CVE-2020-3511 in IOS
Summary
by MITRE
A vulnerability in the ISDN subsystem of Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, adjacent attacker to cause a reload of an affected device, resulting in a denial of service (DoS) condition. The vulnerability is due to insufficient input validation when the ISDN Q.931 messages are processed. An attacker could exploit this vulnerability by sending a malicious ISDN Q.931 message to an affected device. A successful exploit could allow the attacker to cause the process to crash, resulting in a reload of the affected device.
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Analysis
by VulDB Data Team • 09/25/2020
The vulnerability identified as CVE-2020-3511 resides within the ISDN subsystem of Cisco IOS and IOS XE software implementations, representing a critical security flaw that exposes network infrastructure to unauthorized disruption. This weakness specifically targets the processing of ISDN Q.931 messages which are fundamental components of the Integrated Services Digital Network protocol suite used for establishing and managing digital communication connections. The vulnerability manifests when the system fails to properly validate incoming Q.931 messages, creating an exploitable condition that can be leveraged by adversaries positioned within the same network segment. The flaw essentially allows an attacker to manipulate the message parsing logic without requiring authentication credentials, making it particularly dangerous in environments where network adjacency is easily achieved through physical or logical network access.
The technical nature of this vulnerability stems from inadequate input validation mechanisms within the ISDN Q.931 message processing pipeline, which constitutes a classic example of insufficient validation or sanitization as classified under CWE-20. When an attacker crafts and transmits a maliciously formatted Q.931 message, the system's failure to properly validate the message structure causes the processing routine to encounter unexpected data patterns. This leads to memory corruption or buffer overflow conditions that ultimately result in process crashes. The Q.931 protocol messages are essential for call setup, management, and teardown operations in ISDN networks, making this vulnerability particularly impactful as it targets core communication infrastructure. The vulnerability's exploitation requires only network adjacency rather than sophisticated attack vectors, making it accessible to a broad range of potential threat actors including both malicious insiders and external attackers who can establish local network presence.
The operational impact of CVE-2020-3511 extends beyond simple service disruption to potentially compromise network reliability and availability. When successfully exploited, the vulnerability forces the affected Cisco device to undergo an automatic reload process, effectively causing a denial of service condition that can last from several minutes to hours depending on the device configuration and network complexity. This disruption can have cascading effects on network services, particularly in mission-critical environments where continuous availability is paramount. Network administrators may experience significant operational challenges as the device reloads, potentially losing connectivity for users and services dependent on the affected infrastructure. The vulnerability's impact is amplified in environments where multiple devices are interconnected through ISDN links, as a single exploited device can create widespread disruption across the network topology. The DoS condition can also provide cover for more sophisticated attacks by creating operational chaos and diverting administrative attention from other potential threats.
Mitigation strategies for CVE-2020-3511 should prioritize immediate implementation of Cisco's recommended security patches and updates, which address the input validation deficiencies in the ISDN subsystem. Network administrators should implement network segmentation and access controls to limit network adjacency opportunities for unauthorized users, particularly in environments where ISDN functionality is not actively required. The deployment of intrusion detection systems capable of monitoring for anomalous Q.931 message patterns can provide early warning of potential exploitation attempts. Additionally, implementing network access control lists and firewall rules to restrict ISDN protocol traffic to only trusted sources can significantly reduce the attack surface. Organizations should also consider disabling ISDN functionality entirely if it is not required for business operations, following the principle of least privilege and minimizing potential attack vectors. The implementation of monitoring and logging mechanisms specifically designed to track ISDN protocol activity can help detect suspicious patterns and support forensic analysis following any potential exploitation attempts. Security teams should also conduct regular vulnerability assessments and penetration testing to ensure that all network devices are properly patched and configured to prevent exploitation of similar vulnerabilities. This vulnerability aligns with ATT&CK technique T1499.004 which involves network denial of service attacks, and demonstrates the importance of proper input validation in preventing system instability and availability disruptions.