CVE-2020-3408 in IOS
Summary
by MITRE
A vulnerability in the Split DNS feature of Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, remote attacker to cause an affected device to reload, resulting in a denial of service (DoS) condition. The vulnerability occurs because the regular expression (regex) engine that is used with the Split DNS feature of affected releases may time out when it processes the DNS name list configuration. An attacker could exploit this vulnerability by trying to resolve an address or hostname that the affected device handles. A successful exploit could allow the attacker to cause the device to reload, resulting in a DoS condition.
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Analysis
by VulDB Data Team • 09/25/2020
The vulnerability identified as CVE-2020-3408 resides within the Split DNS functionality of Cisco IOS and IOS XE software implementations, representing a critical denial of service weakness that can be exploited remotely without authentication. This flaw specifically targets the regular expression engine utilized by the Split DNS feature, creating a condition where legitimate configuration processing can become indefinitely suspended. The vulnerability affects multiple Cisco device families including routers and switches running affected software versions, making it particularly concerning given the widespread deployment of these networking devices in enterprise and service provider environments.
The technical mechanism behind this vulnerability involves the processing of DNS name list configurations through a flawed regular expression engine implementation. When an affected device encounters certain DNS resolution requests or hostname handling operations within the Split DNS context, the regex engine enters a timeout state that ultimately leads to system instability. This behavior stems from inadequate handling of specific input patterns that cause the engine to consume excessive processing resources or enter an infinite loop scenario. The vulnerability is particularly insidious because it can be triggered through normal DNS resolution operations that the device would typically process without issue, making exploitation both straightforward and difficult to detect.
Operationally, this vulnerability presents a significant risk to network availability and service continuity, as successful exploitation results in complete device reloads that can disrupt network operations for extended periods. Network administrators may experience unexpected outages or service interruptions when attackers leverage this vulnerability, particularly in environments where continuous network availability is critical. The DoS condition impacts not only the immediate device but can cascade through network infrastructure, potentially affecting multiple services that depend on the affected device for routing or DNS resolution. The remote nature of the attack means that adversaries can exploit this vulnerability from outside the network perimeter without requiring any credentials or privileged access, making it a particularly attractive target for malicious actors seeking to disrupt network services.
Mitigation strategies for CVE-2020-3408 primarily involve applying official Cisco security patches and software updates that address the specific regex engine implementation flaw. Network administrators should prioritize patching affected devices through official Cisco update channels and verify that the applied patches resolve the vulnerability without introducing compatibility issues. Additional defensive measures include implementing access control lists to restrict DNS query processing, monitoring network traffic for suspicious DNS resolution patterns, and configuring device logging to detect potential exploitation attempts. The vulnerability aligns with CWE-400, which describes the weakness of unspecified resource exhaustion, and can be mapped to ATT&CK technique T1499.004 for network denial of service attacks. Organizations should also consider implementing network segmentation and redundant routing paths to minimize the impact of potential exploitation, while maintaining comprehensive network monitoring to detect anomalous behavior that might indicate attempted exploitation of this vulnerability.