CVE-2017-12353 in Email Security Appliance
Summary
by MITRE
A vulnerability in the Multipurpose Internet Mail Extensions (MIME) scanner of Cisco AsyncOS Software for Cisco Email Security Appliances (ESA) could allow an unauthenticated, remote attacker to bypass configured user filters on the device. The vulnerability is due to improper error handling of a malformed MIME header in an email attachment. An attacker could exploit this vulnerability by sending an email with a crafted MIME attachment. For example, a successful exploit could allow the attacker to bypass configured user filters to drop the email. The malformed MIME headers may not be RFC compliant. However, some mail clients could still allow users to access the attachment, which may not have been properly filtered by the device. Cisco Bug IDs: CSCvf44666.
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Analysis
by VulDB Data Team • 01/25/2021
The vulnerability identified as CVE-2017-12353 resides within the Multipurpose Internet Mail Extensions scanner of Cisco AsyncOS Software operating on Cisco Email Security Appliances. This flaw represents a critical security weakness that enables unauthenticated remote attackers to circumvent user-defined email filtering policies, fundamentally undermining the security posture of email infrastructure. The vulnerability specifically manifests when the system processes malformed MIME headers within email attachments, creating a pathway for malicious actors to bypass content filtering mechanisms that are designed to protect organizational networks from unwanted or harmful email traffic.
The technical root cause of this vulnerability stems from inadequate error handling procedures within the MIME header parsing functionality of the email security appliance. When encountering malformed MIME headers that do not conform to RFC specifications, the system fails to properly process these irregularities, instead allowing the malformed content to pass through filtering mechanisms. This improper error handling creates a condition where the appliance cannot adequately distinguish between legitimate and malicious content, particularly when dealing with attachments that contain non-compliant MIME structures. The vulnerability exploits the difference between strict RFC compliance requirements and the lenient processing behavior of certain email clients that may still permit users to access problematic attachments despite the appliance's filtering attempts.
The operational impact of this vulnerability extends beyond simple bypass of email filters, potentially enabling attackers to deliver malicious content that would otherwise be blocked by security policies. An attacker could craft emails with specifically designed malformed MIME headers that cause the ESA to improperly process the attachment, allowing the email to be delivered to recipients while appearing to have been filtered. This could result in the delivery of phishing emails, malware attachments, or other malicious content that bypasses the security controls designed to prevent such threats. The vulnerability's exploitation requires minimal privileges and can be executed remotely without authentication, making it particularly dangerous as it allows adversaries to systematically circumvent email security measures without detection.
Organizations utilizing Cisco Email Security Appliances are advised to implement immediate mitigations including applying the latest software patches released by Cisco to address the identified vulnerability. The vulnerability aligns with CWE-20, which describes improper input validation, and represents a specific case where inadequate error handling leads to security bypass. From an ATT&CK framework perspective, this vulnerability maps to techniques involving bypassing security controls and privilege escalation through content filtering manipulation. Network administrators should also consider implementing additional monitoring and logging procedures to detect anomalous email processing behavior that might indicate exploitation attempts, while maintaining awareness of the Cisco Bug ID CSCvf44666 which specifically documents this issue and its resolution path.