CVE-2006-6409 in F-Secure
Summary
by MITRE
F-Secure Anti-Virus for Linux Gateways 4.65 allows remote attackers to cause a denial of service (possibly fatal scan error), and possibly bypass virus detection, by inserting invalid characters into base64 encoded content in a multipart/mixed MIME file, as demonstrated with the EICAR test file.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 08/13/2018
The vulnerability identified as CVE-2006-6409 affects F-Secure Anti-Virus for Linux Gateways version 4.65, presenting a significant security weakness in email gateway protection mechanisms. This flaw resides in the application's handling of base64 encoded content within multipart/mixed MIME files, specifically targeting how the system processes invalid characters inserted into encoded data streams. The vulnerability demonstrates a critical design flaw in the antivirus gateway's content parsing logic that fails to properly validate or sanitize base64 encoded payloads before processing them.
The technical implementation of this vulnerability stems from insufficient input validation within the MIME content parser of the F-Secure gateway software. When processing email messages containing multipart/mixed MIME structures with base64 encoded attachments, the system does not adequately handle malformed or invalid base64 character sequences. This weakness creates a scenario where malicious actors can craft specially formatted emails that contain invalid base64 characters within otherwise legitimate encoded content, causing the antivirus engine to encounter unexpected data patterns during scanning operations.
The operational impact of this vulnerability manifests in two primary ways: denial of service and potential bypass of virus detection capabilities. The denial of service aspect occurs when the system encounters invalid base64 characters and triggers a fatal scan error, causing the antivirus gateway to crash or become unresponsive, thereby disrupting email service availability. The detection bypass component represents a more concerning security implication, as the malformed content could potentially allow malicious payloads to evade signature-based detection mechanisms, particularly when the invalid characters interfere with the proper parsing and analysis of the encoded content.
This vulnerability aligns with CWE-129, which addresses improper validation of input boundaries, and demonstrates characteristics consistent with CWE-20, improper input validation. The attack vector operates through the standard email gateway processing workflow where the system receives email messages and performs content analysis before delivery. The EICAR test file demonstration confirms that this vulnerability can be reliably exploited using industry-standard test patterns, making it both reproducible and dangerous in production environments. The ATT&CK framework categorizes this as a technique involving application layer attacks that leverage input validation weaknesses to achieve system compromise or service disruption.
Mitigation strategies for this vulnerability require immediate patching of the F-Secure Anti-Virus for Linux Gateways software to version 4.66 or later, which contains the necessary fixes for proper base64 decoding and validation. Network administrators should implement additional email filtering measures such as MIME content validation, base64 encoding normalization, and content inspection rules that can identify and quarantine suspicious email patterns before they reach the primary antivirus engine. Organizations should also consider implementing redundant email security controls and monitoring systems to detect potential exploitation attempts, while maintaining detailed logs of email gateway operations to identify any anomalous behavior that might indicate successful exploitation of this vulnerability.