CVE-2004-0051 in MAILsweeper
Summary
by MITRE
Multiple content security gateway and antivirus products allow remote attackers to bypass content restrictions via MIME messages that use non-standard but frequently supported Content-Transfer-Encoding values such as (1) uuencode, (2) mac-binhex40, and (3) yenc, which may be interpreted differently by mail clients.
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Analysis
by VulDB Data Team • 06/21/2018
The vulnerability described in CVE-2004-0051 represents a critical weakness in content security gateways and antivirus systems that process email traffic. This flaw specifically targets the handling of MIME message encoding standards where security appliances fail to properly validate or normalize content transfer encoding values. The vulnerability affects multiple vendors' security products that are deployed to filter and monitor email content for malicious payloads or policy violations. When these systems encounter email messages utilizing non-standard but widely supported Content-Transfer-Encoding values such as uuencode, mac-binhex40, or yenc, they may incorrectly process these messages, leading to potential bypass of security controls.
The technical root cause of this vulnerability lies in the insufficient validation of Content-Transfer-Encoding headers within MIME messages. Security gateways and antivirus systems typically rely on standard encoding values like base64, quoted-printable, or 8bit for proper message processing and content inspection. However, when encountering non-standard encodings such as uuencode, mac-binhex40, or yenc, these systems may either fail to recognize the encoding properly or interpret it inconsistently with the receiving mail client's expectations. This discrepancy creates a window of opportunity for attackers to craft malicious emails that appear benign to security systems while being properly interpreted by end-user mail clients. The vulnerability operates at the protocol level, specifically within the MIME (Multipurpose Internet Mail Extensions) specification handling within security appliances.
The operational impact of this vulnerability is significant and multifaceted. Remote attackers can exploit this weakness to deliver malicious content that would otherwise be blocked by content security systems, effectively bypassing security controls designed to prevent the transmission of harmful payloads. This includes potentially delivering malware, phishing content, or other malicious materials that exploit the difference in how security appliances process these encodings versus how end-user mail clients interpret them. The vulnerability particularly affects organizations that rely heavily on email security gateways for content filtering, as it undermines the fundamental premise that security appliances can reliably detect and block malicious content based on their encoding methods. This creates a dangerous scenario where security systems may incorrectly approve potentially harmful messages, leading to successful delivery to end users.
Organizations can mitigate this vulnerability by implementing comprehensive validation of Content-Transfer-Encoding headers within their email security infrastructure, ensuring that all encoding values are properly normalized and validated before content inspection occurs. Security administrators should configure their content security gateways to explicitly reject or properly handle non-standard encoding values, rather than allowing them to pass through the system. Additionally, regular updates to security appliance firmware and signature databases are essential to address known encoding handling issues. Network security teams should implement monitoring for unusual encoding patterns and establish proper logging of content transfer encoding values to detect potential exploitation attempts. The mitigation strategy should also include regular security assessments of email infrastructure to identify and address similar encoding-related vulnerabilities, aligning with the principle of defense in depth as outlined in cybersecurity frameworks. This vulnerability specifically relates to CWE-20, Improper Input Validation, and can be mapped to ATT&CK technique T1193, Spearphishing Attachments, where attackers leverage encoding bypasses to deliver malicious payloads.