CVE-2021-20813Network attack vector

Script execution via Content Data edit screen

Published Aug 26, 2021 · Updated Aug 3, 2024

Cross-site scripting in Movable Type and Movable Type Advanced 7 r.4903 and earlier allows remote attackers to run script in a user's browser. The flaw is in the Content Data edit screen, but Six Apart has not disclosed which field or output path fails to neutralize attacker-controlled markup. Exploitation requires a logged-in user to load the affected screen; injected HTML or script then runs in that user's site context.

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 and Movable Type Advanced 7 r.4903 and earlier allows remote attackers to run script in a user's browser. The flaw is in the Content Data edit screen, but Six Apart has not disclosed which field or output path fails to neutralize attacker-controlled markup. Exploitation requires a logged-in user to load the affected screen; injected HTML or script then runs in that user's site context.

The record

CVE
CVE-2021-20813
Published
Aug 26, 2021
Updated
Aug 3, 2024
Vendor
Six Apart Ltd.
Product
Movable Type
Classifications
CWE-79, T1059.007
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 7 r.4903 and earlier (Movable Type 7 Series) and Movable Type Advanced 7 r.4903 and earlier (Movable Type Advanced 7 Series)

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

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

Published CVSS scores

2.6JPCERT/CCAV:N/AC:H/Au:N/C:N/I:P/A:N
4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:P/A:N
6.1JPCERT/CCCVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:L/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
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1059.007JavaScript/JScript

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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