CVE-2011-2581 in NX-OS
Summary
by MITRE
The ACL implementation in Cisco NX-OS 5.0(2) and 5.0(3) before 5.0(3)N2(1) on Nexus 5000 series switches, and NX-OS before 5.0(3)U1(2a) on Nexus 3000 series switches, does not properly handle comments in conjunction with deny statements, which allows remote attackers to bypass intended access restrictions in opportunistic circumstances by sending packets, aka Bug IDs CSCto09813 and CSCtr61490.
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Analysis
by VulDB Data Team • 08/06/2024
The vulnerability described in CVE-2011-2581 represents a critical flaw in the access control list implementation within Cisco NX-OS software versions affecting both Nexus 5000 and 3000 series switches. This issue specifically targets the handling of comments within ACL configurations when combined with deny statements, creating a condition where legitimate access restrictions can be circumvented through carefully crafted packet transmission. The flaw exists in NX-OS versions 5.0(2) and 5.0(3) before the specified patches, making a significant portion of Cisco's data center switching infrastructure vulnerable to unauthorized access attempts.
The technical root cause of this vulnerability stems from improper parsing and evaluation of ACL entries that contain comments preceding deny rules. When the NX-OS software processes these specific combinations, it fails to correctly interpret the logical flow of access control decisions, allowing packets that should be denied to traverse the network boundary. This behavior manifests as a bypass of intended security policies, where attackers can exploit the inconsistent processing of comment lines in conjunction with deny statements to gain access to restricted network segments. The vulnerability operates at the network layer where ACLs function as fundamental security controls, making it particularly dangerous as it undermines the core access control mechanisms of enterprise network infrastructure.
From an operational perspective, this vulnerability creates significant risk for organizations relying on Cisco Nexus switches for network segmentation and access control. The opportunistic nature of the attack means that exploitation requires specific conditions and timing, but once successful, it can allow attackers to bypass critical network security controls. The impact extends beyond simple unauthorized access to potentially enable more sophisticated attacks such as lateral movement within the network, data exfiltration, or disruption of network services. Security teams face the challenge of identifying affected devices within their network infrastructure and implementing patches without disrupting critical network operations, as these switches often serve as core network infrastructure components.
Organizations should prioritize patching affected systems to address this vulnerability, with Cisco releasing specific updates for NX-OS versions 5.0(3)N2(1) and 5.0(3)U1(2a) to resolve the ACL processing issue. The mitigation strategy involves upgrading to patched software versions and conducting thorough network assessments to identify all affected Nexus switches within the infrastructure. Security controls should be implemented to monitor for unusual traffic patterns that might indicate exploitation attempts, while network segmentation should be reviewed to minimize potential impact if the vulnerability is successfully exploited. This vulnerability aligns with CWE-252, which addresses "Unchecked Return Value" in security contexts, and represents a specific case where improper handling of access control elements creates security bypass opportunities. The ATT&CK framework would categorize this as a privilege escalation or defense evasion technique, as it allows attackers to bypass existing network security controls through manipulation of ACL configuration parsing logic.