CVE-2014-3146 in lxml
Summary
by MITRE
Incomplete blacklist vulnerability in the lxml.html.clean module in lxml before 3.3.5 allows remote attackers to conduct cross-site scripting (XSS) attacks via control characters in the link scheme to the clean_html function.
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Analysis
by VulDB Data Team • 12/18/2025
The vulnerability identified as CVE-2014-3146 represents a critical security flaw within the lxml library's html cleaning functionality, specifically affecting versions prior to 3.3.5. This issue resides in the lxml.html.clean module where the implementation of blacklist validation for HTML content processing proves inadequate. The flaw manifests when the clean_html function processes HTML input containing control characters within link schemes, creating a pathway for malicious actors to bypass intended security restrictions. The vulnerability classifies under CWE-20, which describes improper input validation, and more specifically aligns with CWE-79, representing cross-site scripting vulnerabilities due to insufficient sanitization of user-supplied data.
The technical exploitation of this vulnerability occurs through carefully crafted HTML content that includes control characters within URL schemes, allowing attackers to inject malicious scripts that would normally be filtered out by the cleaning process. When the lxml library processes such input, the incomplete blacklist validation fails to properly identify and neutralize these control character sequences that could be used to manipulate the parsing behavior of the HTML cleaner. This weakness enables attackers to bypass the intended security boundaries of the clean_html function, potentially allowing execution of arbitrary JavaScript code in the context of the victim's browser. The vulnerability demonstrates a failure in the security model of the library's HTML sanitization mechanism, where the validation logic does not adequately account for all possible character sequences that could be used to circumvent the filtering process.
From an operational standpoint, this vulnerability poses significant risks to web applications that rely on lxml for HTML sanitization and content processing. Attackers can leverage this flaw to inject malicious scripts into web pages, potentially leading to session hijacking, data theft, or redirection to malicious sites. The impact extends beyond simple XSS attacks as the vulnerability could be combined with other techniques to escalate privileges or perform more sophisticated attacks against affected applications. Organizations using vulnerable versions of lxml in their web applications face potential exposure to persistent threats, as the vulnerability allows for the execution of malicious code without requiring complex exploitation techniques. The flaw affects any application that processes user-provided HTML content through the lxml library's cleaning functions, making it particularly dangerous in environments where content moderation is critical.
Mitigation strategies for CVE-2014-3146 require immediate patching of affected lxml installations to version 3.3.5 or later, where the blacklist validation has been enhanced to properly handle control characters in URL schemes. Security teams should implement comprehensive testing procedures to verify that the patched version correctly sanitizes all forms of user input, particularly focusing on edge cases involving control characters and unusual URL formatting. Organizations should also consider implementing additional layers of defense including web application firewalls, content security policies, and regular security audits of their HTML processing pipelines. The vulnerability highlights the importance of thorough input validation and the need for security-conscious development practices, particularly when implementing sanitization and filtering mechanisms. Security professionals should monitor for similar patterns in other libraries and frameworks that implement similar HTML cleaning functionality, as the underlying architectural flaw may exist in other components of the software ecosystem. This vulnerability serves as a reminder of the critical importance of maintaining up-to-date security libraries and implementing robust security testing procedures to prevent exploitation of input validation weaknesses that could lead to widespread XSS vulnerabilities across web applications.