CVE-2006-0486 in Shell
Summary
by MITRE
Certain Cisco IOS releases in 12.2S based trains with maintenance release number 25 and later, 12.3T based trains, and 12.4 based trains reuse a Tcl Shell process across login sessions of different local users on the same terminal if the first user does not use tclquit before exiting, which may cause subsequent local users to execute unintended commands or bypass AAA command authorization checks, aka Bug ID CSCef77770.
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Analysis
by VulDB Data Team • 07/07/2021
The vulnerability described in CVE-2006-0486 represents a critical security flaw in Cisco IOS software that affects multiple release trains including 12.2S, 12.3T, and 12.4. This issue stems from improper process management within the Tcl Shell implementation, creating a persistent security risk that can be exploited by local attackers. The flaw manifests when multiple users share the same terminal session, with the first user's Tcl Shell process remaining active even after they exit the system. This behavior violates fundamental security principles and creates an environment where subsequent users can inherit the context and privileges of the previous session, leading to potential privilege escalation and unauthorized command execution.
The technical root cause of this vulnerability lies in the improper handling of Tcl Shell processes within Cisco IOS. When a user logs into a terminal session and utilizes Tcl commands, the system should properly terminate the Tcl Shell process upon user logout. However, in affected versions, the Tcl Shell process persists and maintains its state across different user sessions. This process reuse creates a scenario where the second user can execute commands within the context of the previous user's session, potentially gaining access to commands or privileges that should be restricted. The vulnerability specifically occurs when the first user fails to explicitly execute the tclquit command before exiting, leaving the Tcl Shell process in a reusable state. This behavior directly contravenes the principle of least privilege and demonstrates a failure in proper session isolation mechanisms.
The operational impact of this vulnerability extends beyond simple command execution, as it fundamentally undermines the AAA (Authentication, Authorization, and Accounting) framework that Cisco IOS relies upon for access control. Local users who gain access to the system can bypass command authorization checks that should normally restrict their actions based on their assigned privileges. This means that an attacker could potentially execute administrative commands or access sensitive system functions that would normally be restricted to authorized personnel. The vulnerability creates a persistent backdoor that remains active as long as the terminal session is shared, making it particularly dangerous in environments where multiple users access the same terminal or console ports. The implications are severe as this allows for unauthorized privilege escalation and potential system compromise.
Cisco IOS implementations following the 12.2S, 12.3T, and 12.4 release trains are affected by this vulnerability, indicating it was present across multiple major versions of the operating system. This widespread impact suggests the flaw was in the core Tcl Shell implementation rather than in specific features or modules. The vulnerability's persistence across multiple release trains indicates that it was not simply a one-time bug but rather a systemic design flaw in how the system managed process lifecycle and session isolation. Organizations running affected Cisco IOS versions should consider this vulnerability as a critical risk to their network infrastructure security, particularly in environments where console access is available to multiple users or where physical security controls are inadequate. The vulnerability aligns with CWE-284 (Improper Access Control) and represents a clear violation of the principle of process isolation that is fundamental to secure operating system design.
The recommended mitigation strategies for this vulnerability include immediate deployment of Cisco IOS patches and updates that address the Tcl Shell process management issue. Organizations should ensure that all affected Cisco devices receive the appropriate maintenance releases that fix the process reuse behavior. Additionally, system administrators should implement strict console access controls and ensure that users properly execute the tclquit command before exiting terminal sessions. Network security teams should also consider implementing additional monitoring and logging of console sessions to detect any unauthorized command execution that might result from this vulnerability. The fix addresses the core issue by ensuring proper process termination and isolation between user sessions, preventing the inheritance of privileges and command contexts that could lead to unauthorized access or privilege escalation. This vulnerability serves as a reminder of the importance of proper process management and session isolation in network operating systems, particularly in environments where multiple users share access to critical infrastructure components.