CVE-2025-29771 in HtmlSanitizer
Summary
by MITRE • 03/14/2025
HtmlSanitizer is a client-side HTML Sanitizer. Versions prior to 2.0.3 have a cross-site scripting vulnerability when the sanitizer is used with a `contentEditable` element to set the elements `innerHTML` to a sanitized string produced by the package. If the code is particularly crafted to abuse the code beautifier, that runs AFTER sanitation. The issue is patched in version 2.0.3.
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Analysis
by VulDB Data Team • 03/14/2025
The HtmlSanitizer library represents a critical client-side security vulnerability that affects applications relying on HTML sanitization for user-generated content processing. This vulnerability specifically targets versions prior to 2.0.3 and demonstrates how seemingly secure sanitization processes can be bypassed through sophisticated exploitation techniques. The flaw emerges when the sanitizer operates within contentEditable elements, creating a dangerous intersection between HTML sanitization and DOM manipulation that traditional security measures fail to address adequately.
The technical exploitation occurs through a sophisticated abuse of the code beautifier component that executes after the initial sanitization process. This timing issue creates a window where maliciously crafted HTML code can bypass sanitization checks and execute unintended operations within the contentEditable context. The vulnerability exploits the sequence where sanitization occurs first, followed by code beautification, allowing attackers to inject payloads that appear harmless during initial sanitization but become active after the beautification process transforms the code structure. This represents a classic case of post-sanitization code execution that undermines the fundamental security assumptions of the sanitization process.
The operational impact of this vulnerability extends beyond simple XSS attacks to potentially enable full client-side compromise of affected applications. When attackers can manipulate contentEditable elements through crafted HTML payloads, they gain the ability to execute arbitrary JavaScript code within the victim's browser context. This capability can lead to session hijacking, data exfiltration, credential theft, and other advanced persistent threats that exploit the trust relationship between the user and the web application. The vulnerability affects any application that uses HtmlSanitizer for contentEditable elements, making it particularly dangerous in rich text editing environments and collaborative platforms.
Security practitioners should immediately upgrade to HtmlSanitizer version 2.0.3 to address this vulnerability, as the patch resolves the timing issue between sanitization and beautification processes. Organizations should conduct comprehensive code reviews to identify all instances where contentEditable elements interact with HtmlSanitizer, particularly in applications handling user-generated content. The vulnerability aligns with CWE-79 which addresses cross-site scripting flaws, and represents a specific instance of ATT&CK technique T1211 which involves code injection attacks. Additional mitigations include implementing Content Security Policy headers, using strict input validation, and employing sandboxing techniques for contentEditable elements to limit the potential impact of any successful exploitation attempts.