CVE-2006-2783 in Thunderbird
Summary
by MITRE
Mozilla Firefox and Thunderbird before 1.5.0.4 strip the Unicode Byte-order-Mark (BOM) from a UTF-8 page before the page is passed to the parser, which allows remote attackers to conduct cross-site scripting (XSS) attacks via a BOM sequence in the middle of a dangerous tag such as SCRIPT.
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Analysis
by VulDB Data Team • 06/20/2019
The vulnerability described in CVE-2006-2783 represents a critical security flaw in Mozilla Firefox and Thunderbird versions prior to 1.5.0.4 that fundamentally compromised the browser's ability to handle Unicode encoded content properly. This issue stems from the improper handling of Unicode Byte-Order-Mark (BOM) sequences within UTF-8 encoded web pages, creating a dangerous condition that could be exploited by remote attackers to bypass security mechanisms designed to prevent cross-site scripting attacks. The flaw specifically manifested when the browser stripped BOM characters from UTF-8 pages before passing the content to the HTML parser, thereby disrupting the expected encoding behavior that security controls rely upon.
The technical exploitation of this vulnerability occurs through carefully crafted malicious content that embeds BOM sequences within dangerous HTML tags such as SCRIPT elements. When a web page containing a BOM character within a script tag is processed by the affected browsers, the BOM is removed during the preprocessing stage before the parser encounters the actual script content. This removal effectively allows attackers to inject malicious code that would otherwise be properly sanitized or blocked by the browser's security mechanisms. The BOM character, which typically serves as an encoding identifier for Unicode text, becomes a vector for bypassing input validation and sanitization processes that are designed to prevent XSS attacks.
From an operational perspective, this vulnerability created a significant risk for users of affected browser versions, as it allowed remote attackers to execute arbitrary JavaScript code within the context of the victim's browser session. The impact extends beyond simple script execution to potentially enable more sophisticated attacks including session hijacking, data theft, and redirection to malicious websites. The vulnerability's exploitation requires minimal user interaction, as it can be triggered simply by visiting a malicious webpage, making it particularly dangerous in phishing campaigns and drive-by download scenarios. Security researchers have classified this issue as a direct violation of secure coding practices that should prevent such encoding-related bypasses in web application security.
The technical flaw aligns with CWE-180, which addresses the issue of allowing a character or sequence of characters to be interpreted in multiple ways, and relates to ATT&CK technique T1059.007 for execution through scripting. This vulnerability demonstrates the critical importance of proper input sanitization and encoding handling in web browsers, as it shows how seemingly benign encoding artifacts can be weaponized to undermine security controls. The fix implemented by Mozilla involved modifying the browser's handling of UTF-8 encoded content to preserve BOM characters during the parsing process, ensuring that security mechanisms could properly validate content regardless of encoding artifacts. This remediation approach aligns with security best practices that emphasize maintaining the integrity of character encoding during processing to prevent such bypasses. Organizations should ensure that all affected systems are updated to versions that address this vulnerability, as the risk remains significant for any system running unsupported browser versions.