CVE-2015-4184 in Email Security Appliance
Summary
by MITRE
The anti-spam scanner on Cisco Email Security Appliance (ESA) devices 3.3.1-09, 7.5.1-gpl-022, and 8.5.6-074 allows remote attackers to bypass intended e-mail restrictions via a malformed DNS SPF record, aka Bug IDs CSCuu35853 and CSCuu37733.
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Analysis
by VulDB Data Team • 05/21/2022
The Cisco Email Security Appliance (ESA) represents a critical component in enterprise email security infrastructure, designed to protect organizations from spam, malware, and other email-based threats. This vulnerability affects specific versions of the ESA platform including 3.3.1-09, 7.5.1-gpl-022, and 8.5.6-074, creating a significant security gap in email filtering capabilities. The anti-spam scanner functionality within these devices is responsible for implementing various email restrictions and validation mechanisms to prevent unwanted communications from reaching end users. When compromised, this vulnerability allows malicious actors to bypass these essential security controls through manipulation of DNS records.
The technical flaw resides in the implementation of the Sender Policy Framework (SPF) validation mechanism within the ESA's anti-spam scanner. SPF records serve as DNS-based email authentication methods that help determine whether an email was sent from an authorized server. The vulnerability occurs when the system fails to properly validate malformed SPF records, allowing attackers to craft email messages with intentionally corrupted SPF data that appears legitimate to the scanner. This parsing error creates a bypass condition where the system incorrectly processes the malformed SPF record and fails to apply the intended email restrictions. The issue stems from insufficient input validation and error handling within the DNS record processing logic, which falls under CWE-20 - Improper Input Validation and CWE-707 - Improper Neutralization of Input During Web Page Generation.
The operational impact of this vulnerability is substantial for organizations relying on Cisco ESA for email protection. Attackers can exploit this weakness to deliver spam emails, phishing attempts, and potentially malicious payloads that would normally be blocked by the appliance's filtering rules. The bypass mechanism specifically targets the email restriction enforcement capabilities, meaning legitimate security policies may be circumvented without detection. This vulnerability enables attackers to abuse the email system for unauthorized communication, potentially leading to data exfiltration, credential theft, or other malicious activities. The implications extend beyond simple spam delivery as this bypass could affect content filtering, sender authentication, and overall email security posture.
Organizations should immediately implement mitigations including applying the latest security patches provided by Cisco to address the specific SPF validation flaw. Network administrators must also consider implementing additional email security controls such as DKIM and DMARC validation to provide layered protection against similar bypass techniques. The vulnerability demonstrates the importance of proper input validation in security appliances and highlights the need for comprehensive testing of authentication mechanisms. From an ATT&CK framework perspective, this vulnerability maps to T1566 - Phishing and T1071.004 - Application Layer Protocol: DNS, representing a technique that enables initial access through email-based attacks. Organizations should also conduct thorough vulnerability assessments of their email security infrastructure to identify similar weaknesses in other security controls and ensure proper monitoring for potential exploitation attempts.