CVE-2006-6405 in BitDefender Mail Protection
Summary
by MITRE
BitDefender Mail Protection for SMB 2.0 allows remote attackers to bypass virus detection by inserting invalid characters into base64 encoded content in a multipart/mixed MIME file, as demonstrated with the EICAR test file.
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Analysis
by VulDB Data Team • 09/26/2017
The vulnerability identified as CVE-2006-6405 affects BitDefender Mail Protection for SMB 2.0, a security solution designed to protect email communications within small and medium business environments. This flaw represents a critical bypass issue that undermines the fundamental purpose of antivirus protection by allowing malicious content to evade detection mechanisms. The vulnerability specifically targets the handling of base64 encoded content within multipart/mixed MIME email structures, which are commonly used for transmitting files alongside email messages. The attack vector leverages the manipulation of encoding standards to create a condition where legitimate security scanning fails to identify known malicious content.
The technical implementation of this vulnerability stems from improper validation of base64 encoded data within MIME boundaries. When processing multipart/mixed email content, the BitDefender system fails to properly handle invalid or malformed base64 character sequences that are inserted into the encoded payload. This processing gap occurs during the content inspection phase where the antivirus engine attempts to decode and analyze the base64 encoded portions of the email. The flaw creates a parsing inconsistency where the system accepts malformed base64 sequences that, when decoded, still produce valid content that bypasses signature-based detection mechanisms. This behavior aligns with CWE-20, which describes improper input validation, and specifically demonstrates weaknesses in data sanitization and encoding handling within security appliances.
The operational impact of this vulnerability extends beyond simple detection bypass, as it fundamentally compromises the security posture of organizations relying on BitDefender Mail Protection for SMB 2.0. Attackers can exploit this weakness by crafting emails containing EICAR test files with manipulated base64 encoding, effectively neutralizing the antivirus protection for those specific messages. The vulnerability demonstrates a classic application-level attack pattern that aligns with ATT&CK technique T1071.004 for application layer protocol manipulation. Organizations using this mail protection system face significant risk of malware delivery through email channels, as the bypass mechanism allows threat actors to circumvent security controls designed to prevent such infections. The attack is particularly concerning because it requires minimal sophistication and can be executed against any email containing multipart/mixed content with manipulated base64 encoding.
Mitigation strategies for this vulnerability must address both immediate remediation and long-term security architecture improvements. Organizations should prioritize applying the vendor-provided patches or updates that correct the base64 decoding and validation logic within the BitDefender system. Network administrators should implement additional email security layers such as content filtering rules that monitor for malformed base64 sequences or implement more robust MIME parsing mechanisms. The solution involves strengthening input validation controls and ensuring that all base64 encoded content undergoes strict verification before analysis. Security teams should also consider implementing email sandboxing techniques that can detect anomalous behavior patterns regardless of the encoding used. This vulnerability highlights the importance of proper encoding validation in security appliances and reinforces the principle that all input processing must account for malformed or unexpected data sequences, which is consistent with security best practices outlined in NIST SP 800-144 for secure coding practices.