CVE-2026-50873Public exploitNetwork attack vector

Stored XSS via active attachment upload

Published Jun 15, 2026 · Updated Jun 15, 2026

Stored XSS in flatnotes 5.5.4 allows remote authenticated users to execute same-origin script via an HTML or SVG attachment. The filesystem attachment backend validates filename shape but accepts active extensions, and FileResponse serves the stored file as browser-executable content under the application's origin. An account permitted to upload attachments and a viewing user opening the crafted file are required; the script acts with the viewer's flatnotes session privileges.

CVSS severity9.8
Critical
EPSS probability0.44%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stored XSS in flatnotes 5.5.4 allows remote authenticated users to execute same-origin script via an HTML or SVG attachment. The filesystem attachment backend validates filename shape but accepts active extensions, and FileResponse serves the stored file as browser-executable content under the application's origin. An account permitted to upload attachments and a viewing user opening the crafted file are required; the script acts with the viewer's flatnotes session privileges.

The record

CVE
CVE-2026-50873
Published
Jun 15, 2026
Updated
Jun 15, 2026
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-434, T1203
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jun 15, 2026CVE publishedPublication date reported by the CVE source.
  2. Jun 15, 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?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

9.8CISA-ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

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
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-434Unrestricted Upload of File with Dangerous Type
T1203Exploitation for Client Execution

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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