CVE-2017-3827 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) and Web Security Appliances (WSA) could allow an unauthenticated, remote attacker to bypass configured user filters on the device. Affected Products: This vulnerability affects all releases prior to the first fixed release of Cisco AsyncOS Software for Cisco ESA and Cisco WSA, both virtual and hardware appliances, that are configured with message or content filters to scan incoming email attachments on the ESA or services scanning content of web access on the WSA. More Information: SCvb91473, CSCvc76500. Known Affected Releases: 10.0.0-203 9.9.9-894 WSA10.0.0-233.
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Analysis
by VulDB Data Team • 12/25/2024
This vulnerability resides within the Multipurpose Internet Mail Extensions scanner of Cisco AsyncOS Software affecting both Email Security Appliances and Web Security Appliances. The flaw represents a critical bypass issue that allows unauthenticated remote attackers to circumvent configured user filters, fundamentally undermining the security posture of affected devices. The vulnerability specifically impacts systems configured with message or content filters that scan incoming email attachments on ESA devices or web content scanning services on WSA appliances. The affected releases span multiple versions including 10.0.0-203, 9.9.9-894, and WSA10.0.0-233, indicating this represents a widespread issue across various product iterations. The vulnerability's classification aligns with CWE-284, which addresses improper access control mechanisms, as it allows unauthorized bypass of security controls that should restrict access to filtered content. This weakness directly impacts the principle of least privilege by enabling attackers to access content that should be restricted through configured filters.
The technical exploitation of this vulnerability occurs through the MIME scanner component which processes email attachments and web content. When the scanner fails to properly validate or process certain MIME structures, it allows malicious content to slip through configured security filters without proper inspection. Attackers can craft specific email attachments or web content that exploits the scanner's parsing logic to bypass intended filtering rules. This creates a scenario where the security appliance fails to perform its core function of content inspection, effectively rendering configured filters ineffective. The vulnerability's impact extends beyond simple content bypass as it represents a fundamental failure in the appliance's security enforcement mechanisms, potentially allowing malicious payloads to reach end users or systems that should have been blocked. The issue demonstrates poor input validation and insufficient sanitization of MIME content structures, creating an attack vector that operates at the protocol level rather than application level.
Operationally, this vulnerability creates significant risk for organizations relying on Cisco ESA and WSA appliances for email and web security. The unauthenticated nature of the attack means that external threat actors can exploit this without requiring valid credentials or prior access to the network. This bypass capability allows attackers to deliver malicious content that would normally be blocked by configured filters, potentially leading to data breaches, malware infections, or other security incidents. The vulnerability affects both virtual and hardware appliances, indicating that organizations using either deployment model face identical risk. The impact is particularly severe for email security appliances where bypassed filters could allow phishing emails, malware attachments, or other malicious content to reach end users. Organizations may experience cascading security failures as the appliance fails to enforce configured security policies, potentially leading to regulatory compliance issues and increased incident response costs. The vulnerability's presence in multiple release versions suggests that organizations may have been exposed for extended periods without detection.
Organizations should immediately implement mitigations including upgrading to the first fixed release of Cisco AsyncOS Software for both ESA and WSA appliances, as specified in the security advisory. The patch addresses the core MIME scanner logic that enables the bypass, restoring proper content inspection functionality. Network segmentation and additional monitoring should be implemented to detect potential exploitation attempts, particularly focusing on unusual email traffic patterns or content delivery that might indicate bypass attempts. Security teams should review existing filter configurations to identify any potential gaps that might be exploited through this vulnerability, ensuring that multiple layers of security are maintained even if one control is compromised. The mitigation strategy should also include enhanced logging and alerting for the MIME scanner component to detect abnormal parsing behavior. Organizations should conduct vulnerability assessments to determine if any exploitation has occurred and implement network-based intrusion detection systems to monitor for known exploit patterns. This vulnerability demonstrates the importance of maintaining up-to-date security appliances and the critical need for regular security assessments to identify potential bypass mechanisms that could compromise network security posture. The issue also highlights the need for robust input validation in security appliances to prevent similar vulnerabilities from emerging in other components.