{"id":"CVE-2021-35955","aliases":["EUVD-2021-1756","GHSA-hr3h-x6gq-rqcp"],"status":"published","title":"Cross site scripting via HTML attributes in the back end","description":"Contao \u003e=4.0.0 allows backend XSS via HTML attributes to an HTML field. Fixed in 4.4.56, 4.9.18, 4.11.7.","severity":{"version":"3.1","score":4.8,"vector":"CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N","level":"medium","assessor":"primary","from":"nvd"},"epss":{"day":"2026-11-10","score":0.00557,"percentile":0.46809,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-79"],"published":"2021-08-12T14:33:44Z","modified":"2026-10-20T10:54:46Z","fixes":[{"ecosystem":"Packagist","package":"contao/core-bundle","fixed_in":["4.4.56","4.9.18","4.11.7"],"sources":["ghsa","osv"]},{"ecosystem":"Packagist","package":"contao/contao","fixed_in":["4.4.56","4.9.18","4.11.7"],"sources":["ghsa","osv"]}],"affected":[{"source":"ghsa","ecosystem":"Packagist","package":"contao/core-bundle","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.0.0"},{"fixed":"4.4.56"}]}]},{"source":"ghsa","ecosystem":"Packagist","package":"contao/contao","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.0.0"},{"fixed":"4.4.56"}]}]},{"source":"ghsa","ecosystem":"Packagist","package":"contao/contao","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.5.0"},{"fixed":"4.9.18"}]}]},{"source":"ghsa","ecosystem":"Packagist","package":"contao/contao","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.10.0"},{"fixed":"4.11.7"}]}]},{"source":"ghsa","ecosystem":"Packagist","package":"contao/core-bundle","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.5.0"},{"fixed":"4.9.18"}]}]},{"source":"ghsa","ecosystem":"Packagist","package":"contao/core-bundle","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.10.0"},{"fixed":"4.11.7"}]}]},{"source":"osv","ecosystem":"Packagist","package":"contao/core-bundle","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.0.0"},{"fixed":"4.4.56"}]}],"versions":["4.0.0","4.0.1","4.0.2","4.0.3","4.0.4","4.1.0","4.1.0-RC1","4.1.0-beta1","4.1.1","4.1.2","4.1.3","4.2.0","4.2.0-RC1","4.2.0-beta1","4.2.1","4.2.2","4.2.3","4.2.4","4.2.5","4.3.0","4.3.0-RC1","4.3.1","4.3.10","4.3.11","4.3.2","4.3.3","4.3.4","4.3.5","4.3.6","4.3.7","4.3.8","4.3.9","4.4.0","4.4.0-RC1","4.4.0-RC2","4.4.0-beta1","4.4.1","4.4.10","4.4.11","4.4.12","4.4.13","4.4.14","4.4.15","4.4.16","4.4.17","4.4.18","4.4.19","4.4.2","4.4.20","4.4.21","4.4.22","4.4.23","4.4.24","4.4.25","4.4.26","4.4.27","4.4.28","4.4.29","4.4.3","4.4.30","4.4.31","4.4.32","4.4.33","4.4.34","4.4.35","4.4.36","4.4.37","4.4.38","4.4.39","4.4.4","4.4.40","4.4.41","4.4.42","4.4.43","4.4.44","4.4.45","4.4.46","4.4.47","4.4.48","4.4.49","4.4.5","4.4.50","4.4.51","4.4.52","4.4.53","4.4.54","4.4.55","4.4.6","4.4.7","4.4.8","4.4.9"]},{"source":"osv","ecosystem":"Packagist","package":"contao/contao","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.0.0"},{"fixed":"4.4.56"}]}],"versions":["4.4.22","4.4.23","4.4.24","4.4.25","4.4.26","4.4.27","4.4.28","4.4.29","4.4.30","4.4.31","4.4.32","4.4.33","4.4.34","4.4.35","4.4.36","4.4.37","4.4.38","4.4.39","4.4.40","4.4.41","4.4.42","4.4.43","4.4.44","4.4.45","4.4.46","4.4.47","4.4.48","4.4.49","4.4.50","4.4.51","4.4.52","4.4.53","4.4.54","4.4.55"]},{"source":"osv","ecosystem":"Packagist","package":"contao/contao","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.5.0"},{"fixed":"4.9.18"}]}],"versions":["4.5.13","4.5.14","4.6.0","4.6.1","4.6.10","4.6.11","4.6.12","4.6.13","4.6.14","4.6.2","4.6.3","4.6.4","4.6.5","4.6.