CVE-2025-20159: Cisco IOS XR ACL Bypass Vulnerability

CVE-2025-20159 Overview

CVE-2025-20159 is an access control weakness in the management interface access control list (ACL) processing feature of Cisco IOS XR Software. An unauthenticated, remote attacker can bypass configured ingress ACLs applied to the management interface for the Secure Shell (SSH), Network Configuration Protocol (NetConf), and gRPC services. The flaw exists because management interface ACLs are not supported on Cisco IOS XR Packet I/O infrastructure platforms for Linux-handled features. The vulnerability is tracked under [CWE-284] Improper Access Control and affects the network-facing management plane.

Critical Impact
Attackers can reach SSH, NetConf, and gRPC management services on affected Cisco IOS XR devices even when administrators believe restrictive ACLs are enforced.

Affected Products

Cisco IOS XR Software running on Packet I/O infrastructure platforms
Devices exposing SSH, NetConf, or gRPC on management interfaces
Cisco IOS XR deployments relying on management interface ACLs for ingress filtering

Discovery Timeline

2025-09-10 - CVE-2025-20159 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-20159

Vulnerability Analysis

The vulnerability resides in how Cisco IOS XR Software processes management interface ACLs on Packet I/O infrastructure platforms. Administrators typically configure ingress ACLs on management interfaces to restrict which source addresses may reach management services. On the affected platforms, ACL enforcement is not applied to traffic destined for Linux-handled features, including SSH, NetConf, and gRPC.

As a result, traffic bound to those services reaches the control plane without ACL evaluation. An unauthenticated remote attacker can send packets to the exposed service ports and interact with those daemons directly. Successful exploitation does not grant code execution or credential compromise on its own, but it removes a network-layer control that many operators rely on to reduce the attack surface. The attacker gains reachability to authentication interfaces that should be network-restricted, enabling follow-on activity such as credential brute force, protocol fuzzing, or exploitation of downstream service vulnerabilities.

Root Cause

The root cause is missing ACL support in the Packet I/O infrastructure code path for Linux-handled management features. Packets destined for SSH, NetConf, and gRPC daemons are handed to Linux user-space services without transiting the ACL evaluation logic used for other management traffic. This is a design-level access control gap categorized as [CWE-284].

Attack Vector

Exploitation requires only network reachability to the management interface. The attacker sends TCP traffic to the SSH, NetConf, or gRPC listening port on an affected Cisco IOS XR device. Because ACL processing is skipped for those Linux-handled services, packets that would otherwise be denied are accepted and processed. No authentication, user interaction, or privileged position is required to trigger the bypass.

No verified public exploit code is available. The vulnerability is described in Cisco's advisory rather than through third-party proof of concept. See the Cisco Security Advisory for technical details.

Detection Methods for CVE-2025-20159

Indicators of Compromise

Successful TCP connections to SSH (port 22), NetConf (port 830), or gRPC ports on management interfaces from source addresses that should be blocked by configured ACLs
Authentication attempts in show logging or AAA logs originating from unexpected source prefixes
gRPC or NetConf session establishment events from hosts outside the intended management network

Detection Strategies

Compare NetFlow or sFlow telemetry from management interfaces against the configured ingress ACL and alert on traffic that should have been dropped
Enable and centralize logging for SSH, NetConf, and gRPC session events, then correlate source IPs against the management ACL policy
Deploy upstream infrastructure ACLs or firewall rules that mirror the intended management ACL and generate alerts on denied traffic

Monitoring Recommendations

Forward Cisco IOS XR syslog and AAA events to a centralized data lake for source-address correlation and long-term retention
Baseline expected management-plane sources and alert on deviations, especially connections from internet-facing address space
Track configuration changes to management ACLs and validate policy effectiveness through periodic connectivity tests from disallowed source addresses

How to Mitigate CVE-2025-20159

Immediate Actions Required

Apply the fixed Cisco IOS XR Software release identified in the Cisco Security Advisory for your platform
Enforce ingress filtering upstream of affected devices using infrastructure ACLs on adjacent routers or firewalls until patching is complete
Restrict management interface exposure to dedicated out-of-band management networks
Audit AAA and session logs for unexpected SSH, NetConf, or gRPC connections from prohibited source ranges

Patch Information

Cisco has published fixed software versions in the security advisory cisco-sa-acl-packetio-Swjhhbtz. Operators should identify the affected platform, confirm exposure via Cisco's Software Checker, and upgrade to a release that implements ACL enforcement for Linux-handled features. Consult the Cisco Security Advisory for the authoritative fixed-release list.

Workarounds

Place affected devices behind an upstream firewall or router that enforces the intended management ACL policy
Move management traffic to a dedicated management VRF with strict route filtering, limiting reachability to authorized jump hosts
Disable NetConf and gRPC services on devices where they are not required, reducing the exposed Linux-handled surface

bash

# Configuration example: apply an upstream infrastructure ACL as a compensating control
ipv4 access-list MGMT-PLANE-FILTER
 10 permit tcp 10.10.0.0/24 host <mgmt-ip> eq 22
 20 permit tcp 10.10.0.0/24 host <mgmt-ip> eq 830
 30 permit tcp 10.10.0.0/24 host <mgmt-ip> eq 57400
 40 deny   ipv4 any host <mgmt-ip> log
!
interface <upstream-interface>
 ipv4 access-group MGMT-PLANE-FILTER ingress