CVE-2017-6671 in Email Security Appliance
Summary
by MITRE
A vulnerability in the email message scanning of Cisco AsyncOS Software for Cisco Email Security Appliance (ESA) could allow an unauthenticated, remote attacker to bypass configured filters on the device, as demonstrated by the Attachment Filter. More Information: CSCvd34632. Known Affected Releases: 10.0.1-087 9.7.1-066. Known Fixed Releases: 10.0.2-020 9.8.1-015.
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Analysis
by VulDB Data Team • 12/26/2020
The vulnerability identified as CVE-2017-6671 resides within the email message scanning functionality of Cisco AsyncOS Software operating on Cisco Email Security Appliance devices. This flaw represents a critical security weakness that enables unauthenticated remote attackers to circumvent the device's configured email filtering policies, specifically targeting the Attachment Filter mechanism. The vulnerability stems from improper validation of email message content during the scanning process, allowing malicious actors to craft emails that bypass security controls designed to block specific file types or attachments. The affected versions include Cisco ESA software releases 10.0.1-087 and 9.7.1-066, with remediation available through updates to versions 10.0.2-020 and 9.8.1-015 respectively.
This security flaw operates at the application layer of the email security infrastructure and represents a direct violation of the principle of least privilege and defense in depth. The vulnerability is classified under CWE-20 as "Improper Input Validation" and aligns with ATT&CK technique T1190 "Exploit Public-Facing Application" as it allows remote exploitation of a web-facing email security appliance. The impact extends beyond simple bypass of filters to potentially enable more sophisticated attacks including the delivery of malicious attachments that would normally be blocked by the security appliance. Attackers can leverage this vulnerability to deliver phishing emails, malware payloads, or other malicious content that would otherwise be prevented by the configured security policies, effectively neutralizing the protective mechanisms of the email security appliance.
The operational consequences of this vulnerability are severe for organizations relying on Cisco ESA for email protection, as it fundamentally undermines the trust model of the email security infrastructure. When exploited, the vulnerability allows attackers to bypass critical security controls that are meant to prevent the delivery of harmful content to end users, potentially leading to data breaches, system compromises, or the spread of malware throughout the organization. The attack vector requires no authentication credentials and can be executed remotely, making it particularly dangerous as it can be exploited by threat actors without requiring insider access or credentials. Organizations may experience increased incident response costs due to the need to investigate and remediate compromised systems that were supposed to be protected by the email security appliance.
Organizations should immediately implement the vendor-provided security patches for affected Cisco ESA software versions to remediate this vulnerability. The mitigation strategy should include verifying that the updated software versions have been successfully deployed across all email security appliances within the network infrastructure. Network segmentation and monitoring should be enhanced to detect potential exploitation attempts, while security teams should review email security logs for signs of anomalous behavior or successful bypass attempts. Additionally, organizations should conduct comprehensive security assessments of their email infrastructure to identify any other potential vulnerabilities that might be exploited in conjunction with this flaw, ensuring that the overall email security posture remains robust against evolving threats.