CVE-2011-4006 in ASA
Summary
by MITRE
The ESMTP inspection feature on Cisco Adaptive Security Appliances (ASA) 5500 series devices with software 8.2 through 8.5 allows remote attackers to cause a denial of service (CPU consumption) via an unspecified closing sequence, aka Bug ID CSCtt32565.
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Analysis
by VulDB Data Team • 01/14/2018
The vulnerability identified as CVE-2011-4006 affects Cisco Adaptive Security Appliances (ASA) 5500 series devices running software versions 8.2 through 8.5, specifically targeting the ESMTP inspection feature. This flaw represents a critical denial of service weakness that can be exploited remotely by attackers to consume excessive CPU resources on affected systems. The vulnerability manifests through an unspecified closing sequence that triggers abnormal processing behavior within the ASA's email inspection module, leading to sustained high CPU utilization that can ultimately render the device non-functional or significantly degraded in performance. The issue was documented under Bug ID CSCtt32565, indicating it was recognized and tracked by Cisco's internal bug tracking system.
The technical implementation of this vulnerability resides within the ESMTP inspection functionality of the ASA's packet processing engine. When the device encounters specific email traffic patterns that involve improper closing sequences during SMTP communication, the inspection engine enters a processing loop or consumes excessive computational resources to handle what should be standard termination procedures. This behavior creates a resource exhaustion condition where the CPU utilization spikes to critical levels, effectively preventing the device from processing legitimate network traffic and maintaining normal security operations. The flaw demonstrates poor input validation and error handling within the email protocol inspection module, where the system fails to properly manage abnormal termination sequences that occur during email transaction completion.
From an operational impact perspective, this vulnerability poses significant risk to organizations relying on Cisco ASA 5500 series devices for network security. The remote exploit capability means attackers can initiate the denial of service condition from outside the network perimeter without requiring authentication or physical access to the device. The sustained CPU consumption can lead to complete service disruption, potentially allowing attackers to bypass network security controls and disrupt business operations. Network administrators may experience difficulty in identifying the source of the issue due to the subtle nature of the attack vector, as the symptoms manifest as normal device behavior but with degraded performance rather than immediate system failure. This vulnerability particularly affects email security monitoring and filtering capabilities, as the ESMTP inspection feature is designed to examine and control email traffic passing through the appliance.
The vulnerability aligns with CWE-400, which addresses "Uncontrolled Resource Consumption," and demonstrates characteristics consistent with resource exhaustion attacks that can be classified under the MITRE ATT&CK framework's T1499.1 technique for "Network Denial of Service." Organizations should implement immediate mitigations including applying Cisco's security patches and updates to affected software versions, configuring rate limiting for email inspection processes, and implementing network monitoring to detect unusual CPU utilization patterns. Additionally, network segmentation strategies can help limit the impact of successful exploitation by isolating critical network segments from potentially compromised email inspection services. The vulnerability underscores the importance of maintaining up-to-date security patches and implementing proper network monitoring to detect anomalous behavior that could indicate exploitation attempts.