CVE-2020-3548 in Secure Email
Summary
by MITRE • 11/18/2024
A vulnerability in the Transport Layer Security (TLS) protocol implementation of Cisco AsyncOS software for Cisco Email Security Appliance (ESA) could allow an unauthenticated, remote attacker to cause high CPU usage on an affected device, resulting in a denial of service (DoS) condition. The vulnerability is due to inefficient processing of incoming TLS traffic. An attacker could exploit this vulnerability by sending a series of crafted TLS packets to an affected device. A successful exploit could allow the attacker to trigger a prolonged state of high CPU utilization. The affected device would still be operative, but response time and overall performance may be degraded.There are no workarounds that address this vulnerability.
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Analysis
by VulDB Data Team • 07/31/2025
This vulnerability resides within the TLS protocol implementation of Cisco AsyncOS software running on Cisco Email Security Appliances, representing a significant denial of service risk that affects network security infrastructure. The flaw manifests as inefficient processing of incoming TLS traffic, creating a condition where malicious actors can trigger sustained high CPU utilization without requiring authentication or privileged access. The vulnerability specifically targets the handling of TLS packets during the connection establishment process, where the software fails to properly manage resource allocation when processing malformed or specially crafted TLS handshakes. This weakness allows an unauthenticated remote attacker to consume excessive computational resources, effectively degrading the appliance's performance while maintaining operational status, creating a subtle but impactful service disruption that can persist for extended periods.
The technical exploitation mechanism involves sending a series of carefully crafted TLS packets designed to trigger the inefficient processing logic within the AsyncOS software. This type of vulnerability aligns with CWE-20, which describes improper input validation in software implementations, and represents a classic example of resource exhaustion through protocol manipulation. The attack vector is particularly concerning because it requires no authentication credentials and can be executed remotely, making it accessible to any attacker with network connectivity to the affected appliance. The implementation flaw likely occurs in the TLS handshake processing module where the software does not adequately validate or limit the processing of certain TLS packet sequences, allowing malicious input to cause disproportionate CPU consumption. The vulnerability demonstrates poor defensive programming practices in handling network protocol data, where the system fails to implement proper rate limiting or resource management during TLS connection establishment phases.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromising the overall email security posture of affected organizations. While the appliance remains operational, the sustained high CPU utilization can significantly degrade response times for legitimate email traffic, creating delays in email processing and potentially affecting business continuity. The performance degradation may manifest as increased email delivery latency, reduced throughput for legitimate mail processing, and potential cascading effects on downstream email services that depend on the appliance's functionality. Organizations may experience increased operational overhead as administrators must monitor system performance and potentially implement emergency measures to maintain service levels. The lack of workarounds means that organizations cannot mitigate the vulnerability through configuration changes or temporary operational procedures, forcing them to rely entirely on vendor-provided patches or firmware updates.
Organizations should prioritize immediate patch management to address this vulnerability, as the DoS condition can persist for extended periods and may be difficult to distinguish from legitimate high-traffic scenarios. The vulnerability's classification under the ATT&CK framework would fall under T1499.004, which covers network denial of service attacks, and T1595.001 for reconnaissance activities involving network infrastructure. Security teams should implement monitoring solutions to detect unusual CPU utilization patterns that may indicate exploitation attempts, particularly during peak email processing periods when the appliance is already under stress. Network segmentation and access control measures can help limit the attack surface, though these measures do not provide complete protection against this specific vulnerability. The incident highlights the critical importance of maintaining up-to-date security patches for network infrastructure components and demonstrates how seemingly minor implementation flaws in core protocols can create significant operational risks for enterprise email security systems.