CVE-2021-20664Network attack vector

Browser script execution via Asset registration screen

Published Mar 5, 2021 · Updated Aug 3, 2024

Cross-site scripting in Movable Type's Asset registration screen allows remote attackers to execute script in a logged-in user's browser. Published vendor and JVN material identifies the affected screen but does not disclose which asset field accepts the payload or how its output reaches the browser without neutralization. Exploitation requires user interaction with the affected administrative content, and the resulting script runs with the access available to that browser session.

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 Movable Type's Asset registration screen allows remote attackers to execute script in a logged-in user's browser. Published vendor and JVN material identifies the affected screen but does not disclose which asset field accepts the payload or how its output reaches the browser without neutralization. Exploitation requires user interaction with the affected administrative content, and the resulting script runs with the access available to that browser session.

The record

CVE
CVE-2021-20664
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

Compare your product and version with the public record. A matching version still requires validation against your environment.

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 6.7.5 and earlier (Movable Type 6.7 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 6.7.5 and earlier (Movable Type 6.7 Series), Movable Type Premium 1.39 and earlier, and Movable Type Premium Advanced 1.39 and earlier

Published CVSS scores

2.6JPCERT/CCAV:N/AC:H/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
4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:P/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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 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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