CVE-2017-6672 in ASR 5000
Summary
by MITRE
A vulnerability in certain filtering mechanisms of access control lists (ACLs) for Cisco ASR 5000 Series Aggregation Services Routers through 21.x could allow an unauthenticated, remote attacker to bypass ACL rules that have been configured for an affected device. More Information: CSCvb99022 CSCvc16964 CSCvc37351 CSCvc54843 CSCvc63444 CSCvc77815 CSCvc88658 CSCve08955 CSCve14141 CSCve33870.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 01/06/2021
The vulnerability identified as CVE-2017-6672 affects Cisco ASR 5000 Series Aggregation Services Routers operating on software versions through 21.x, representing a critical flaw in the device's access control list filtering mechanisms. This weakness creates a significant security gap that allows unauthenticated remote attackers to bypass configured ACL rules, effectively undermining the network's perimeter defense. The vulnerability stems from improper validation of packet processing within the router's ACL implementation, where specific conditions can cause the filtering system to skip critical access control checks. The affected devices are widely deployed in telecommunications environments where network security is paramount, making this vulnerability particularly concerning for service providers and enterprise networks that rely on these routers for traffic management and security enforcement. Multiple Cisco bug IDs associated with this vulnerability indicate the complexity and widespread nature of the issue across different software releases and deployment scenarios.
The technical flaw manifests when certain packet processing conditions trigger a bypass in the ACL evaluation logic, allowing malicious traffic to pass through the router without proper authorization checks. This occurs due to a specific interaction between packet headers and the router's internal processing state, where the ACL mechanism fails to properly validate incoming traffic against configured rules. The vulnerability specifically impacts the router's ability to enforce access control policies, enabling attackers to establish unauthorized connections or transmit data that would normally be blocked by configured ACLs. The flaw does not require authentication credentials or privileged access to exploit, making it particularly dangerous as it can be leveraged by any remote attacker with network access to the affected device. This represents a fundamental failure in the router's security architecture where the expected behavior of ACL enforcement is circumvented through predictable packet processing patterns.
The operational impact of this vulnerability extends beyond simple unauthorized network access, potentially enabling attackers to conduct reconnaissance activities, establish persistent backdoors, or launch further attacks against internal network segments that would normally be protected by ACL rules. Network administrators face the challenge of maintaining visibility into their network traffic while dealing with a security gap that could allow sophisticated attackers to bypass multiple layers of network security. The vulnerability's remote exploitability means that attackers can target these devices from outside the network perimeter without requiring physical access or local credentials, significantly expanding the attack surface. Organizations that rely on these routers for traffic filtering and security enforcement may find their network segmentation policies compromised, potentially leading to data breaches, service disruptions, or unauthorized access to sensitive network resources. The impact is particularly severe in environments where these routers serve as primary traffic gateways or where ACLs are used to enforce strict access controls between different network zones.
Mitigation strategies for CVE-2017-6672 require immediate software updates to the affected Cisco ASR 5000 Series routers, with administrators prioritizing deployment of the latest security patches provided by Cisco. Network segmentation should be enhanced through additional filtering mechanisms implemented at higher network layers, while monitoring systems should be configured to detect unusual traffic patterns that might indicate exploitation attempts. Organizations should conduct thorough network assessments to identify all affected devices and implement temporary workarounds such as disabling unnecessary services or implementing alternative access control measures. The vulnerability aligns with CWE-284 Access Control Bypass, where insufficient access control mechanisms allow unauthorized access to protected resources, and corresponds to ATT&CK technique T1071.004 Application Layer Protocol: DNS, as attackers may leverage the bypassed ACLs to establish covert communication channels. Regular security audits and vulnerability assessments should be implemented to identify similar weaknesses in other network infrastructure components, while incident response procedures should be updated to address potential exploitation of this vulnerability. The remediation process requires careful planning to minimize network disruption while ensuring complete protection against this specific bypass vulnerability.