CVE-2020-12409 in Firefox
Summary
by MITRE
When using certain blank characters in a URL, they where incorrectly rendered as spaces instead of an encoded URL. This vulnerability affects Firefox < 77.
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Analysis
by VulDB Data Team • 08/10/2025
This vulnerability represents a critical URL parsing and encoding issue that existed in Mozilla Firefox versions prior to 77. The flaw occurred when browsers encountered specific blank characters within Uniform Resource Locators, leading to improper handling of these characters during URL processing. The technical implementation failed to correctly encode certain whitespace characters that should have been converted to their proper URL encoding formats such as %20 for spaces. This misconfiguration created a scenario where malicious actors could exploit the inconsistent rendering behavior to manipulate URL parsing and potentially redirect users to unintended destinations.
The vulnerability stems from the browser's URL handling mechanism where certain blank characters including tab characters, non-breaking spaces, and other Unicode whitespace sequences were not being properly encoded during URL processing. When these characters appeared in URLs, they were rendered visually as spaces but remained unencoded in the actual URL structure. This inconsistency created a security risk because it allowed attackers to craft URLs that would appear normal to users while actually containing encoded or unencoded characters that could be interpreted differently by the browser's rendering engine. The flaw essentially created a bypass mechanism for URL validation and sanitization processes that rely on proper encoding standards.
The operational impact of this vulnerability extends beyond simple rendering issues and presents significant security implications for web browsing sessions. Users could be subjected to phishing attacks where malicious URLs appear legitimate due to the visual representation while containing hidden encoded characters that redirect to harmful destinations. This vulnerability particularly affects web applications that perform URL validation or redirect handling, as the inconsistent behavior could allow bypassing security controls designed to prevent unauthorized access. The risk is amplified in environments where users navigate to untrusted websites or where automated URL processing systems depend on consistent URL encoding behavior. This issue directly relates to CWE-185, which addresses improper handling of regular expressions and URL encoding inconsistencies.
Mitigation strategies for this vulnerability require immediate browser updates to Firefox version 77 or later where the URL encoding and parsing logic has been corrected. Organizations should implement comprehensive URL sanitization policies that validate all incoming URLs against proper encoding standards regardless of visual representation. Network security controls including web application firewalls and proxy servers should be configured to enforce strict URL validation rules that normalize whitespace characters before processing. Additionally, security teams should conduct regular vulnerability assessments to identify similar encoding inconsistencies in other browser components or web applications that might present similar attack vectors. This vulnerability demonstrates the importance of consistent URL handling across all web technologies and aligns with ATT&CK technique T1071.004 for application layer protocol manipulation. The fix implemented in Firefox 77 addresses the underlying parsing logic to ensure that all blank characters are consistently encoded according to RFC 3986 standards, preventing the visual representation mismatch that previously enabled exploitation.