CVE-2016-6463 in Email Security Appliance
Summary
by MITRE
A vulnerability in the email filtering functionality of Cisco AsyncOS Software for Cisco Email Security Appliances could allow an unauthenticated, remote attacker to bypass Advanced Malware Protection (AMP) filters that are configured for an affected device. This vulnerability affects all releases prior to the first fixed release of Cisco AsyncOS Software for both virtual and hardware versions of Cisco Email Security Appliances, if the AMP feature is configured to scan incoming email attachments. More Information: CSCuz85823. Known Affected Releases: 10.0.0-082 9.7.0-125 9.7.1-066. Known Fixed Releases: 10.0.0-203 9.7.2-131.
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Analysis
by VulDB Data Team • 06/03/2019
The vulnerability identified as CVE-2016-6463 represents a critical security flaw within Cisco AsyncOS Software that specifically impacts the email filtering capabilities of Cisco Email Security Appliances. This weakness resides in the Advanced Malware Protection functionality, which is designed to scan and block malicious email attachments before they reach end users. The vulnerability creates a pathway for unauthenticated remote attackers to circumvent the protective measures that organizations rely upon to defend against sophisticated malware threats. The flaw is particularly concerning because it affects all versions prior to the specified fixed releases, encompassing both virtual and hardware implementations of the security appliance, thereby amplifying its potential impact across diverse deployment scenarios.
The technical nature of this vulnerability stems from insufficient validation mechanisms within the email filtering pipeline that processes incoming attachments. When the Advanced Malware Protection feature is enabled, the system should rigorously inspect all email attachments against known malware signatures and behavioral patterns. However, the flaw allows malicious actors to craft specific email payloads that can bypass these inspection processes, effectively rendering the AMP protection ineffective. This bypass occurs at the filtering layer where legitimate security controls are supposed to intercept and neutralize threats before they can be delivered to users' mailboxes. The vulnerability operates by exploiting a gap in the validation sequence that occurs during attachment processing, enabling attackers to deliver malware-laden emails that would normally be flagged and quarantined.
The operational impact of this vulnerability extends beyond simple email security breaches, as it fundamentally undermines the trust model that organizations place in their email security infrastructure. Organizations that have configured AMP protection on their Cisco Email Security Appliances are left with a false sense of security, believing that their systems are actively protecting against advanced malware threats. In practice, attackers can now deliver malicious attachments that bypass these protections, potentially leading to successful phishing campaigns, ransomware infections, and other malware delivery methods. The vulnerability affects all releases prior to the specified fixed versions, meaning that organizations with older software configurations remain exposed and must urgently assess their risk exposure. This creates a significant operational burden for security teams who must evaluate their current deployments and prioritize remediation efforts across their email security infrastructure.
The exploitation of this vulnerability aligns with tactics described in the attack framework, particularly those involving initial access and execution phases. According to the MITRE ATT&CK framework, this weakness enables techniques such as spearphishing with attachments and malicious file delivery, where attackers leverage the bypassed security controls to deliver payloads that would normally be blocked. The vulnerability can be classified under CWE-20, which addresses improper input validation, specifically in the context of email processing and attachment handling. Organizations affected by this vulnerability should immediately implement the recommended fixed releases to restore proper security controls, while also conducting comprehensive security assessments to identify any potential compromise that may have occurred during the period of vulnerability. The remediation process requires careful planning and execution to ensure that the updated software maintains all existing email security policies while properly addressing the identified bypass mechanism.