6","4.6.7","4.6.8","4.6.9","4.7.0","4.7.0-RC1","4.7.0-RC2","4.7.0-RC3","4.7.0-RC4","4.7.1","4.7.2","4.7.3","4.7.4","4.7.5","4.7.6","4.7.7","4.8.0","4.8.0-RC1","4.8.0-RC2","4.8.1","4.8.2","4.8.3","4.8.4","4.8.5","4.8.6","4.8.7","4.8.8","4.9.0","4.9.0-RC1","4.9.0-RC2","4.9.1","4.9.10","4.9.11","4.9.12","4.9.13","4.9.14","4.9.15","4.9.16","4.9.17","4.9.2","4.9.3","4.9.4","4.9.5","4.9.6","4.9.7","4.9.8","4.9.9"]},{"source":"osv","ecosystem":"Packagist","package":"contao/contao","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.10.0"},{"fixed":"4.11.7"}]}],"versions":["4.10.0","4.10.1","4.10.2","4.10.3","4.10.4","4.10.5","4.10.6","4.10.7","4.11.0","4.11.0-RC1","4.11.0-RC2","4.11.1","4.11.2","4.11.3","4.11.4","4.11.5","4.11.6"]},{"source":"osv","ecosystem":"Packagist","package":"contao/core-bundle","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.5.0"},{"fixed":"4.9.18"}]}],"versions":["4.5.0","4.5.1","4.5.10","4.5.11","4.5.12","4.5.13","4.5.14","4.5.2","4.5.3","4.5.4","4.5.5","4.5.6","4.5.7","4.5.8","4.5.9","4.6.0","4.6.0-RC1","4.6.0-RC2","4.6.0-RC3","4.6.1","4.6.10","4.6.11","4.6.12","4.6.13","4.6.14","4.6.2","4.6.3","4.6.4","4.6.5","4.6.6","4.6.7","4.6.8","4.6.9","4.7.0","4.7.0-RC1","4.7.0-RC2","4.7.0-RC3","4.7.0-RC4","4.7.1","4.7.2","4.7.3","4.7.4","4.7.5","4.7.6","4.7.7","4.8.0","4.8.0-RC1","4.8.0-RC2","4.8.1","4.8.2","4.8.3","4.8.4","4.8.5","4.8.6","4.8.7","4.8.8","4.9.0","4.9.0-RC1","4.9.0-RC2","4.9.1","4.9.10","4.9.11","4.9.12","4.9.13","4.9.14","4.9.15","4.9.16","4.9.17","4.9.2","4.9.3","4.9.4","4.9.5","4.9.6","4.9.7","4.9.8","4.9.9"]},{"source":"osv","ecosystem":"Packagist","package":"contao/core-bundle","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.10.0"},{"fixed":"4.11.7"}]}],"versions":["4.10.0","4.10.1","4.10.2","4.10.3","4.10.4","4.10.5","4.10.6","4.10.7","4.11.0","4.11.0-RC1","4.11.0-RC2","4.11.1","4.11.2","4.11.3","4.11.4","4.11.5","4.11.6"]}],"products":[{"vendor":"contao","product":"contao"}],"references":[{"url":"https://contao.org/en/news/contao-4-9-16-and-4-11-5-are-available.html","tags":["x_refsource_MISC","x_transferred","Vendor Advisory","advisory","web"]},{"url":"https://github.com/contao/contao/security/advisories/GHSA-hr3h-x6gq-rqcp","tags":["x_refsource_MISC","x_transferred","Third Party Advisory","advisory","web"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2021-35955","tags":["advisory"]},{"url":"https://contao.org/en/security-advisories/cross-site-scripting-via-html-attributes-in-the-back-end.html","tags":["web"]},{"url":"https://github.com/FriendsOfPHP/security-advisories/blob/master/contao/contao/CVE-2021-35955.yaml","tags":["web"]},{"url":"https://github.com/FriendsOfPHP/security-advisories/blob/master/contao/core-bundle/CVE-2021-35955.yaml","tags":["web"]},{"url":"https://github.com/advisories/GHSA-hr3h-x6gq-rqcp"},{"url":"https://github.com/contao/contao","tags":["package"]}],"sources":{"cve":[{"id":"CVE-2021-35955","description":"Contao \u003e=4.0.0 allows backend XSS via HTML attributes to an HTML field. Fixed in 4.4.56, 4.9.18, 4.11.7.","status":"published","published":"2021-08-12T14:33:44Z","modified":"2024-08-04T00:47:42.583Z","updated_at":"2026-11-02T14:33:09.864034Z"}],"euvd":[{"id":"EUVD-2021-1756","description":"Contao \u003e=4.0.0 allows backend XSS via HTML attributes to an HTML field. Fixed in 4.4.56, 4.9.18, 4.11.7.","status":"published","published":"2021-08-12T14:33:44Z","modified":"2024-08-04T00:47:42Z","updated_at":"2026-11-03T03:24:30.441845Z"}],"ghsa":[{"id":"GHSA-hr3h-x6gq-rqcp","title":"Cross site scripting via HTML attributes in the back end","description":"### Impact\n\nIt is possible for untrusted users to inject malicious code into HTML attributes in the back end, which will be executed both in the element preview (back end) and on the website (front end).