CVE-2016-6356 in Email Security Appliance
Summary
by MITRE
A vulnerability in the email message filtering feature of Cisco AsyncOS Software for Cisco Email Security Appliances could allow an unauthenticated, remote attacker to cause an affected device to stop scanning and forwarding email messages due to a denial of service (DoS) condition. Affected Products: This vulnerability affects all releases prior to the first fixed release of Cisco AsyncOS Software for Cisco Email Security Appliances, both virtual and hardware appliances, if the software is configured to apply a message filter or content filter to incoming email attachments. The vulnerability is not limited to any specific rules or actions for a message filter or content filter. More Information: CSCuz63143. Known Affected Releases: 8.5.7-042 9.7.0-125. Known Fixed Releases: 10.0.0-125 9.1.1-038 9.7.2-047.
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Analysis
by VulDB Data Team • 09/25/2024
The vulnerability described in CVE-2016-6356 represents a critical denial of service weakness within Cisco AsyncOS Software that specifically targets the email message filtering capabilities of Cisco Email Security Appliances. This flaw exists in the software's handling of message filters and content filters applied to incoming email attachments, creating a condition where an unauthenticated remote attacker can deliberately disrupt the appliance's core functionality. The vulnerability affects both virtual and hardware implementations of the Cisco Email Security Appliances, making it particularly concerning given the widespread deployment of these security devices across enterprise networks. The impact extends beyond simple service interruption as the affected system ceases to scan and forward email messages entirely, effectively creating a complete communication breakdown that can compromise organizational email infrastructure and potentially expose networks to additional security risks.
The technical nature of this vulnerability stems from improper handling of message filtering operations within the AsyncOS software stack, where specific conditions can trigger a state where the email scanning engine becomes unresponsive. This flaw operates at the application layer and leverages the existing message filtering functionality as an attack vector rather than exploiting a separate software component. The vulnerability manifests when processing email attachments that are subject to message filters or content filters, and it is not limited to any particular filter rules or actions, indicating a fundamental flaw in how the software manages filter execution states. According to the Cisco bug ID CSCuz63143, the issue occurs during the processing of filtered messages, where the system fails to properly maintain its operational state, leading to complete service disruption. This behavior aligns with CWE-400 vulnerability classification related to resource exhaustion and improper error handling, where the system cannot recover from malformed or specially crafted filter conditions.
The operational impact of CVE-2016-6356 extends far beyond simple availability disruption as it fundamentally compromises the security posture of organizations relying on Cisco Email Security Appliances. When an appliance becomes unresponsive to email scanning and forwarding, it creates a complete communication blackout that can affect business operations, customer service, and internal communications. The vulnerability's ability to affect all releases prior to specific fixed versions means that organizations with legacy deployments face prolonged exposure, while the fact that it impacts both virtual and hardware appliances suggests that the flaw exists at the software architecture level rather than being a deployment-specific issue. This makes the vulnerability particularly dangerous for organizations with mixed appliance environments, as the attack could potentially impact their entire email security infrastructure. The unauthenticated nature of the attack means that even organizations with strong perimeter security measures could be affected, as no credentials or access privileges are required to exploit the vulnerability.
Organizations affected by CVE-2016-6356 should prioritize immediate remediation through the application of the specified fixed releases, including version 10.0.0-125 for the 10.0 release line, 9.1.1-038 for the 9.1 release line, and 9.7.2-047 for the 9.7 release line. The remediation process should include thorough testing of the updated software in non-production environments to ensure compatibility with existing email filtering rules and policies. Security teams should also implement network monitoring to detect potential exploitation attempts and establish incident response procedures for rapid deployment of patches when vulnerabilities are identified. From an ATT&CK framework perspective, this vulnerability maps to the T1499.004 technique related to network denial of service, and organizations should consider implementing additional network segmentation and monitoring to prevent lateral movement if exploitation occurs. The vulnerability also highlights the importance of maintaining current software versions and following vendor security advisories as part of comprehensive vulnerability management programs, since the affected releases span multiple major version lines, indicating a persistent flaw that required multiple release-specific fixes to address properly.