CVE-2016-9910 in html5lib
Summary
by MITRE
The serializer in html5lib before 0.99999999 might allow remote attackers to conduct cross-site scripting (XSS) attacks by leveraging mishandling of special characters in attribute values, a different vulnerability than CVE-2016-9909.
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Analysis
by VulDB Data Team • 09/29/2020
The vulnerability identified as CVE-2016-9910 represents a critical cross-site scripting weakness within the html5lib library, specifically affecting versions prior to 0.99999999. This security flaw resides in the library's serializer component which is responsible for converting parsed HTML5 documents back into serialized string format. The vulnerability stems from improper handling of special characters when processing attribute values during the serialization process, creating an avenue for malicious actors to inject harmful scripts into web applications that utilize this library for HTML processing. Unlike CVE-2016-9909 which addressed different aspects of html5lib's handling of special characters, this vulnerability specifically targets the serializer's treatment of attribute values, making it particularly dangerous for web applications that rely on html5lib for sanitizing or processing user-provided HTML content. The issue manifests when the serializer encounters certain special characters within attribute values that are not properly escaped or handled, allowing attackers to inject malicious payloads that can execute in the context of a victim's browser session.
The technical implementation of this vulnerability involves the html5lib library's failure to adequately sanitize attribute values during the serialization phase of HTML processing. When attribute values contain specific combinations of special characters, the serializer does not properly escape or encode these characters, resulting in malformed HTML output that can be exploited by attackers. This flaw typically occurs when user input containing potentially malicious content is processed through html5lib's parsing and serialization pipeline, particularly in scenarios where attribute values are not properly validated or escaped before being rendered in web documents. The vulnerability is categorized under CWE-79 as "Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')", which specifically addresses the failure to properly escape or encode user-controllable data before including it in web page output. The attack vector leverages the fact that html5lib's serializer does not perform adequate character escaping for attribute values, allowing attackers to inject script tags or other malicious content that executes when the malformed HTML is rendered in browsers.
The operational impact of CVE-2016-9910 extends beyond simple XSS exploitation, as it can enable attackers to perform a wide range of malicious activities including session hijacking, data theft, and credential harvesting. Web applications that use html5lib for HTML processing, sanitization, or content generation become vulnerable to attacks where user-provided data containing special characters in attribute values can be manipulated to inject malicious scripts. This vulnerability is particularly concerning because html5lib is widely used in Python-based web applications and content management systems, making the attack surface quite broad. The potential for exploitation increases when applications fail to properly validate or sanitize user input before processing it through html5lib, creating a direct pathway for attackers to bypass security measures that rely on html5lib for HTML sanitization. The vulnerability can be leveraged in various attack scenarios including web application attacks, social engineering campaigns, and even more sophisticated attacks that combine this XSS vulnerability with other exploitation techniques.
Mitigation strategies for CVE-2016-9910 primarily focus on updating to html5lib version 0.99999999 or later, which contains the necessary fixes to properly handle special characters in attribute values. Organizations should conduct comprehensive vulnerability assessments to identify all applications and systems that utilize html5lib, particularly those that process user-provided HTML content or rely on html5lib for HTML sanitization. Additional defensive measures include implementing proper input validation and sanitization at multiple layers of the application architecture, including server-side validation before any processing occurs. Security teams should also consider implementing content security policies and other browser-based protections to limit the impact of potential XSS exploits. The remediation process should include thorough testing to ensure that the updated html5lib version does not introduce compatibility issues with existing applications, while also validating that proper character escaping is implemented throughout the application's HTML processing pipeline. This vulnerability demonstrates the importance of maintaining up-to-date security libraries and the critical need for proper input validation and sanitization in web applications, aligning with ATT&CK technique T1203 for "Exploitation for Client Execution" and emphasizing the necessity of secure coding practices for preventing XSS vulnerabilities in web applications.