CVE-2026-104828 in Neko
Summary
by MITRE • 10/15/2026
HtmlUnit Neko is an HtmlUnit adaptation of NekoHTML. Prior to 5.4.0, HTMLWriterFilter writes decoded TEXTAREA and TITLE character content without entity encoding because both elements are treated as special content. During a parse-and-serialize round trip, entity references inside these escapable raw-text elements are decoded by the parser and then emitted as raw markup by printCharacters(), changing the document's semantics. Applications that use htmlunit-neko in an HTML sanitizer, content filter, or other parse-transform-serialize pipeline can therefore turn attacker-controlled encoded text into executable markup when the output is rendered by a browser. This issue is fixed in version 5.4.0.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability identified in HtmlUnit Neko prior to version 5.4.0 represents a critical failure in input validation and output encoding mechanisms within HTML parsing and serialization pipelines. HtmlUnit Neko serves as an adaptation of the NekoHTML parser, widely utilized for processing HTML documents in Java-based applications. The core technical flaw resides specifically within the HtmlWriterFilter component, which is responsible for converting parsed DOM trees back into serialized HTML markup. When encountering specific raw-text elements such as TEXTAREA and TITLE tags, the filter incorrectly treats their content as special data that does not require entity encoding. This design oversight leads to a scenario where character references are decoded by the parser during the initial ingestion phase but are subsequently emitted as literal characters rather than being re-encoded into safe HTML entities during the serialization phase.
This behavior creates a classic parse-and-serialize round-trip vulnerability, often categorized under CWE-830: Inclusion of Functionality from Untrusted Control Sources or more specifically CWE-79: Improper Neutralization of Input During Web Page Generation known as Cross-Site Scripting. The operational impact is severe because it allows attacker-controlled encoded text to be transformed into executable markup when the output is rendered by a web browser. For instance, if an application uses htmlunit-neko in a capacity such as an HTML sanitizer or content filter, an input string containing escaped script tags like <script> might be decoded to <script> and then written directly to the output stream without being converted back to <script>. Consequently, when this sanitized but flawed output is served to users, browsers interpret it as active code rather than static text, leading to successful cross-site scripting attacks.
The security implications extend beyond simple script execution if these elements are used in contexts that allow for other forms of injection or data exfiltration. Since the vulnerability affects both TEXTAREA and TITLE elements, attackers can potentially bypass filters designed to protect form inputs or metadata fields. This undermines the integrity of any application relying on this library for content transformation, as the sanitization process effectively becomes a vector for attack rather than a defense mechanism. The flaw highlights the importance of ensuring that all output destined for HTML contexts is properly encoded according to the specific context in which it will be rendered, regardless of whether the source data was originally escaped or not.
To mitigate this vulnerability, organizations must immediately upgrade HtmlUnit Neko to version 5.4.0 or later, where the issue has been resolved by ensuring that character content within these raw-text elements is correctly entity-encoded during serialization. For applications unable to update immediately due to dependency constraints, alternative sanitization libraries should be evaluated for compliance with OWASP guidelines on output encoding. Developers implementing custom parsers and serializers must ensure that all dynamic data inserted into HTML documents undergoes rigorous context-aware encoding. This includes verifying that raw-text elements are handled consistently with other DOM nodes regarding escape sequences. Regular security audits of parse-transform-serialize pipelines can help identify similar logic errors where decoded content is not re-encoded before output, thereby preventing the transformation of safe input into malicious executable code.