CVE-2009-0777 in Firefox
Summary
by MITRE
Mozilla Firefox before 3.0.7, Thunderbird before 2.0.0.21, and SeaMonkey before 1.1.15 decode invisible characters when they are displayed in the location bar, which causes an incorrect address to be displayed and makes it easier for remote attackers to spoof URLs and conduct phishing attacks.
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Analysis
by VulDB Data Team • 08/29/2019
This vulnerability resides in the web browser and email client software from Mozilla Foundation, specifically affecting versions prior to critical security updates. The flaw manifests in how these applications handle URL display in their location bars, creating a deceptive user interface that can mislead users about the actual web address they are visiting or about to interact with. The vulnerability stems from the improper handling of invisible characters within URLs, which are typically non-printable or visually hidden characters that should not be displayed to users but are instead processed by the underlying application logic. This behavior creates a critical security gap that enables attackers to craft malicious URLs that appear legitimate to users while actually directing them to different destinations.
The technical implementation of this vulnerability involves the rendering engine's interpretation of URL components that contain invisible characters such as zero-width characters, non-breaking spaces, or other Unicode characters that are normally suppressed from visual display. When these applications decode and display such URLs in their location bars, they fail to properly sanitize or filter these invisible characters, resulting in a discrepancy between what the user perceives and what the actual URL resolves to. This creates a scenario where an attacker can register a domain name that includes invisible characters, making it appear as though they are visiting a legitimate website while actually being directed to a malicious one. The flaw operates at the application layer of the OSI model, specifically within the user interface rendering and URL parsing components that handle web address display.
The operational impact of this vulnerability is severe and directly enables sophisticated phishing attacks that can bypass traditional security measures. Attackers can exploit this weakness to create convincing fake websites that appear legitimate to users who are not aware of the invisible character manipulation. The vulnerability particularly affects users who rely on visual verification of URLs when browsing, as the location bar display becomes misleading and untrustworthy. This creates a significant risk for financial transactions, personal data entry, and other sensitive online activities where users might be tricked into believing they are interacting with legitimate services while actually engaging with malicious actors. The attack surface extends beyond simple phishing to include more complex social engineering campaigns where the visual deception enhances the overall attack effectiveness.
Organizations and individuals should immediately update their Mozilla Firefox, Thunderbird, and SeaMonkey installations to versions 3.0.7, 2.0.0.21, and 1.1.15 respectively, as these releases contain the necessary patches to address the invisible character handling issue. System administrators should implement comprehensive patch management procedures to ensure all affected software installations are updated promptly, particularly in enterprise environments where multiple users may be exposed to this vulnerability. Additional protective measures include implementing browser security extensions that can detect and warn about suspicious URL patterns, conducting user awareness training on phishing attack recognition, and establishing monitoring procedures to detect potential exploitation attempts. This vulnerability aligns with CWE-170, which addresses improper handling of potentially dangerous characters, and maps to ATT&CK technique T1566.001 for credential harvesting through spearphishing with malicious attachments and T1566.002 for spearphishing with malicious links, demonstrating the broader attack implications beyond simple URL spoofing.
The root cause of this vulnerability demonstrates a fundamental flaw in input validation and user interface design within web browser applications. The security issue arises from insufficient sanitization of URL components before display, creating a trust boundary violation where the user interface fails to accurately represent the underlying security state. This represents a classic case of inadequate character encoding handling and improper output filtering, where the application's user-facing components do not properly account for the security implications of displaying potentially malicious input. The vulnerability's persistence across multiple Mozilla products indicates a systemic issue in how these applications handle URL parsing and display logic, requiring comprehensive code review and security testing of similar functionality throughout the application architecture.