CVE-2017-3818 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, aka a Malformed MIME Header Filtering Bypass. 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 apply a message filter or content filter to incoming email attachments. More Information: CSCvb65245. Known Affected Releases: 9.7.1-066. Known Fixed Releases: 9.8.0-092.
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Analysis
by VulDB Data Team • 08/28/2024
The vulnerability described in CVE-2017-3818 represents a critical weakness in the MIME scanning functionality of Cisco AsyncOS Software running on Cisco Email Security Appliances. This flaw specifically targets the Multipurpose Internet Mail Extensions scanner component responsible for processing email attachments and filtering content based on configured policies. The vulnerability stems from improper handling of malformed MIME headers during the scanning process, creating a potential bypass mechanism that allows malicious actors to circumvent security controls that should prevent certain email content from being processed or delivered. The issue affects both virtual and hardware implementations of the Cisco ESA platform, making it particularly concerning given the widespread deployment of these appliances in enterprise email security environments.
The technical nature of this vulnerability falls under CWE-129, which describes improper validation of input boundaries, and more specifically relates to CWE-20, which covers insufficient input validation. Attackers can exploit this weakness by crafting specially formatted email messages containing malformed MIME headers that the scanner fails to properly parse or validate. This failure allows the malicious content to bypass configured user filters and content protection rules that should normally prevent such messages from being processed or delivered to end users. The vulnerability is particularly dangerous because it operates at the message inspection layer, where the security appliance is designed to act as a gatekeeper for incoming email traffic, effectively allowing attackers to inject content that would normally be blocked by security policies.
Operationally, this vulnerability creates a significant risk for organizations relying on Cisco ESA appliances for email security, as it enables unauthenticated remote attackers to bypass critical filtering mechanisms without requiring any credentials or privileged access. The impact extends beyond simple content bypass, as it could potentially allow attackers to deliver phishing emails, malware attachments, or other malicious content that would normally be rejected by the security appliance. Organizations using the affected versions of Cisco AsyncOS Software, specifically those running release 9.7.1-066 or earlier, face a heightened risk of security breaches, data exfiltration, and credential theft through email-based attack vectors. The vulnerability affects all message filters and content filters configured on the appliance, making it particularly dangerous for organizations with strict email security policies and compliance requirements.
Mitigation strategies for this vulnerability require immediate software updates to the fixed releases, specifically version 9.8.0-092 or later, which contain patches addressing the malformed MIME header processing issue. Organizations should conduct thorough vulnerability assessments to identify systems running the affected software versions and prioritize patch deployment across all Cisco ESA appliances in their environment. Network administrators should also implement additional monitoring and logging procedures to detect potential exploitation attempts, as the bypass mechanism may not generate obvious alert conditions. Security teams should review and validate existing email security policies to ensure that alternative protection mechanisms are in place, while also considering the implementation of additional email security layers such as sandboxing or advanced threat protection solutions. The vulnerability highlights the importance of maintaining up-to-date security software and demonstrates how flaws in core scanning functionality can undermine entire email security infrastructures, aligning with ATT&CK technique T1190 for Exploit Public-Facing Application and T1078 for Valid Accounts to maintain persistent access through compromised email systems.