CVE-2026-20118 in IOS XR
Summary
by MITRE • 03/11/2026
A vulnerability in the handling of an Egress Packet Network Interface (EPNI) Aligner interrupt in Cisco IOS XR Software for Cisco Network Convergence System (NCS) 5500 Series with NC57 line cards and Cisco NCS 5700 Routers and Cisco IOS XR Software for Third Party Software could allow an unauthenticated, remote attacker to cause the network processing unit (NPU) and ASIC to stop processing, preventing traffic from traversing the interface.
This vulnerability is due to the corruption of packets in specific cases when an EPNI Aligner interrupt is triggered while an affected device is experiencing heavy transit traffic. An attacker could exploit this vulnerability by sending a continuous flow of crafted packets to an interface of the affected device. A successful exploit could allow the attacker to cause persistent, heavy packet loss, resulting in a denial of service (DoS) condition. Note: If active exploitation of this vulnerability is suspected, contact the Cisco Technical Assistance Center (TAC) or your contracted maintenance provider. Cisco has assigned this security advisory a Security Impact Rating (SIR) of High rather than Medium as the score indicates. This change was made because the affected device operates within a critical network segment where compromise could lead to significant disruption or exposure, thereby elevating the overall risk beyond the base technical severity.
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Analysis
by VulDB Data Team • 03/14/2026
The vulnerability described in CVE-2026-20118 represents a critical flaw in the network processing architecture of Cisco's NCS 5500 Series and NCS 5700 Routers running IOS XR Software. This issue manifests within the Egress Packet Network Interface (EPNI) Aligner interrupt handling mechanism, which is fundamental to the proper functioning of network interfaces in high-performance routing equipment. The vulnerability specifically targets the interaction between the Network Processing Unit (NPU) and Application-Specific Integrated Circuits (ASIC) that manage packet forwarding operations. The flaw exists in how the system processes interrupt signals during periods of heavy traffic, creating a condition where packet corruption occurs during critical processing phases. This represents a significant concern for network infrastructure as it directly impacts the reliability and availability of critical network services.
The technical execution of this vulnerability requires an attacker to exploit specific timing conditions during high transit traffic periods when EPNI Aligner interrupts are triggered. The attack vector is remote and unauthenticated, meaning any external party can potentially exploit this weakness without requiring prior access credentials or privileged positions within the network. The attacker must maintain a continuous flow of crafted packets directed at vulnerable interfaces, which then triggers the corrupt packet processing sequence. This mechanism operates at the hardware-software boundary where the IOS XR operating system interacts with the underlying ASIC and NPU components, creating a pathway for persistent disruption. The vulnerability's exploitation is not limited to a single interface but can potentially affect multiple network paths depending on the device configuration and traffic patterns. The underlying cause relates to improper state management during interrupt processing, which falls under the category of memory corruption vulnerabilities as defined by CWE-129 and CWE-787.
The operational impact of CVE-2026-20118 extends beyond simple packet loss to create a complete denial of service condition that can severely disrupt network operations. When the NPU and ASIC stop processing packets, the entire network interface becomes non-functional, leading to complete traffic blocking at the affected device. This creates cascading effects throughout the network topology as routing decisions become impossible and services dependent on the affected paths experience complete interruption. The vulnerability's designation as High severity by Cisco reflects its potential for significant business impact, particularly in mission-critical network segments where downtime translates directly to financial losses and service degradation. Network administrators must consider that this vulnerability can be exploited continuously without detection, making it particularly dangerous in environments where network monitoring may not immediately identify the specific packet patterns causing the disruption.
The security implications of this vulnerability align with ATT&CK framework techniques related to denial of service attacks and system manipulation. The attack leverages the system's interrupt handling mechanisms to create a persistent state where normal packet processing cannot occur, which corresponds to techniques such as process injection and system service manipulation. Organizations should implement immediate mitigation strategies including traffic filtering at network boundaries, interface isolation, and monitoring for unusual packet patterns that might indicate exploitation attempts. The vulnerability's nature suggests that defensive measures should focus on network traffic analysis and the implementation of traffic shaping policies that prevent the specific packet patterns from reaching vulnerable interfaces. Additionally, Cisco recommends applying the latest software patches and updates to address the root cause of the interrupt handling flaw, which typically involves correcting the state management logic in the EPNI Aligner component. The high security impact rating indicates that organizations must treat this vulnerability as a priority for remediation, particularly in environments where network availability is critical to business operations.