What is CVE-2021-35955?

CVE-2021-35955 is a medium-severity vulnerability in Contao, classified under Cross-site Scripting. CVSS score: 4.8/10. Published 2021-08-12.

How severe is CVE-2021-35955?

Medium severity. CVSS v3 base score is 4.8 out of 10.

XSS in Contao

CVE-2021-35955

Contao >=4.0.0 allows backend XSS via HTML attributes to an HTML field. Fixed in 4.4.56, 4.9.18, 4.11.7.

Vulnerability class: XSS (Cross-Site Scripting)

Published 2021-08-12 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 4.8 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.006 (44.3th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 44th percentile — 44.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.006 · newest: 0.006 · change: 0.000

Affected products

Contao

Weakness classification (CWE)

CWE-79 — Cross-site Scripting

References

contao.org/en/news/contao-4-9-16-and-4-11-5-are-available.html (Vendor Advisory)
github.com/contao/contao/security/advisories/GHSA-hr3h-x6gq-rqcp (Third Party Advisory)

Frequently asked questions

What is CVE-2021-35955?: CVE-2021-35955 is a medium-severity vulnerability in Contao, classified under Cross-site Scripting. CVSS score: 4.8/10. Published 2021-08-12.
How severe is CVE-2021-35955?: Medium severity. CVSS v3 base score is 4.8 out of 10.