CVE-2016-6393 in IOS
Summary
by MITRE
The AAA service in Cisco IOS 12.0 through 12.4 and 15.0 through 15.6 and IOS XE 2.1 through 3.18 and 16.2 allows remote attackers to cause a denial of service (device reload) via a failed SSH connection attempt that is mishandled during generation of an error-log message, aka Bug ID CSCuy87667.
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Analysis
by VulDB Data Team • 09/21/2022
The vulnerability identified as CVE-2016-6393 represents a critical denial of service flaw within Cisco's IOS and IOS XE operating systems that affects multiple version ranges including IOS 12.0 through 12.4 and 15.0 through 15.6 along with IOS XE 2.1 through 3.18 and 16.2. This weakness specifically resides within the AAA service component which is responsible for authentication, authorization, and accounting functions within network devices. The flaw manifests when the system encounters failed SSH connection attempts that are improperly handled during the error-log message generation process, leading to complete device reloads and service disruption. This vulnerability operates at the system level and demonstrates how improper error handling in network infrastructure software can result in catastrophic operational failures.
The technical mechanism behind this vulnerability involves the AAA service's inadequate processing of failed SSH authentication attempts that occur during the connection establishment phase. When an SSH connection fails, the system attempts to generate an error log message to document the failed attempt, but the error handling code contains a flaw that causes the device to crash and reboot. This particular issue maps directly to CWE-248, which describes an "Uncaught Exception" scenario where an exception that should be properly handled instead causes the application or system to terminate unexpectedly. The vulnerability is particularly dangerous because it can be triggered remotely without requiring authentication or specific privileges, making it accessible to any attacker with network connectivity to the affected device.
The operational impact of CVE-2016-6393 extends beyond simple service interruption to encompass complete network infrastructure disruption. Network administrators face the risk of unannounced device reboots that can occur at any time, potentially during critical network operations or maintenance windows. This vulnerability affects the availability aspect of network security by creating a condition where legitimate network services become unavailable due to the device reloading itself. The flaw can be exploited repeatedly, allowing attackers to maintain persistent denial of service conditions that can severely impact network reliability and business continuity. The vulnerability also has implications for network monitoring and logging systems that depend on consistent device operation, as the sudden reboots can disrupt logging processes and create gaps in security monitoring.
Mitigation strategies for this vulnerability require immediate implementation of Cisco's security advisories and patches that address the specific error handling flaw in the AAA service. Network administrators should prioritize applying the relevant IOS updates that contain fixes for the SSH error handling code and ensure that all affected devices receive the necessary security patches. Additionally, implementing network segmentation and access controls can help reduce the attack surface by limiting remote access to devices that are not essential for public network connectivity. The remediation process should include thorough testing of patches in lab environments before deployment to production systems to avoid potential compatibility issues. Organizations should also consider implementing network monitoring solutions that can detect unusual device reboot patterns and alert administrators to potential exploitation attempts. This vulnerability serves as a reminder of the critical importance of proper error handling in network infrastructure software and the potential for seemingly minor coding flaws to result in major security incidents that can compromise entire network operations and availability.