CVE-2015-0776 in IOS XR
Summary
by MITRE
telnetd in Cisco IOS XR 5.0.1 on Network Convergence System 6000 devices allows remote attackers to cause a denial of service (device reload) via a malformed TELNET packet, aka Bug ID CSCuq31566.
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Analysis
by VulDB Data Team • 06/30/2017
The vulnerability described in CVE-2015-0776 represents a critical denial of service flaw within Cisco IOS XR 5.0.1 operating on Network Convergence System 6000 devices. This issue specifically targets the telnetd service component that handles remote terminal connections, creating a scenario where malicious actors can exploit a malformed telnet packet to trigger an unauthorized device reload. The vulnerability exists in the protocol handling mechanism of the telnet daemon, which fails to properly validate incoming packet structures before processing them, leading to an abrupt system restart that disrupts network operations and service availability.
The technical implementation of this vulnerability stems from insufficient input validation within the telnetd service of the IOS XR operating system. When the system receives a malformed telnet packet containing crafted data sequences, the parsing routine does not adequately sanitize or reject invalid packet structures. This processing error causes the telnet daemon to enter an unstable state where it either crashes or triggers an automatic system reboot as part of its error handling mechanism. The flaw is classified as a buffer over-read or improper state handling issue that can be exploited through network-based attacks without requiring authentication or privileged access. According to CWE classification, this vulnerability maps to CWE-129: Improper Validation of Array Index, as the system fails to validate packet boundaries and data lengths before processing, and potentially CWE-248: Uncaught Exception, since the system does not properly handle malformed input sequences.
The operational impact of this vulnerability extends beyond simple service disruption, as it can lead to significant network reliability issues and potential business continuity concerns. Network convergence system 6000 devices serve as critical infrastructure components in telecommunications environments, where device uptime directly correlates to service availability for end users. When exploited, the vulnerability can cause unexpected device reloads that may result in temporary network outages, traffic disruption, and service degradation. Attackers can leverage this vulnerability to perform sustained denial of service attacks by repeatedly sending malformed packets, potentially causing cascading failures in network infrastructure. The automatic reload process can also result in data loss, configuration changes, or disruption of ongoing network operations, particularly in environments where multiple devices are interconnected and rely on stable communication paths.
Organizations operating affected Cisco IOS XR 5.0.1 systems should implement immediate mitigations to protect their network infrastructure from exploitation. The primary recommended approach involves applying the vendor-provided security patches and software updates that address the specific telnet packet validation issues. Network administrators should also consider implementing access control measures such as firewall rules that restrict telnet access to trusted networks only, or alternatively disable telnet services entirely in favor of more secure remote access protocols like SSH. Additionally, network monitoring solutions should be configured to detect and alert on unusual packet patterns or device restart events that may indicate exploitation attempts. From an ATT&CK framework perspective, this vulnerability aligns with techniques such as T1499.004: Endpoint Denial of Service and T1566.001: Phishing, as attackers may use network-based attacks to trigger the vulnerability while potentially combining it with social engineering to maintain persistent access to compromised systems. Network segmentation and intrusion detection systems should be deployed to prevent lateral movement if exploitation occurs, and regular security assessments should be conducted to identify and remediate similar vulnerabilities in other network components.