CVE-2016-6372 in Email Security Appliance
Summary
by MITRE
A vulnerability in the email message and content filtering for malformed Multipurpose Internet Mail Extensions (MIME) headers of Cisco AsyncOS Software for Cisco Email Security Appliances (ESA) and Web Security Appliances (WSA) could allow an unauthenticated, remote attacker to bypass the filtering functionality of the targeted device. Emails that should have been quarantined could instead be processed. Affected Products: This vulnerability affects all releases prior to the first fixed release of Cisco AsyncOS Software for Cisco ESA and Cisco WSA on both virtual and hardware appliances that are configured with message or content filters to scan incoming email attachments. More Information: CSCuy54740, CSCuy75174. Known Affected Releases: 9.7.1-066 9.5.0-575 WSA10.0.0-000. Known Fixed Releases: 10.0.0-125 9.1.1-038 9.7.2-047.
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Analysis
by VulDB Data Team • 09/25/2024
This vulnerability resides in the email message and content filtering mechanisms of Cisco AsyncOS Software, specifically targeting the handling of malformed Multipurpose Internet Mail Extensions headers within the Cisco Email Security Appliances and Web Security Appliances. The flaw represents a critical bypass issue that undermines the core security functionality designed to protect organizations from malicious email content. The vulnerability stems from insufficient validation of MIME header structures, allowing attackers to craft specially formatted email messages that can evade detection and quarantine processes. This weakness affects both virtual and hardware implementations of the security appliances, making it particularly concerning given the widespread deployment of these solutions across enterprise environments. The issue is particularly dangerous because it operates at the protocol level where email filtering mechanisms are designed to intercept and neutralize threats before they reach end users.
The technical exploitation of this vulnerability occurs when malformed MIME headers are processed by the filtering software, causing the system to misinterpret the message structure and subsequently bypass content scanning mechanisms. Attackers can leverage this flaw to deliver malicious payloads that would normally be detected and quarantined, effectively rendering the security appliance's filtering capabilities ineffective. The vulnerability's impact is amplified by the fact that it affects multiple product lines and software versions, with specific releases identified as both affected and fixed. The flaw essentially creates a condition where the security appliance fails to properly parse and evaluate email content, leading to false negatives in threat detection. This type of vulnerability is categorized under CWE-119, which deals with improper restriction of operations within a memory buffer, and aligns with ATT&CK technique T1190 for Exploit Public-Facing Application, as it represents an unauthenticated remote attack vector targeting the appliance's filtering functionality.
The operational consequences of this vulnerability are severe for organizations relying on Cisco security appliances for email and web filtering protection. When the filtering bypass occurs, malicious attachments that should be quarantined can successfully reach end users, potentially leading to data breaches, malware infections, and other security incidents. The vulnerability impacts the fundamental integrity of the security infrastructure, as it allows attackers to circumvent the very protections that organizations have invested in implementing. Organizations may experience false confidence in their security posture due to the appliance's apparent normal operation while simultaneously allowing threats to pass through undetected. The attack surface is particularly broad since it affects both email and web security appliances, and the vulnerability can be exploited without authentication, making it accessible to any remote attacker with basic network connectivity to the affected devices. The potential for supply chain attacks or lateral movement within compromised networks increases significantly when this vulnerability is present, as the attacker can effectively use the appliance itself as a vector for delivering malicious content.
Organizations must implement immediate mitigation strategies to address this vulnerability, beginning with upgrading to the fixed releases identified in the advisory. The recommended fixed versions include 10.0.0-125 for WSA, 9.1.1-038 for ESA, and 9.7.2-047 for the affected software versions. Network segmentation and access control measures should be reinforced to limit exposure of the affected appliances to untrusted networks. Additionally, organizations should conduct thorough security assessments to verify that their appliances are properly configured and that no unauthorized modifications have occurred. Monitoring and logging capabilities should be enhanced to detect potential exploitation attempts, including unusual patterns in email processing or filtering bypass events. Security teams should also implement network-based intrusion detection systems to monitor for traffic patterns consistent with exploitation attempts targeting this specific vulnerability. The remediation process should include comprehensive testing of the updated software to ensure that the fix does not introduce compatibility issues with existing security policies or network configurations. Regular vulnerability assessments and patch management procedures should be strengthened to prevent similar issues from occurring in the future, particularly focusing on input validation and header parsing mechanisms within security appliances.