CVE-2023-36823 in Sanitize
Summary
by MITRE • 07/06/2023
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.
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Analysis
by VulDB Data Team • 07/25/2023
The vulnerability identified as CVE-2023-36823 affects the Sanitize library, a widely-used allowlist-based HTML and CSS sanitizer designed to prevent cross-site scripting attacks by filtering potentially dangerous content. This security flaw exists in versions 3.0.0 through 6.0.1 of the library, creating a significant risk for applications that rely on Sanitize to protect against malicious input. The vulnerability specifically targets configurations that utilize the built-in "relaxed" configuration or custom configurations that permit style elements and CSS at-rules, making it particularly dangerous for web applications that process user-generated content and sanitize HTML output.
The technical implementation flaw stems from inadequate handling of CSS content within style elements during the sanitization process. When Sanitize processes HTML containing style elements that include CSS at-rules, the library fails to properly escape or filter certain character sequences that could allow attackers to inject malicious code. This weakness creates a path for attackers to bypass the sanitizer's intended protections by carefully crafting input that exploits the parser's handling of CSS content, particularly around the interpretation of CSS at-rules. The vulnerability is classified under CWE-79 as a Cross-Site Scripting (XSS) weakness, specifically manifesting as a failure to sanitize output properly. The attack vector aligns with ATT&CK technique T1068 which involves the exploitation of vulnerabilities in applications to execute malicious code.
The operational impact of this vulnerability is substantial as it allows attackers to inject arbitrary HTML and CSS code that can be executed in browsers when the sanitized content is rendered. This could enable a range of malicious activities including but not limited to session hijacking, defacement of web pages, data exfiltration, and execution of malicious scripts. The vulnerability is particularly concerning because it affects commonly used configurations that prioritize flexibility over security, making it likely to be present in many production environments. When exploited, the vulnerability could result in unauthorized access to user sessions, data breaches, and compromise of entire web applications that rely on the Sanitize library for content filtering.
The fix implemented in version 6.0.2 addresses this issue through enhanced escaping of CSS content within style elements, specifically targeting the problematic character sequences that attackers could exploit. This remediation aligns with industry best practices for XSS prevention by ensuring that CSS content is properly sanitized before being rendered in browsers. Organizations unable to immediately upgrade to the patched version can implement several mitigation strategies including configuring Sanitize to disallow style elements entirely, restricting the use of CSS at-rules in custom configurations, or manually escaping the character sequence `</` as `<\/` within style element content. These workarounds provide temporary protection while allowing organizations to maintain their current software versions. The vulnerability demonstrates the critical importance of proper input validation and output encoding in web applications, particularly when handling user-generated content that must be safely rendered in browsers.