CVE-2018-15460 in Email Security Appliance
Summary
by MITRE
A vulnerability in the email message filtering feature of Cisco AsyncOS Software for Cisco Email Security Appliances (ESA) could allow an unauthenticated, remote attacker to cause the CPU utilization to increase to 100 percent, causing a denial of service (DoS) condition on an affected device. The vulnerability is due to improper filtering of email messages that contain references to whitelisted URLs. An attacker could exploit this vulnerability by sending a malicious email message that contains a large number of whitelisted URLs. A successful exploit could allow the attacker to cause a sustained DoS condition that could force the affected device to stop scanning and forwarding email messages.
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Analysis
by VulDB Data Team • 06/26/2023
The vulnerability identified as CVE-2018-15460 affects Cisco AsyncOS Software running on Cisco Email Security Appliances which represents a critical denial of service weakness in email security infrastructure. This flaw specifically targets the email message filtering mechanism within the AsyncOS platform, creating a scenario where an unauthenticated remote attacker can trigger excessive system resource consumption. The vulnerability stems from inadequate processing of email messages containing whitelisted URL references, demonstrating a fundamental flaw in how the system handles legitimate network resources that should normally be trusted. This represents a classic example of improper input validation where the system fails to properly sanitize or limit the processing of reference data that should be benign.
The technical exploitation mechanism involves crafting malicious email messages containing an excessive number of references to whitelisted URLs, which triggers the CPU utilization to reach 100 percent capacity. This occurs because the email security appliance continuously processes and validates each URL reference in the message, even when those URLs are part of the trusted whitelist. The system's inability to properly limit or batch these validation operations creates a resource exhaustion condition that cascades through the appliance's processing pipeline. The vulnerability specifically impacts the message scanning and forwarding capabilities of the device, effectively rendering the email security appliance unable to perform its core function of filtering and delivering email communications.
The operational impact of this vulnerability extends beyond simple service disruption, creating a persistent threat to email infrastructure availability and business continuity. When the CPU reaches maximum utilization, the affected device becomes incapable of processing new email messages, leading to message queuing and potential delivery failures that can impact critical business communications. Organizations relying on Cisco Email Security Appliances for email filtering and security may experience complete email service outages, forcing administrators to manually intervene and potentially causing significant operational disruption. The sustained nature of this denial of service condition means that once exploited, the appliance remains compromised until manual intervention occurs, creating a window of vulnerability that could be exploited repeatedly.
This vulnerability aligns with CWE-400, which describes improper resource management where systems fail to properly handle resource consumption patterns, and maps to ATT&CK technique T1499.004 for resource exhaustion attacks. The flaw demonstrates poor input sanitization practices and inadequate rate limiting mechanisms within the email appliance's processing pipeline. Organizations should implement immediate mitigations including applying Cisco's security patches, configuring rate limiting on URL validation processes, and implementing network segmentation to isolate email security appliances from critical network segments. Additionally, monitoring for unusual CPU utilization patterns and implementing automated alerting systems can help detect exploitation attempts before they cause complete service disruption. The vulnerability underscores the importance of proper resource management in security appliances and highlights the need for comprehensive testing of input validation mechanisms in email filtering systems to prevent similar issues in the future.