence

  1. Review inbound connections to the affected service from unexpected sources since the advisory date.Telemetry: DeviceNetworkEvents · Hunting hypothesis from the CVSS vector; it does not show that exploitation occurred. · Rule network-unauthenticated

Response

  1. Confirm the source-stated update is installed and the CVE is no longer reported.Telemetry: DeviceTvmSoftwareVulnerabilities · A follow-up inventory check verifies remediation. · Rule network-unauthenticated

CISA KEV Record

CISA KEV status was not determined for this record.

Change History & Source Data

— No recorded material change

CNA / source
disclosure@vulncheck.com
Record status · verification
UPDATED · NVD API
CPE
Unknown

Evidence & Provenance

3 public sources
nvd.nist.govSource recordgithub.comSource recordvulncheck.comSource record
Detection Hunting Query

— No published hunting query names this record. Browse community and library queries instead.

Open KQL Library

Correlated briefings

— No published briefing references this CVE

SecOps Signal surfaces public advisory facts only. A missing value means the source has not published it, not that your environment is unaffected. No tenant inventory is connected.