CVE-2018-0484 in IOS
Summary
by MITRE
A vulnerability in the access control logic of the Secure Shell (SSH) server of Cisco IOS and IOS XE Software may allow connections sourced from a virtual routing and forwarding (VRF) instance despite the absence of the vrf-also keyword in the access-class configuration. The vulnerability is due to a missing check in the SSH server. An attacker could use this vulnerability to open an SSH connection to an affected Cisco IOS or IOS XE device with a source address belonging to a VRF instance. Once connected, the attacker would still need to provide valid credentials to access the device.
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Analysis
by VulDB Data Team • 06/26/2023
The vulnerability described in CVE-2018-0484 represents a critical access control flaw within Cisco IOS and IOS XE Software implementations of the Secure Shell protocol. This issue specifically targets the SSH server component where the access control logic fails to properly validate source addresses when connections are made from virtual routing and forwarding instances. The vulnerability stems from a missing validation check that should have enforced the presence of the vrf-also keyword in access-class configurations, which serves as a critical security control for managing VRF-based access restrictions.
The technical flaw manifests when an attacker exploits the absence of proper VRF source address validation within the SSH server implementation. Normally, when VRF instances are configured with access control lists, the vrf-also keyword is required to explicitly permit connections from those VRF sources. Without this keyword, the system should reject connections originating from VRF instances, but the missing validation check allows unauthorized connections to proceed. This represents a fundamental breakdown in the principle of least privilege and proper network segmentation enforcement. The vulnerability aligns with CWE-284, which addresses improper access control, and demonstrates how inadequate input validation can lead to privilege escalation and unauthorized system access.
The operational impact of this vulnerability extends beyond simple unauthorized access attempts, as it fundamentally undermines the security posture of network devices implementing VRF-based access controls. Attackers can exploit this weakness to establish SSH sessions to affected devices using source addresses belonging to VRF instances, potentially bypassing network segmentation controls that are critical for protecting sensitive network zones. Even though valid credentials remain required for successful authentication, the ability to connect to the device from a VRF source provides attackers with an additional attack vector that could be combined with other exploitation techniques. This vulnerability particularly affects enterprise networks where VRF instances are commonly used for network segmentation and security isolation, making it a significant concern for organizations relying on Cisco networking equipment.
Mitigation strategies for CVE-2018-0484 should prioritize immediate implementation of software patches provided by Cisco, as these updates address the missing validation check in the SSH server access control logic. Network administrators must also review and properly configure access-class statements to ensure that the vrf-also keyword is appropriately implemented for VRF-based access controls. Additional defensive measures include implementing network segmentation controls beyond VRF configurations, monitoring SSH connection attempts from VRF sources, and maintaining comprehensive audit logs of access control events. The vulnerability demonstrates the importance of proper access control implementation as outlined in the NIST Cybersecurity Framework and aligns with ATT&CK technique T1078 which covers valid accounts and legitimate credentials as attack vectors. Organizations should also consider implementing additional security controls such as SSH key-based authentication, two-factor authentication, and regular security assessments to reduce the overall risk exposure associated with this and similar access control vulnerabilities.