CVE-2004-1166 in Internet Explorer
Summary
by MITRE
CRLF injection vulnerability in Microsoft Internet Explorer 6.0.2800.1106 and earlier allows remote attackers to execute arbitrary FTP commands via an ftp:// URL that contains a URL-encoded newline ("%0a") before the FTP command, which causes the commands to be inserted into the resulting FTP session, as demonstrated using a PORT command.
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Analysis
by VulDB Data Team • 09/14/2025
The CVE-2004-1166 vulnerability represents a critical cross-site request forgery and command injection flaw in Microsoft Internet Explorer versions up to 6.0.2800.1106. This vulnerability specifically targets the handling of ftp:// URLs within the browser's protocol processing mechanism, creating a pathway for remote attackers to manipulate FTP session commands through carefully crafted malicious URLs. The flaw exploits the browser's insufficient input validation when processing URL-encoded characters, particularly newline sequences that are not properly sanitized during URL parsing operations. The vulnerability's exploitation mechanism relies on the insertion of URL-encoded newline characters "%0a" before legitimate FTP commands, which then get interpreted and executed by the underlying FTP client component of the browser.
The technical implementation of this vulnerability stems from improper validation of special characters within URL parsing routines, specifically failing to normalize or sanitize newline sequences that appear in the URL path or query parameters. When Internet Explorer encounters an ftp:// URL containing a URL-encoded newline character before an FTP command, the browser processes these characters without adequate sanitization, allowing the injected newline to be interpreted as a command separator by the FTP protocol handler. This creates a situation where attackers can inject arbitrary FTP commands such as PORT, PASV, or other control commands directly into the FTP session, potentially enabling unauthorized access to FTP servers or manipulation of file transfer operations. The vulnerability operates at the application layer and demonstrates a classic case of improper input handling that violates fundamental security principles of data validation and sanitization.
The operational impact of this vulnerability extends beyond simple command injection, as it can be leveraged to perform various malicious activities including unauthorized FTP server access, data exfiltration, or even server compromise through command execution. Attackers can craft malicious web pages containing specially formatted ftp:// URLs that, when clicked by victims, automatically initiate FTP sessions with injected commands. The PORT command demonstration shows how attackers can potentially redirect FTP traffic to malicious servers or establish unauthorized connections, making this vulnerability particularly dangerous in environments where users browse untrusted websites or receive phishing emails. This vulnerability also demonstrates the risk of protocol-level injection attacks that can bypass traditional web application firewalls and security measures, as the injection occurs at the browser's native protocol handling layer rather than through web application code.
The vulnerability aligns with CWE-113, which addresses improper neutralization of CRLF characters in HTTP headers, and relates to ATT&CK technique T1059.007 for command and scripting interpreter. Microsoft's response included a security update that addressed the URL parsing behavior and implemented proper character validation for URL-encoded sequences. Organizations should implement network-level filtering to block suspicious FTP URL patterns and ensure all systems have the latest security patches installed. Browser security configurations should include enhanced URL validation and sanitization policies, while network administrators should monitor for unusual FTP traffic patterns that might indicate exploitation attempts. The vulnerability serves as a reminder of the importance of input validation at all layers of application processing and demonstrates how seemingly minor parsing flaws can create significant security risks when combined with protocol-level vulnerabilities.