CVE-2020-27783 in python-lxml
Summary
by MITRE • 12/03/2020
A XSS vulnerability was discovered in python-lxml's clean module. The module's parser didn't properly imitate browsers, which caused different behaviors between the sanitizer and the user's page. A remote attacker could exploit this flaw to run arbitrary HTML/JS code.
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Analysis
by VulDB Data Team • 12/18/2025
The vulnerability identified as CVE-2020-27783 represents a cross-site scripting weakness within the python-lxml library's clean module, specifically affecting applications that utilize this library for HTML sanitization purposes. This flaw resides in the library's ability to properly emulate browser parsing behavior, creating a dangerous discrepancy between how the sanitizer processes content and how actual web browsers render it. The python-lxml library serves as a popular Python binding for the libxml2 and libxslt libraries, commonly used for processing XML and HTML documents in web applications and data processing pipelines. When developers rely on the clean module to sanitize user-provided HTML content, they expect the output to be safe from malicious scripts, but this vulnerability undermines that security guarantee.
The technical root cause of this vulnerability stems from the clean module's parser not accurately mimicking browser parsing mechanisms, particularly in how it handles malformed or edge-case HTML constructs. This parsing inconsistency creates a scenario where HTML content that appears safe to the sanitizer may be interpreted differently by web browsers, potentially executing malicious JavaScript code. The flaw essentially allows attackers to craft input that passes through the sanitizer's validation but still results in executable code when rendered in a browser environment. This behavior aligns with CWE-79, which describes cross-site scripting vulnerabilities where untrusted data is improperly incorporated into web pages without proper sanitization or encoding.
The operational impact of this vulnerability extends beyond simple code execution, as it can enable attackers to perform various malicious activities including session hijacking, data theft, defacement of web applications, and redirection to malicious sites. Remote attackers can exploit this vulnerability by injecting specially crafted HTML content that bypasses the sanitizer's protections, then executing arbitrary JavaScript code when the sanitized content is rendered in a user's browser. The attack surface is particularly broad since many web applications use python-lxml for content sanitization, making this vulnerability potentially exploitable across numerous platforms and services. This vulnerability directly maps to attack techniques described in the MITRE ATT&CK framework under the T1059.001 category for Command and Scripting Interpreter, specifically targeting JavaScript execution in web browsers.
Organizations using python-lxml should immediately update to versions that address this vulnerability, as no effective workarounds exist that preserve the library's intended functionality while mitigating this specific parsing inconsistency. Security teams should conduct comprehensive audits of applications that utilize the clean module to identify potential exposure points and ensure proper patching across all affected systems. The vulnerability demonstrates the critical importance of maintaining strict browser compatibility in security tools, as the gap between sanitizer behavior and actual browser rendering creates exploitable attack vectors. Additionally, developers should consider implementing additional layers of input validation and output encoding beyond relying solely on third-party sanitization libraries to provide defense in depth against similar parsing discrepancies.