CVE-2016-6458 in Email Security Appliance
Summary
by MITRE
A vulnerability in the content filtering functionality of Cisco AsyncOS Software for Cisco Email Security Appliances could allow an unauthenticated, remote attacker to bypass content filters configured on an affected device. Email that should have been filtered could instead be forwarded by the device. 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 use a content filter for email attachments that are protected or encrypted. More Information: CSCva52546. Known Affected Releases: 10.0.0-125 9.7.1-066.
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Analysis
by VulDB Data Team • 10/04/2022
The vulnerability identified as CVE-2016-6458 represents a critical security flaw in Cisco AsyncOS Software that impacts the content filtering capabilities of Cisco Email Security Appliances. This weakness specifically targets the software's handling of email attachments that are protected or encrypted, creating a scenario where unauthorized attackers can bypass configured security controls. The vulnerability exists within the content filtering functionality of the email security appliance, which is designed to inspect and filter potentially malicious email content before it reaches end users. The flaw allows an unauthenticated remote attacker to exploit the system's filtering mechanisms, effectively undermining the security posture that organizations rely upon to protect against malicious email content.
The technical nature of this vulnerability stems from improper handling of encrypted or protected email attachments within the content filtering process. When email security appliances are configured to apply content filters to attachments that are protected or encrypted, the system fails to properly validate or process these specific attachment types, creating a bypass path that allows filtered content to be forwarded undetected. This issue affects all versions prior to the first fixed release of Cisco AsyncOS Software, with specific affected releases including 10.0.0-125 and 9.7.1-066 for both virtual and hardware appliance implementations. The vulnerability's impact is particularly severe because it operates at the core filtering mechanism of the email security appliance, potentially allowing malicious content to traverse security controls that were specifically implemented to prevent such delivery.
The operational impact of CVE-2016-6458 extends beyond simple bypass of content filters, as it fundamentally compromises the integrity of email security policies that organizations have configured to protect against various threats. Organizations utilizing Cisco Email Security Appliances with content filtering enabled for encrypted attachments face the risk of malware delivery, data exfiltration attempts, and other malicious activities that would normally be blocked by the security controls. This vulnerability creates a persistent threat vector that remains active until the affected software versions are updated, potentially allowing attackers to maintain access to filtered content streams over extended periods. The remote nature of the attack means that threat actors do not require local access or authentication credentials to exploit the vulnerability, making it particularly dangerous in environments where email security is a primary defense mechanism.
Organizations should immediately implement mitigations including upgrading to patched versions of Cisco AsyncOS Software as recommended by Cisco's security advisories. The vulnerability aligns with CWE-284 Access Control Issues, specifically relating to insufficient access control mechanisms in content filtering systems. From an ATT&CK framework perspective, this vulnerability maps to techniques involving privilege escalation and evasion of security controls, particularly T1566.100 (Phishing with Spoofed Credentials) and T1071.004 (Application Layer Protocol: DNS). Administrators should also consider implementing additional monitoring and logging controls to detect potential exploitation attempts, while reviewing existing email security policies to ensure that encrypted attachment handling is properly addressed. The vulnerability demonstrates the critical importance of maintaining up-to-date security software and the potential consequences of relying on outdated systems that may contain unpatched security flaws.