CVE-2021-26540

Name: CVE-2021-26540
Creator: NVD / NIST
Published: 2021-02-08T17:15:13.737+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 5.3/10EPSS 1.75%

Last modified Jun 17, 2026

CVE-2021-26540 is a medium-severity vulnerability rated 5.3/10 on the CVSS scale. Apostrophe Technologies sanitize-html before 2.3.2 does not properly validate the hostnames set by the "allowedIframeHostnames" option when the "allowIframeRelativeUrls" is set to true, which allows attackers to bypass hostname whitelist for iframe element, related using an src value that starts with "/\\example.com".. EPSS estimates a 1.75% chance of exploitation in the next 30 days.

Description

Apostrophe Technologies sanitize-html before 2.3.2 does not properly validate the hostnames set by the "allowedIframeHostnames" option when the "allowIframeRelativeUrls" is set to true, which allows attackers to bypass hostname whitelist for iframe element, related using an src value that starts with "/\\example.com".

Metrics

CVSS 3.1

5.3/10

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

EPSS Probability

1.75%

75.1th percentile

Probability of exploitation in the next 30 days. Learn more

Affected Software

Vendor	Product	Versions
Apostrophecms	Sanitize-Html	< 2.3.2

References

https://advisory.checkmarx.net/advisory/CX-2021-4309Exploit, Patch, Third Party Advisory
https://github.com/apostrophecms/sanitize-html/blob/main/CHANGELOG.md#232-2021-01-26Release Notes, Third Party Advisory
https://github.com/apostrophecms/sanitize-html/pull/460Patch, Third Party Advisory
https://advisory.checkmarx.net/advisory/CX-2021-4309Exploit, Patch, Third Party Advisory
https://github.com/apostrophecms/sanitize-html/blob/main/CHANGELOG.md#232-2021-01-26Release Notes, Third Party Advisory
https://github.com/apostrophecms/sanitize-html/pull/460Patch, Third Party Advisory

Timeline

Published: Feb 8, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-26540?

How severe is CVE-2021-26540?

CVE-2021-26540 has a CVSS score of 5.3/10 (MEDIUM severity). The EPSS model estimates a 1.75% probability of exploitation in the next 30 days.

How do I fix CVE-2021-26540?

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-2021-26540?

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

Scan your code now

Source: NVD / NIST