CVE-2021-39221Network attack vector

Stored XSS via unsanitized SVG contact photo

Published Oct 25, 2021 · Updated Aug 4, 2024

Stored XSS in Nextcloud Contacts before 4.0.3 allows remote authenticated users to execute script through a malicious SVG contact photo. The contact-photo rendering path created an SVG blob URL from uploaded avatar data without first sanitizing the SVG content. Execution requires a victim to right-click the malicious file and open it in a new tab, and strict CSP prevents exploitation in modern supporting browsers.

CVSS severity6.4
Medium
EPSS probability0.52%
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 Contacts before 4.0.3 allows remote authenticated users to execute script through a malicious SVG contact photo. The contact-photo rendering path created an SVG blob URL from uploaded avatar data without first sanitizing the SVG content. Execution requires a victim to right-click the malicious file and open it in a new tab, and strict CSP prevents exploitation in modern supporting browsers.

The record

CVE
CVE-2021-39221
Published
Oct 25, 2021
Updated
Aug 4, 2024
Vendor
Nextcloud GmbH
Product
Nextcloud Security Advisories
Classifications
CWE-79, CWE-434, T1203
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Oct 25, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 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=< 4.0.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Nextcloud GmbH · Nextcloud Security Advisoriesversion=< 4.0.3

Published CVSS scores

5.4NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
3.5NIST NVDAV:N/AC:M/Au:S/C:N/I:P/A:N
6.4GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/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')
CWE-434Unrestricted Upload of File with Dangerous Type
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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