CVE-2021-20812Network attack vector

Script execution via Server Sync settings XSS

Published Aug 26, 2021 · Updated Aug 3, 2024

Cross-site scripting in Movable Type Advanced 7 r.4903 and earlier allows remote attackers to execute script in a logged-in user's browser. The flaw is in the Server Sync settings screen, but Six Apart and JVN have not disclosed which input is mishandled or where output escaping fails. Movable Type Premium Advanced 1.44 and earlier is also affected, and exploitation requires a logged-in user to view crafted content; the reported impact is browser-side script execution.

CVSS severity6.1
Medium
EPSS probability0.90%
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 Movable Type Advanced 7 r.4903 and earlier allows remote attackers to execute script in a logged-in user's browser. The flaw is in the Server Sync settings screen, but Six Apart and JVN have not disclosed which input is mishandled or where output escaping fails. Movable Type Premium Advanced 1.44 and earlier is also affected, and exploitation requires a logged-in user to view crafted content; the reported impact is browser-side script execution.

The record

CVE
CVE-2021-20812
Published
Aug 26, 2021
Updated
Aug 3, 2024
Vendor
Six Apart Ltd.
Product
Movable Type
Classifications
CWE-79, T1189
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 26, 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 Advanced 7 r.4903 and earlier (Movable Type Advanced 7 Series) and Movable Type Premium Advanced 1.44 and earlier

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Six Apart Ltd. · Movable Typeversion=Movable Type Advanced 7 r.4903 and earlier (Movable Type Advanced 7 Series) and Movable Type Premium Advanced 1.44 and earlier

Published CVSS scores

6.1JPCERT/CCCVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
2.6JPCERT/CCAV: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')
T1189Drive-by Compromise

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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