What is CVE-2020-26870?

CVE-2020-26870 is a medium-severity vulnerability in Cure53 Dompurify, classified under Cross-site Scripting. CVSS score: 6.1/10. Published 2020-10-07.

How severe is CVE-2020-26870?

Medium severity. CVSS v3 base score is 6.1 out of 10.

Is CVE-2020-26870 known to be exploited?

3 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

XSS in Cure53 Dompurify

CVE-2020-26870

Cure53 DOMPurify before 2.0.17 allows mutation XSS. This occurs because a serialize-parse roundtrip does not necessarily return the original DOM tree, and a namespace can change from HTML to MathML, as demonstrated by nesting of FORM elements.

Vulnerability class: XSS (Cross-Site Scripting)

Published 2020-10-07 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 6.1 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.049 (92.0th percentile), scored 2026-10-22.

Low probability of exploitation in the next 30 days, but still tracked. 92nd percentile — 92.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.049 · newest: 0.049 · change: 0.000

Affected products

Cure53 Dompurify
Microsoft Visual Studio 2017 — versions 15.9
Microsoft Visual Studio 2019 — versions 16.0, 16.4, 16.7
Oracle Application Express
Debian Linux — versions 9.0

Weakness classification (CWE)

CWE-79 — Cross-site Scripting

Public proof-of-concept exploits

References

research.securitum.com/mutation-xss-via-mathml-mutation-dompurify-2-0-17-bypass/ (Exploit, Third Party Advisory)
github.com/cure53/DOMPurify/commit/02724b8eb048dd219d6725b05c3000936f11d62d (Patch, Third Party Advisory)
github.com/cure53/DOMPurify/compare/2.0.16...2.0.17 (Patch, Third Party Advisory)
lists.debian.org/debian-lts-announce/2020/10/msg00029.html (mailing-list, Mailing List, Third Party Advisory)
portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2020-26870 (vendor-advisory, Patch, Vendor Advisory)
www.oracle.com//security-alerts/cpujul2021.html (Patch, Third Party Advisory)

Frequently asked questions

What is CVE-2020-26870?: CVE-2020-26870 is a medium-severity vulnerability in Cure53 Dompurify, classified under Cross-site Scripting. CVSS score: 6.1/10. Published 2020-10-07.
How severe is CVE-2020-26870?: Medium severity. CVSS v3 base score is 6.1 out of 10.
Is CVE-2020-26870 known to be exploited?: 3 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.