CVE-2017-3876 in IOS XR
Summary
by MITRE
A vulnerability in the Event Management Service daemon (emsd) of Cisco IOS XR routers could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on the affected device. The vulnerability is due to improper handling of gRPC requests. An attacker could exploit this vulnerability by repeatedly sending unauthenticated gRPC requests to the affected device. A successful exploit could allow the attacker to crash the device in such a manner that manual intervention is required to recover. This vulnerability affects all Cisco IOS XR platforms that are running release 6.1.1 of Cisco IOS XR Software when the gRPC service is enabled on the device. The gRPC service is not enabled by default. Cisco Bug IDs: CSCvb14441.
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Analysis
by VulDB Data Team • 12/22/2020
The vulnerability identified as CVE-2017-3876 resides within the Event Management Service daemon (emsd) of Cisco IOS XR routers, representing a critical denial of service weakness that can be exploited remotely without authentication. This flaw specifically manifests in the improper handling of gRPC (Google Remote Procedure Call) requests, which forms a core component of the device's service architecture. The vulnerability affects all Cisco IOS XR platforms operating on release 6.1.1 of the software when the gRPC service is enabled, making it particularly concerning given the widespread deployment of these networking devices in critical infrastructure environments. The gRPC service, while not enabled by default, represents a potential attack vector that security administrators must consider in their risk assessments.
The technical implementation of this vulnerability stems from insufficient input validation and error handling within the emsd daemon when processing gRPC communication protocols. When an attacker sends repeated unauthenticated gRPC requests to the affected device, the system fails to properly manage these malformed or excessive requests, leading to a condition where the Event Management Service daemon becomes unstable. This improper request handling creates a resource exhaustion scenario where the daemon's memory management or thread handling mechanisms become compromised, ultimately resulting in a system crash. The nature of the vulnerability demonstrates a classic lack of robustness in protocol handling, where the system does not adequately implement rate limiting, request validation, or proper error recovery mechanisms. This weakness directly maps to CWE-20, "Improper Input Validation," and CWE-400, "Uncontrolled Resource Consumption," which are fundamental security principles that should be implemented in any network service handling external communications.
The operational impact of this vulnerability extends beyond simple service disruption, as it requires manual intervention for system recovery, significantly increasing the operational burden on network administrators and potentially leading to extended downtime in mission-critical environments. The attack vector is particularly dangerous because it requires no authentication credentials, making it accessible to any remote attacker who can reach the device's gRPC service port. This vulnerability can be exploited through automated scanning tools that identify devices with the gRPC service enabled, potentially leading to widespread DoS attacks across networks that have not properly secured their IOS XR devices. The requirement for manual intervention indicates that the crash state may not be recoverable through automated restart mechanisms, further complicating incident response procedures. From an ATT&CK framework perspective, this vulnerability aligns with T1499.004, "Cloud Compute Disruption," and T1498, "Network Denial of Service," where the attacker leverages a service daemon flaw to cause system-wide disruption.
Mitigation strategies for CVE-2017-3876 should focus on both immediate defensive measures and long-term architectural improvements. The most effective immediate solution involves disabling the gRPC service on affected devices when it is not actively required, as the vulnerability only manifests when this service is enabled. Network administrators should implement network segmentation to restrict access to gRPC ports to only trusted management networks, while also considering firewall rules that limit the number of connections from any single source. Additionally, implementing monitoring solutions that can detect unusual patterns of gRPC requests or rapid connection attempts can provide early warning of potential exploitation attempts. The vulnerability highlights the importance of following security best practices such as the principle of least privilege, where services are only enabled when absolutely necessary. Organizations should also consider implementing intrusion detection systems that can identify and alert on malformed gRPC traffic patterns, and maintain current patch management procedures to ensure that all devices are updated to versions that address this specific vulnerability. Regular security assessments of network infrastructure should include verification of service configurations and proper implementation of access controls to prevent unauthorized exploitation of similar service daemon vulnerabilities.