CVE-2016-6357 in Email Security Appliance
Summary
by MITRE
A vulnerability in the configured security policies, including drop email filtering, in Cisco AsyncOS for Cisco Email Security Appliance (ESA) could allow an unauthenticated, remote attacker to bypass a configured drop filter by using an email with a corrupted attachment. More Information: CSCuz01651. Known Affected Releases: 10.0.9-015 9.7.1-066 9.9.6-026.
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Analysis
by VulDB Data Team • 09/25/2024
The vulnerability identified as CVE-2016-6357 represents a critical flaw in Cisco AsyncOS for Cisco Email Security Appliance (ESA) that undermines the integrity of email filtering policies. This security weakness specifically targets the drop email filtering mechanism, which is designed to prevent certain email content from reaching end users by blocking it at the network boundary. The flaw allows an unauthenticated remote attacker to circumvent these protective measures through the deliberate corruption of email attachments, effectively bypassing the configured security policies that should have prevented such content from being delivered.
The technical implementation of this vulnerability stems from insufficient validation mechanisms within the email processing pipeline of the Cisco ESA appliance. When an email arrives with a corrupted attachment, the system fails to properly identify the malicious or unwanted content that should have been blocked by the drop filter policies. This failure occurs at the application layer where email security policies are enforced, creating a pathway for attackers to exploit the system's inability to properly validate attachment integrity and content classification. The vulnerability specifically affects versions 10.0.9-015, 9.7.1-066, and 9.9.6-026 of the Cisco AsyncOS, indicating a widespread issue across multiple release branches that were in use within enterprise email security infrastructures.
The operational impact of this vulnerability extends far beyond simple bypass of email filters, creating significant risks for organizations relying on Cisco ESA for email protection. An attacker could potentially deliver phishing emails, malware payloads, or other malicious content that would normally be blocked by the configured drop filters, thereby compromising the security posture of the entire email ecosystem. This vulnerability essentially undermines the fundamental purpose of email security appliances, which is to act as a protective barrier against unwanted and potentially harmful email traffic. The implications are particularly severe in enterprise environments where email is a primary communication channel and where the compromise of email security can lead to data breaches, credential theft, and other serious security incidents.
Organizations affected by CVE-2016-6357 should immediately implement mitigations including applying the relevant Cisco security patches and updates that address the validation flaw in the email attachment processing. Network administrators should also consider implementing additional monitoring and alerting mechanisms to detect potential exploitation attempts, as well as reviewing and strengthening email security policies to compensate for the temporary vulnerability. The mitigation strategy should align with industry best practices and standards such as those outlined in the CWE (Common Weakness Enumeration) catalog, specifically addressing weaknesses related to insufficient input validation and security policy bypass mechanisms. From an ATT&CK framework perspective, this vulnerability maps to techniques involving bypassing security controls and privilege escalation through manipulation of system validation processes, highlighting the need for comprehensive security architecture reviews and continuous monitoring of email security appliances to prevent exploitation of similar weaknesses.