CVE-2017-3820 in ASR 1000
Summary
by MITRE
A vulnerability in Simple Network Management Protocol (SNMP) functions of Cisco ASR 1000 Series Aggregation Services Routers running Cisco IOS XE Software Release 3.13.6S, 3.16.2S, or 3.17.1S could allow an authenticated, remote attacker to cause high CPU usage on an affected device, resulting in a denial of service (DoS) condition. More Information: CSCux68796. Known Affected Releases: 15.5(3)S2.1 15.6(1)S1.1. Known Fixed Releases: 15.4(3)S6.1 15.4(3)S6.2 15.5(3)S2.2 15.5(3)S3 15.6(0.22)S0.23 15.6(1)S2 16.2(0.295) 16.3(0.94) 15.5.3S3.
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Analysis
by VulDB Data Team • 11/11/2022
The vulnerability described in CVE-2017-3820 represents a significant denial of service weakness within the Simple Network Management Protocol implementation of Cisco ASR 1000 Series Aggregation Services Routers. This issue specifically affects devices operating on Cisco IOS XE Software versions 3.13.6S, 3.16.2S, and 3.17.1S, creating a scenario where authenticated remote attackers can exploit the SNMP functionality to generate excessive CPU utilization. The flaw manifests as a condition where normal network management operations become distorted, leading to system resource exhaustion that ultimately results in complete service disruption for legitimate network users.
The technical mechanism behind this vulnerability stems from improper handling of SNMP requests within the router's management subsystem. When an authenticated attacker crafts specific SNMP packets targeting the affected router, the device's processing logic becomes overwhelmed with computational overhead, causing the central processing unit to consume excessive resources. This behavior aligns with CWE-400, which classifies uncontrolled resource consumption as a critical weakness in software systems, particularly in network infrastructure devices where resource management is paramount. The vulnerability specifically impacts the SNMPv1 and SNMPv2c protocol implementations, where the router fails to properly validate or limit the processing of certain management requests that trigger recursive or iterative operations within the SNMP engine.
From an operational perspective, this vulnerability presents a severe risk to network availability and reliability, particularly in enterprise and service provider environments where ASR 1000 Series routers serve as critical aggregation points. The authenticated nature of the attack means that adversaries must first establish valid credentials, but this requirement does not significantly reduce the threat level given that many network management systems maintain credentials in accessible locations or that attackers may obtain valid authentication through other means such as compromised accounts or insider threats. The resulting high CPU usage can persist for extended periods, potentially causing cascading failures throughout the network infrastructure as the affected router becomes unresponsive to legitimate management traffic and may fail to forward normal data packets.
The impact extends beyond simple service interruption to encompass potential business disruption and increased operational costs for network administrators who must respond to the DoS condition. Network monitoring systems may detect the high CPU utilization as an anomaly, triggering alerts that require manual intervention to restore normal operations. In large-scale deployments, the vulnerability could affect multiple routers simultaneously if similar configurations exist across the network, potentially creating widespread disruption. The attack vector through the SNMP protocol also aligns with ATT&CK technique T1078 which covers legitimate credentials use for persistence and privilege escalation, though in this case the technique manifests as a DoS rather than unauthorized access.
Mitigation strategies should prioritize immediate patch deployment to affected devices, with network administrators upgrading to the fixed software releases specified in the advisory including versions 15.4(3)S6.1, 15.5(3)S2.2, 15.6(0.22)S0.23, and subsequent releases. Additionally, implementing network segmentation to isolate SNMP traffic, configuring access control lists to limit SNMP access to trusted management stations, and monitoring for unusual CPU utilization patterns can provide additional defensive layers. Network administrators should also consider disabling SNMPv1 and SNMPv2c if SNMPv3 is available and properly configured, as these older protocols lack the security features to prevent exploitation. The vulnerability highlights the importance of regular security updates and the need for comprehensive network monitoring to detect anomalous behavior that could indicate exploitation attempts.