CVE-2019-1983 in Email Security Appliance
Summary
by MITRE
A vulnerability in the email message filtering feature of Cisco AsyncOS Software for Cisco Email Security Appliance (ESA) and Cisco Content Security Management Appliance (SMA) could allow an unauthenticated, remote attacker to cause repeated crashes in some internal processes that are running on the affected devices, resulting in a denial of service (DoS) condition. The vulnerability is due to insufficient input validation of email attachments. An attacker could exploit this vulnerability by sending an email message with a crafted attachment through an affected device. A successful exploit could allow the attacker to cause specific processes to crash repeatedly, resulting in the complete unavailability of both the Cisco Advanced Malware Protection (AMP) and message tracking features and in severe performance degradation while processing email. After the affected processes restart, the software resumes filtering for the same attachment, causing the affected processes to crash and restart again. A successful exploit could also allow the attacker to cause a repeated DoS condition. Manual intervention may be required to recover from this situation.
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Analysis
by VulDB Data Team • 09/23/2020
The vulnerability identified as CVE-2019-1983 represents a critical denial of service weakness within Cisco's email security infrastructure, specifically affecting the AsyncOS software running on Cisco Email Security Appliances and Content Security Management Appliances. This flaw resides in the email message filtering functionality where inadequate validation mechanisms fail to properly handle maliciously crafted email attachments. The vulnerability stems from insufficient input validation processes that do not adequately sanitize or verify the integrity of email attachments before processing them through the security appliance's internal systems.
The technical exploitation of this vulnerability occurs when an unauthenticated remote attacker crafts a malicious email attachment and delivers it to an affected device. The flaw manifests when the appliance processes the malformed attachment through its internal filtering mechanisms, causing specific system processes to crash repeatedly in a continuous loop. This creates a cascading failure effect where the crashing processes automatically restart, only to encounter the same malformed attachment and crash again, establishing a persistent denial of service condition. The vulnerability specifically impacts the Cisco Advanced Malware Protection capabilities and message tracking features, rendering these critical security functions completely non-operational while simultaneously degrading overall email processing performance to unacceptable levels.
From an operational standpoint, this vulnerability presents a severe threat to email security infrastructure reliability and business continuity. The repeated crashing and restarting cycles of internal processes create sustained degradation that can persist until manual intervention occurs, potentially leaving organizations without critical email security protections for extended periods. The impact extends beyond simple service disruption as the affected appliance loses its ability to properly filter malware and track email messages, creating security gaps that could allow malicious traffic to bypass protection mechanisms. This vulnerability directly maps to CWE-20, which describes improper input validation, and aligns with ATT&CK technique T1499.004 for network denial of service attacks. The repetitive nature of the crashes also demonstrates characteristics consistent with resource exhaustion attacks that consume system processing power and memory resources unnecessarily.
Organizations should implement immediate mitigations including network segmentation to limit access to affected appliances, deployment of temporary email filtering rules that reject suspicious attachment types, and regular monitoring of system logs for signs of process crashes. Cisco recommends applying the latest software patches and updates as soon as they become available, while network administrators should establish automated alerting systems to detect unusual process restart patterns. Manual recovery procedures should be documented and tested regularly, including procedures for safely restarting affected services and clearing corrupted process states. The vulnerability also highlights the importance of implementing layered security approaches where multiple security controls work together to prevent single points of failure, ensuring that even if one security mechanism is compromised, others can maintain operational integrity.