\n\nInstallations are only affected if there are untrusted back end users who have the rights to modify HTML fields (e.g. TinyMCE).\n\n### Patches\n\nUpdate to Contao 4.4.56, 4.9.18 or 4.11.7\n\n### Workarounds\n\nDisable all fields that allow HTML for untrusted back end users or disable the login for these users.\n\n### References\n\nhttps://contao.org/en/security-advisories/cross-site-scripting-via-html-attributes-in-the-back-end\n\n### For more information\n\nIf you have any questions or comments about this advisory, open an issue in [contao/contao](https://github.com/contao/contao/issues/new/choose).\n\n### Credits\n\nThanks to Mikhail Khramenkov and Moritz Vondano for reporting this security issue.","status":"published","published":"2021-08-25T14:45:01Z","modified":"2024-02-05T10:37:32Z","cvss":[{"version":"3.1","score":5.9,"vector":"CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:L","level":"medium"}],"cwes":["CWE-79"],"updated_at":"2026-11-02T14:37:35.293818Z"}],"nvd":[{"id":"CVE-2021-35955","description":"Contao \u003e=4.0.0 allows backend XSS via HTML attributes to an HTML field. Fixed in 4.4.56, 4.9.18, 4.11.7.","status":"published","published":"2021-08-12T15:15:07.96Z","modified":"2026-06-17T03:58:01.78Z","cvss":[{"version":"3.1","score":4.8,"vector":"CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N","level":"medium","assessor":"primary"},{"version":"2.0","score":3.5,"vector":"AV:N/AC:M/Au:S/C:N/I:P/A:N","level":"low","assessor":"primary"}],"cwes":["CWE-79"],"updated_at":"2026-11-02T14:06:56.943671Z"}],"osv":[{"id":"CVE-2021-35955","description":"Contao \u003e=4.0.0 allows backend XSS via HTML attributes to an HTML field. Fixed in 4.4.56, 4.9.18, 4.11.7.","status":"published","published":"2021-08-12T15:15:07.96Z","modified":"2026-10-20T10:54:46.910100415Z","cvss":[{"version":"3.1","score":4.8,"vector":"CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N","level":"medium"}],"updated_at":"2026-11-02T15:44:20.13324Z"},{"id":"GHSA-hr3h-x6gq-rqcp","title":"Cross site scripting via HTML attributes in the back end","description":"### Impact\n\nIt is possible for untrusted users to inject malicious code into HTML attributes in the back end, which will be executed both in the element preview (back end) and on the website (front end).\n\nInstallations are only affected if there are untrusted back end users who have the rights to modify HTML fields (e.g. TinyMCE).\n\n### Patches\n\nUpdate to Contao 4.4.56, 4.9.18 or 4.11.7\n\n### Workarounds\n\nDisable all fields that allow HTML for untrusted back end users or disable the login for these users.\n\n### References\n\nhttps://contao.org/en/security-advisories/cross-site-scripting-via-html-attributes-in-the-back-end\n\n### For more information\n\nIf you have any questions or comments about this advisory, open an issue in [contao/contao](https://github.com/contao/contao/issues/new/choose).\n\n### Credits\n\nThanks to Mikhail Khramenkov and Moritz Vondano for reporting this security issue.","status":"published","published":"2021-08-25T14:45:01Z","modified":"2026-07-08T06:28:14.331157245Z","cvss":[{"version":"3.1","score":5.9,"vector":"CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:L","level":"medium"}],"cwes":["CWE-79"],"updated_at":"2026-11-02T15:46:49.621585Z"}]}}