CVE-2026-77540: UniFi OS Server RCE Vulnerability

CVE-2026-77540 Overview

CVE-2026-77540 is a command injection vulnerability in Ubiquiti UniFi OS Server. The flaw stems from improper input validation [CWE-20] and allows an authenticated attacker with high privileges and network access to execute arbitrary commands on the host device. Successful exploitation impacts confidentiality, integrity, and availability, and the scope changes to affect resources beyond the vulnerable component. Ubiquiti disclosed the issue in Ubiquiti Security Advisory Bulletin 067.

Critical Impact
Authenticated high-privileged attackers can execute arbitrary operating system commands on the UniFi OS Server host, resulting in full compromise of the device and connected network infrastructure.

Affected Products

Ubiquiti UniFi OS Server
Refer to Ubiquiti Security Advisory Bulletin 067 for affected versions
Deployments running UniFi OS Server on self-hosted hardware

Discovery Timeline

2026-08-26 - CVE-2026-77540 published to NVD
2026-08-27 - Last updated in NVD database

Technical Details for CVE-2026-77540

Vulnerability Analysis

The vulnerability resides in UniFi OS Server, the management operating system that runs Ubiquiti UniFi Network deployments. An input handler within the server fails to validate or sanitize user-supplied data before passing it to a system command execution context. An authenticated actor with high privileges can inject shell metacharacters or command sequences that are executed by the underlying operating system on the host device.

Because the scope changes to affect other components, executed commands run with the privileges of the UniFi OS Server process and can pivot into adjacent managed devices, configuration stores, and network segments controlled by the appliance. The CWE-20 classification indicates the root defect is missing or incomplete input validation rather than a downstream encoding failure.

Root Cause

The root cause is improper input validation on a request field that flows into a command execution path within UniFi OS Server. Ubiquiti has not published the specific parameter, endpoint, or code path in the public advisory. See the Ubiquiti Security Advisory Bulletin 067 for vendor details.

Attack Vector

The attack is delivered over the network against the UniFi OS Server management interface. The attacker must already hold high-privilege credentials on the server, which means the vulnerability chains most naturally after credential theft, privilege escalation, or abuse of a compromised administrator account. No user interaction is required. Once the crafted request is processed, the injected command runs on the host and can be used to install persistence, exfiltrate configuration data, or manipulate managed UniFi devices.

No public proof-of-concept exploit is currently available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Detection Methods for CVE-2026-77540

Indicators of Compromise

Unexpected child processes spawned by the UniFi OS Server service or web application worker
Outbound network connections from the UniFi host to unfamiliar destinations following administrative API activity
New or modified cron jobs, systemd units, or startup scripts on the UniFi OS Server host
Administrative API requests containing shell metacharacters such as ;, |, &&, backticks, or $(...) in request parameters

Detection Strategies

Monitor UniFi OS Server audit logs for administrative actions executed by high-privileged accounts outside of normal change windows
Alert on process creation events where the UniFi service process is the parent of shells such as /bin/sh, /bin/bash, or interpreters like python and perl
Inspect HTTP request bodies and query strings sent to the UniFi management interface for command injection payload patterns

Monitoring Recommendations

Forward UniFi OS Server host logs and API access logs to a centralized analytics platform for correlation with authentication events
Baseline normal administrative behavior and alert on deviations such as new source IPs, unusual API endpoints, or off-hours activity
Track privileged account usage and require multi-factor authentication events to correlate with UniFi administrative sessions

How to Mitigate CVE-2026-77540

Immediate Actions Required

Apply the fixed UniFi OS Server release identified in Ubiquiti Security Advisory Bulletin 067 as soon as possible
Rotate credentials for all high-privileged UniFi OS Server accounts and enforce multi-factor authentication
Restrict network access to the UniFi OS Server management interface to trusted administrative networks only
Review recent administrative activity and host process history for signs of prior exploitation

Patch Information

Ubiquiti has published fixed builds in Ubiquiti Security Advisory Bulletin 067. Administrators should consult the advisory for the exact version numbers that resolve CVE-2026-77540 and upgrade all self-hosted UniFi OS Server instances.

Workarounds

Place the UniFi OS Server management interface behind a VPN or bastion host so it is not reachable from untrusted networks
Reduce the number of accounts with high administrative privileges and apply least-privilege role assignments
Enable network-layer access control lists on the UniFi host to limit inbound management traffic to specific administrator source addresses

bash

# Configuration example: restrict management interface access with host firewall rules
# Replace 10.0.0.0/24 with your administrative network
iptables -A INPUT -p tcp --dport 443 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP