CVE-2021-20665Network attack vector

Arbitrary script execution in logged-in browsers

Published Mar 5, 2021 · Updated Aug 3, 2024

Cross-site scripting in Six Apart Movable Type 7 r.4705 and earlier allows remote attackers to execute script in a logged-in user's browser. Six Apart and JVN identify the Add Asset screen for a Contents field but do not disclose the input parameter or faulty rendering operation. The same issue affects Advanced r.4705 and earlier and Premium 1.39 and earlier; exploitation requires a logged-in user to open the affected screen.

CVSS severity6.1
Medium
EPSS probability0.81%
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 Six Apart Movable Type 7 r.4705 and earlier allows remote attackers to execute script in a logged-in user's browser. Six Apart and JVN identify the Add Asset screen for a Contents field but do not disclose the input parameter or faulty rendering operation. The same issue affects Advanced r.4705 and earlier and Premium 1.39 and earlier; exploitation requires a logged-in user to open the affected screen.

The record

CVE
CVE-2021-20665
Published
Mar 5, 2021
Updated
Aug 3, 2024
Vendor
Six Apart Ltd.
Product
Movable Type
Classifications
CWE-79, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 5, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 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=Movable Type 7 r.4705 and earlier (Movable Type 7 Series), Movable Type Advanced 7 r.4705 and earlier (Movable Type Advanced 7 Series), Movable Type Premium 1.39 and earlier, and Movable Type Premium Advanced 1.39 and earlier

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Six Apart Ltd. · Movable Typeversion=Movable Type 7 r.4705 and earlier (Movable Type 7 Series), Movable Type Advanced 7 r.4705 and earlier (Movable Type Advanced 7 Series), Movable Type Premium 1.39 and earlier, and Movable Type Premium Advanced 1.39 and earlier

Published CVSS scores

6.1JPCERT/CC and IPACVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
2.6JPCERT/CC and IPAAV:N/AC:H/Au:N/C:N/I:P/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
Privilegesunauthenticated
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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