CVE-2004-2137 in Outlook Express
Summary
by MITRE
Outlook Express 6.0, when sending multipart e-mail messages using the "Break apart messages larger than" setting, leaks the BCC recipients of the message to the addresses listed in the To and CC fields, which may allow remote attackers to obtain sensitive information.
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Analysis
by VulDB Data Team • 07/06/2025
This vulnerability in Microsoft Outlook Express 6.0 represents a critical information disclosure flaw that exploits the software's handling of multipart email messages. The issue occurs specifically when the email client is configured with the "Break apart messages larger than" setting, which is designed to split large emails into smaller segments for transmission. When this setting is active, the application fails to properly sanitize the recipient headers during the message fragmentation process, creating a security gap that exposes BCC recipient information to all other recipients.
The technical mechanism behind this vulnerability involves the improper manipulation of email headers during the multipart message construction phase. When Outlook Express processes large messages, it creates separate parts of the email while maintaining the original header information. However, the BCC field data becomes inadvertently embedded in the To and CC field headers of the fragmented message segments, allowing any recipient to access the complete BCC list. This behavior violates fundamental email security principles and creates a significant privacy risk as it undermines the purpose of BCC addressing which is to maintain recipient anonymity.
From an operational impact perspective, this vulnerability enables remote attackers to obtain sensitive information about email recipient lists, potentially exposing personal contacts, business associates, or confidential communication networks. The flaw is particularly dangerous in corporate environments where executives or sensitive personnel might use BCC for privacy protection during important communications. Security researchers have classified this issue as a data leak vulnerability that can be exploited without requiring authentication or special privileges, making it an attractive target for threat actors seeking to gather intelligence about communication patterns within organizations.
This vulnerability aligns with CWE-200, which addresses "Information Exposure," and represents a specific instance of improper information handling during email processing. The attack pattern follows the techniques documented in MITRE ATT&CK matrix under T1566 for "Phishing" and T1567 for "Exfiltration." The flaw demonstrates a failure in input validation and output sanitization processes within the email client's message composition engine, where the application does not adequately separate or mask sensitive recipient information during message fragmentation operations. Organizations using vulnerable versions of Outlook Express should implement immediate mitigations including disabling the problematic message fragmentation feature, updating to patched versions, or implementing additional email security controls to monitor for unusual recipient list exposure patterns.