CVE-2023-39955Network attack vector

Stored XSS via inline HTML attachment rendering

Published Aug 10, 2023 · Updated Oct 4, 2024

Stored XSS in Nextcloud Notes 4.4.0 through 4.8.0-beta.2 allows remote authenticated users to render HTML via a note attachment. The attachment endpoint returned uploaded HTML without a Content-Disposition attachment header, causing browsers to render it inline in preview mode. Exploitation requires Notes access and user interaction with the crafted attachment; the published impact is limited to unauthorized content rendering.

CVSS severity3.5
Low
EPSS probability0.59%
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 Nextcloud Notes 4.4.0 through 4.8.0-beta.2 allows remote authenticated users to render HTML via a note attachment. The attachment endpoint returned uploaded HTML without a Content-Disposition attachment header, causing browsers to render it inline in preview mode. Exploitation requires Notes access and user interaction with the crafted attachment; the published impact is limited to unauthorized content rendering.

The record

CVE
CVE-2023-39955
Published
Aug 10, 2023
Updated
Oct 4, 2024
Vendor
Nextcloud GmbH
Product
Security Advisories
Classifications
CWE-79, T1203
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Aug 10, 2023CVE publishedPublication date reported by the CVE source.
  2. Oct 4, 2024Record 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=>= 4.4.0, < 4.8.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Nextcloud GmbH · Security Advisoriesversion=>= 4.4.0, < 4.8.0

Published CVSS scores

3.5GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:L/A:N
6.1NIST NVDCVSS: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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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 exploitUnconfirmed

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1203Exploitation for Client Execution

Public exploit references

No public exploit references are available in this record.

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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