CVE-2026-109345 in DOMPurify
Summary
by MITRE • 10/12/2026
DOMPurify before 3.4.16 contains a cross-site scripting vulnerability that allows attackers to bypass sanitization because IN_PLACE mode returns a force-removed style element whose text content still carries markup. Attackers can supply a style element containing a closing style tag and an img onerror payload, which executes script when the application serializes and re-parses the returned node.
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Analysis
by VulDB Data Team • 10/12/2026
The vulnerability identified in DOMPurify versions prior to 3.4.16 represents a critical failure in input sanitization logic within its IN_PLACE processing mode. This cross-site scripting flaw arises from an inconsistency between how elements are removed and how their textual content is handled during the serialization process. Specifically, when the sanitizer operates in IN_PLACE mode, it attempts to remove specific dangerous nodes by stripping them from the document tree while preserving surrounding text or attributes. However, due to a logic error, the implementation fails to properly sanitize the inner text of these force-removed style elements before returning them for re-insertion into the DOM. This oversight allows malicious markup embedded within the text content of a removed element to persist and subsequently execute when the application serializes the node back into HTML and parses it again by the browser engine.
The technical mechanism relies on the specific behavior of CSS parsing contexts versus HTML parsing contexts in modern browsers. An attacker can craft a payload consisting of a style element that contains a closing style tag followed immediately by an image element with an onerror event handler, such as <style></style><img src=x onerror=alert(1)>. When DOMPurify processes this input in IN_PLACE mode, it identifies the style block as potentially dangerous and removes it from the active node list. However, because the text content of that removed element is not fully sanitized or escaped before being returned to the calling context, the raw string containing both the closing tag and the subsequent HTML payload remains intact. When this string is serialized back into the document structure, the browser interprets the sequence correctly: it closes the style block and then parses the remaining <img> tag as standard HTML, thereby triggering the onerror event handler and executing arbitrary JavaScript code within the context of the victim's session.
This vulnerability has significant operational impacts for web applications relying heavily on client-side sanitization to prevent stored or reflected cross-site scripting attacks. Since DOMPurify is widely used in frameworks like React, Vue, and Angular as a defense-in-depth measure against XSS, its failure can lead to full account compromise, session hijacking, defacement of user interfaces, or the delivery of further malware payloads such as keyloggers or credential harvesters. The severity is exacerbated by the fact that many developers treat DOMPurify output as trusted input after sanitization, assuming no malicious code remains. Consequently, attackers can bypass security controls designed to filter out script tags and event handlers, effectively neutralizing a primary layer of defense against web-based attacks.
From a classification perspective, this flaw aligns with CWE-79: Improper Neutralization of Input During Web Page Generation, commonly known as Cross-site Scripting (XSS). More specifically, it falls under the sub-category of DOM-Based XSS where the vulnerability exists in client-side JavaScript code rather than server-side processing. In terms of the MITRE ATT&CK framework for browser-based attacks, this technique corresponds to T1059: Command and Control via Browser Execution or more broadly to techniques involving script injection through input validation failures. The specific vector leverages the interaction between CSS parsing rules and HTML tokenization, which is a known area where sanitizers often struggle due to complex edge cases in browser rendering engines.
Mitigation strategies must prioritize immediate software updates as well as defensive coding practices. The primary remediation is upgrading DOMPurify to version 3.4.16 or later, where the developers have addressed this specific logic error by ensuring that text content within removed elements is properly escaped or stripped of any residual markup before being returned. For applications unable to update immediately due to dependency constraints, a temporary workaround involves avoiding the use of IN_PLACE mode if possible and instead using standard sanitization modes that return new DOM nodes rather than modifying existing ones in place. Additionally, developers should implement Content Security Policy (CSP) headers with strict script-src directives to limit the execution context of any scripts that might slip through client-side filters. Regular security audits focusing on how sanitized data is re-introduced into the DOM can also help identify similar patterns where serialization and parsing cycles create opportunities for injection attacks.