CVE-2019-11720 in Firefox
Summary
by MITRE
Some unicode characters are incorrectly treated as whitespace during the parsing of web content instead of triggering parsing errors. This allows malicious code to then be processed, evading cross-site scripting (XSS) filtering. This vulnerability affects Firefox < 68.
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Analysis
by VulDB Data Team • 07/10/2020
This vulnerability represents a critical parsing inconsistency in Firefox's web content handling mechanism that directly impacts the browser's ability to properly validate and sanitize input data. The flaw manifests when certain unicode characters that should trigger parsing errors are instead interpreted as whitespace characters during content parsing operations. This misclassification creates a bypass mechanism that allows malicious actors to inject potentially harmful code sequences that would normally be detected and filtered by XSS protection systems. The vulnerability specifically affects Firefox versions prior to 68, indicating a widespread issue within the browser's rendering engine that could have enabled sophisticated cross-site scripting attacks. The technical nature of this flaw places it squarely within the realm of input validation and sanitization failures that are commonly associated with CWE-20 Improper Input Validation.
The operational impact of this vulnerability extends beyond simple XSS evasion to encompass broader security implications for web application integrity and user protection. When unicode characters are incorrectly processed as whitespace, it effectively undermines the security boundaries that browsers establish to prevent malicious code execution. Attackers can exploit this weakness by crafting payloads that utilize specific unicode sequences to bypass existing XSS filters, potentially leading to session hijacking, data theft, or unauthorized access to user accounts. The vulnerability demonstrates how seemingly minor parsing inconsistencies can create significant security gaps in web browsers, particularly when these gaps intersect with established security controls. This flaw aligns with ATT&CK technique T1211 Exploitation for Defense Evasion, as it leverages the parsing behavior to circumvent security measures that are designed to prevent malicious code execution.
The technical implementation of this vulnerability involves the browser's content parser encountering specific unicode character sequences that should result in parsing errors or invalid content detection. Instead, these characters are processed as legitimate whitespace, allowing embedded malicious code to pass through validation checkpoints that would normally block such content. This behavior creates a pathway for attackers to inject JavaScript code or other malicious payloads that would otherwise be filtered by standard XSS protection mechanisms. The vulnerability's impact is particularly concerning given Firefox's widespread use and the browser's role as a primary gateway for web content access. Security researchers have identified that this issue stems from the browser's failure to properly distinguish between valid whitespace characters and potentially malicious unicode sequences during the parsing phase of web content processing. The flaw represents a fundamental breakdown in the browser's content sanitization pipeline, where the distinction between legitimate content and malicious input becomes blurred due to improper character classification during parsing operations.