Headline
CVE-2023-36823: Insufficient neutralization of `style` element content may allow XSS in Sanitize
Sanitize is an allowlist-based HTML and CSS sanitizer. Using carefully crafted input, an attacker may be able to sneak arbitrary HTML and CSS through Sanitize starting with version 3.0.0 and prior to version 6.0.2 when Sanitize is configured to use the built-in “relaxed” config or when using a custom config that allows style elements and one or more CSS at-rules. This could result in cross-site scripting or other undesired behavior when the malicious HTML and CSS are rendered in a browser. Sanitize 6.0.2 performs additional escaping of CSS in style element content, which fixes this issue. Users who are unable to upgrade can prevent this issue by using a Sanitize config that doesn’t allow style elements, using a Sanitize config that doesn’t allow CSS at-rules, or by manually escaping the character sequence </ as <\/ in style element content.
Impact
Using carefully crafted input, an attacker may be able to sneak arbitrary HTML and CSS through Sanitize >= 3.0.0, < 6.0.2 when Sanitize is configured to use the built-in “relaxed” config or when using a custom config that allows style elements and one or more CSS at-rules. This could result in XSS (cross-site scripting) or other undesired behavior when the malicious HTML and CSS are rendered in a browser.
Patches
Sanitize >= 6.0.2 performs additional escaping of CSS in style element content, which fixes this issue.
Workarounds
Users who are unable to upgrade can prevent this issue by using a Sanitize config that doesn’t allow style elements, using a Sanitize config that doesn’t allow CSS at-rules, or by manually escaping the character sequence </ as </ in style element content.
Credit
This issue was found by @cure53 during an audit of a project that uses Sanitize and was reported by one of that project’s maintainers. Thank you!
Related news
Ubuntu Security Notice 6748-1 - It was discovered that Sanitize incorrectly handled noscript elements under certain circumstances. An attacker could possibly use this issue to execute a cross-site scripting attack. This issue only affected Ubuntu 22.04 LTS. It was discovered that Sanitize incorrectly handled style elements under certain circumstances. An attacker could possibly use this issue to execute a cross-site scripting attack.
Debian Linux Security Advisory 5616-1 - It was discovered that ruby-sanitize, a whitelist-based HTML sanitizer, insufficiently sanitized style elements, which may result in cross-site scripting.
### Impact Using carefully crafted input, an attacker may be able to sneak arbitrary HTML and CSS through Sanitize `>= 3.0.0, < 6.0.2` when Sanitize is configured to use the built-in "relaxed" config or when using a custom config that allows `style` elements and one or more CSS at-rules. This could result in XSS (cross-site scripting) or other undesired behavior when the malicious HTML and CSS are rendered in a browser. ### Patches Sanitize `>= 6.0.2` performs additional escaping of CSS in `style` element content, which fixes this issue. ### Workarounds Users who are unable to upgrade can prevent this issue by using a Sanitize config that doesn't allow `style` elements, using a Sanitize config that doesn't allow CSS at-rules, or by manually escaping the character sequence `</` as `<\/` in `style` element content. ### Credit This issue was found by @cure53 during an audit of a project that uses Sanitize and was reported by one of that project's maintainers. Thank you!