CVE-2026-16876: UNIVERGE IX Authentication Bypass Vulnerability

CVE-2026-16876 Overview

CVE-2026-16876 is an authentication bypass vulnerability affecting the WebGUI of NEC UNIVERGE IX-R and IX-V series routers. An unauthenticated remote attacker can tamper with WebGUI messages and send them to the device over the internet to bypass authentication controls. Once authentication is bypassed, the attacker can execute arbitrary command-line interface (CLI) commands on the affected device. The flaw is classified under [CWE-306] Missing Authentication for a Critical Function, indicating that a security-sensitive function fails to require credentials before processing requests.

Critical Impact

Unauthenticated remote attackers can execute arbitrary CLI commands on affected UNIVERGE IX-R/IX-V devices exposed to the internet, enabling full network device compromise.

Affected Products

• NEC UNIVERGE IX-R Series
• NEC UNIVERGE IX-V Series
• Refer to the NEC Security Information Update for the complete list of affected firmware versions

Discovery Timeline

• 2026-09-07 - CVE-2026-16876 published to the National Vulnerability Database (NVD)
• 2026-09-09 - Last updated in NVD database

Technical Details for CVE-2026-16876

Vulnerability Analysis

The vulnerability resides in the WebGUI management interface of UNIVERGE IX-R and IX-V routers. The WebGUI processes messages without adequately verifying that the sender has successfully authenticated. An attacker crafts or modifies WebGUI protocol messages and submits them directly to the device. The device honors these tampered requests and forwards the embedded commands to the CLI handler.

Because the affected component is a network-facing management interface, exploitation requires no local access, user interaction, or prior credentials. Successful exploitation results in arbitrary CLI command execution, which grants an attacker administrative control over routing, access-control lists, VPN configurations, and traffic inspection functions on the compromised device.

Root Cause

The root cause is Missing Authentication for a Critical Function [CWE-306]. The WebGUI request handler does not enforce authentication state on privileged operations that reach CLI execution paths. Message tampering allows the attacker to reach code that should be restricted to authenticated administrators.

Attack Vector

The attack vector is network based. An attacker sends specially crafted WebGUI messages to the device's management interface over the internet. No credentials, user interaction, or elevated privileges are required. Devices exposing the WebGUI to untrusted networks are directly reachable and exploitable. NEC has not published proof-of-concept code, and no public exploit is currently listed in ExploitDB or CISA's Known Exploited Vulnerabilities catalog.

For technical specifics on affected versions and patch levels, see the NEC Security Information Update.

Detection Methods for CVE-2026-16876

Indicators of Compromise

• Unexpected CLI configuration changes on UNIVERGE IX-R/IX-V devices, including new administrative accounts, altered access-control lists, or modified routing entries
• Anomalous HTTP or HTTPS requests targeting the WebGUI endpoint from external or unrecognized source addresses
• Outbound connections from the router to unfamiliar destinations, potentially indicating post-exploitation command and control
• Unexpected reboots, configuration reloads, or gaps in device logging

Detection Strategies

• Inspect network flows to and from the management interface of UNIVERGE IX-R/IX-V devices for unauthorized WebGUI traffic
• Compare running configurations against known-good baselines to detect unsolicited CLI changes
• Correlate WebGUI access logs with authenticated administrator sessions to identify requests that bypass login
• Alert on any WebGUI access originating from public IP ranges when management should be restricted to internal networks

Monitoring Recommendations

• Forward device syslog and configuration change events to a centralized SIEM for continuous review
• Enable and review authentication and authorization logs on affected devices at least daily
• Monitor for repeated malformed WebGUI requests that may indicate tampering attempts
• Track internet-exposed management interfaces using external attack surface scanning

How to Mitigate CVE-2026-16876

Immediate Actions Required

• Restrict WebGUI access to trusted management networks using access-control lists or firewall rules, and block all inbound WebGUI traffic from the internet
• Apply the firmware update published by NEC as soon as it is available for your device model
• Audit device configurations for unauthorized changes and rotate all administrative credentials
• Disable the WebGUI interface entirely on devices where CLI or out-of-band management is sufficient

Patch Information

NEC has published a security advisory covering CVE-2026-16876. Consult the NEC Security Information Update for firmware versions, patched releases, and detailed upgrade guidance for each affected UNIVERGE IX-R/IX-V model.

Workarounds

• Place the WebGUI behind a VPN or jump host so it is not reachable from the public internet
• Enforce source-address filtering at the network edge to permit management traffic only from designated administrator subnets
• Segment management traffic onto a dedicated out-of-band network isolated from production data paths
• Continuously monitor device configurations and authentication logs until the patched firmware is deployed

# Example: restrict WebGUI access to a trusted management subnet
# Replace 192.0.2.0/24 with your administrator network
# Consult NEC documentation for exact IX-R/IX-V syntax

access-list mgmt-acl permit tcp 192.0.2.0/24 any eq 443
access-list mgmt-acl deny   tcp any any eq 443
interface GigaEthernet0.0
 ip access-group mgmt-acl in