CVE-2021-20815

Name: CVE-2021-20815
Creator: NVD / NIST
Published: 2021-08-26T02:15:11.827+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.1/10EPSS 0.90%

Last modified Jun 17, 2026

CVE-2021-20815 is a medium-severity vulnerability rated 6.1/10 on the CVSS scale. Cross-site scripting vulnerability in Edit Boilerplate screen of Movable Type (Movable Type 7 r.4903 and earlier (Movable Type 7 Series), Movable Type 6.8.0 and earlier (Movable Type 6 Series), Movable Type Advanced 7 r.4903 and earlier (Movable Type Advanced 7 Series), Movable Type Premium 1.44 and earlier, and Movable Type Premium Advanced 1.44 and earlier) allows remote attackers to inject arbitrary script or HTML via unspecified vectors.. EPSS estimates a 0.90% chance of exploitation in the next 30 days.

Description

Cross-site scripting vulnerability in Edit Boilerplate screen of Movable Type (Movable Type 7 r.4903 and earlier (Movable Type 7 Series), Movable Type 6.8.0 and earlier (Movable Type 6 Series), Movable Type Advanced 7 r.4903 and earlier (Movable Type Advanced 7 Series), Movable Type Premium 1.44 and earlier, and Movable Type Premium Advanced 1.44 and earlier) allows remote attackers to inject arbitrary script or HTML via unspecified vectors.

Metrics

CVSS 3.1

6.1/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

EPSS Probability

0.90%

55.1th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-79

Affected Software

Vendor	Product	Versions
Sixapart	Movable Type	<= 1.44
Sixapart	Movable Type	>= 6.0, <= 6.8.0
Sixapart	Movable Type	>= 7.0, < 7.8.0

References

https://jvn.jp/en/jp/JVN97545738/index.htmlThird Party Advisory
https://movabletype.org/news/2021/08/mt-780-681-released.htmlRelease Notes, Vendor Advisory
https://jvn.jp/en/jp/JVN97545738/index.htmlThird Party Advisory
https://movabletype.org/news/2021/08/mt-780-681-released.htmlRelease Notes, Vendor Advisory

Timeline

Published: Aug 26, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-20815?

How severe is CVE-2021-20815?

CVE-2021-20815 has a CVSS score of 6.1/10 (MEDIUM severity). The EPSS model estimates a 0.90% probability of exploitation in the next 30 days.

How do I fix CVE-2021-20815?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-20815?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST